Genetics is the study of
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s,
genetic variation, and
heredity in
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
s.
[Hartl D, Jones E (2005)] It is an important branch in
biology
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
because
heredity is vital to organisms'
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
.
Gregor Mendel
Gregor Johann Mendel, OSA (; cs, Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was a biologist, meteorologist, mathematician, Augustinian friar and abbot of St. Thomas' Abbey in Brünn (''Brno''), Margraviate of Moravia. Mendel was ...
, a
Moravia
Moravia ( , also , ; cs, Morava ; german: link=yes, Mähren ; pl, Morawy ; szl, Morawa; la, Moravia) is a historical region in the east of the Czech Republic and one of three historical Czech lands, with Bohemia and Czech Silesia.
The m ...
n
Augustinian friar working in the 19th century in
Brno, was the first to study genetics scientifically. Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring over time. He observed that organisms (pea plants) inherit traits by way of discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene.
Trait inheritance and
molecular inheritance mechanisms of genes are still primary principles of genetics in the 21st century, but modern genetics has expanded to study the function and behavior of genes. Gene structure and function, variation, and distribution are studied within the context of the
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
, the organism (e.g.
dominance), and within the context of a population. Genetics has given rise to a number of subfields, including
molecular genetics
Molecular genetics is a sub-field of biology that addresses how differences in the structures or expression of DNA molecules manifests as variation among organisms. Molecular genetics often applies an "investigative approach" to determine the ...
,
epigenetics
In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are ...
and
population genetics
Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as Adaptation (biology), adaptation, ...
. Organisms studied within the broad field span the domains of life (
archaea,
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
, and
eukarya
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bact ...
).
Genetic processes work in combination with an organism's environment and experiences to influence development and
behavior, often referred to as
nature versus nurture. The
intracellular
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
or
extracellular
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
environment of a living cell or organism may switch gene transcription on or off. A classic example is two seeds of genetically identical corn, one placed in a temperate climate and one in an arid climate (lacking sufficient waterfall or rain). While the average height of the two corn stalks may be genetically determined to be equal, the one in the
arid climate
The desert climate or arid climate (in the Köppen climate classification ''BWh'' and ''BWk''), is a dry climate sub-type in which there is a severe excess of evaporation over precipitation. The typically bald, rocky, or sandy surfaces in deser ...
only grows to half the height of the one in the temperate climate due to lack of water and nutrients in its environment.
We need to know a lot more about the biology of viruses because of genetic analysis. The virus's genome, which is made up of 11 double-stranded RNA segments, serves as its defining characteristic. The primary characteristic of viral genetics is the genome's segmented structure's ability to reassign genome segments during mixed infections
Etymology
The word ''genetics'' stems from the
ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...
' meaning "genitive"/"generative", which in turn derives from ' meaning "origin".
History
The observation that living things inherit
traits from their parents has been used since prehistoric times to improve crop plants and animals through
selective breeding
Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant m ...
.
The modern science of genetics, seeking to understand this process, began with the work of the
Augustinian friar
Gregor Mendel
Gregor Johann Mendel, OSA (; cs, Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was a biologist, meteorologist, mathematician, Augustinian friar and abbot of St. Thomas' Abbey in Brünn (''Brno''), Margraviate of Moravia. Mendel was ...
in the mid-19th century.
Prior to Mendel,
Imre Festetics
Count Imre Festetics de Tolna (1764 – 1847) was a noble landowner and geneticist.
Scientific works
Many of the central principles the discipline of genetics were formulated by Imre Festetics through the study of sheep. Festetics formulated a nu ...
, a
Hungarian noble, who lived in Kőszeg before Mendel, was the first who used the word "genetic" in hereditarian context. He described several rules of biological inheritance in his works ''The genetic laws of the Nature'' (Die genetischen Gesetze der Natur, 1819).
His second law is the same as what Mendel published.
In his third law, he developed the basic principles of mutation (he can be considered a forerunner of
Hugo de Vries
Hugo Marie de Vries () (16 February 1848 – 21 May 1935) was a Dutch botanist and one of the first geneticists. He is known chiefly for suggesting the concept of genes, rediscovering the laws of heredity in the 1890s while apparently unaware o ...
). Festetics argued that changes observed in the generation of farm animals, plants, and humans are the result of scientific laws.
Festetics empirically deduced that organisms inherit their characteristics, not acquire them. He recognized recessive traits and inherent variation by postulating that traits of past generations could reappear later, and organisms could produce progeny with different attributes.
These observations represent an important prelude to Mendel’s theory of particulate inheritance insofar as it features a transition of heredity from its status as myth to that of a scientific discipline, by providing a fundamental theoretical basis for genetics in the twentieth century.
[ ]
Text was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
![Blending Inheritance](https://upload.wikimedia.org/wikipedia/commons/3/3b/Blending_Inheritance.svg)
Other theories of inheritance preceded Mendel's work. A popular theory during the 19th century, and implied by
Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
's 1859 ''
On the Origin of Species
''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'', was
blending inheritance
Blending may refer to:
* The process of mixing in process engineering
* Mixing paints to achieve a greater range of colors
* Blending (alcohol production), a technique to produce alcoholic beverages by mixing different brews
* Blending (linguisti ...
: the idea that individuals inherit a smooth blend of traits from their parents.
Mendel's work provided examples where traits were definitely not blended after hybridization, showing that traits are produced by combinations of distinct genes rather than a continuous blend. Blending of traits in the progeny is now explained by the action of multiple genes with
quantitative effects. Another theory that had some support at that time was the
inheritance of acquired characteristics
Lamarckism, also known as Lamarckian inheritance or neo-Lamarckism, is the notion that an organism can pass on to its offspring physical characteristics that the parent organism acquired through use or disuse during its lifetime. It is also calle ...
: the belief that individuals inherit traits strengthened by their parents. This theory (commonly associated with
Jean-Baptiste Lamarck
Jean-Baptiste Pierre Antoine de Monet, chevalier de Lamarck (1 August 1744 – 18 December 1829), often known simply as Lamarck (; ), was a French naturalist, biologist, academic, and soldier. He was an early proponent of the idea that biolo ...
) is now known to be wrong—the experiences of individuals do not affect the genes they pass to their children. Other theories included Darwin's
pangenesis
Pangenesis was Charles Darwin's hypothetical mechanism for heredity, in which he proposed that each part of the body continually emitted its own type of small organic particles called gemmules that aggregated in the gonads, contributing herita ...
(which had both acquired and inherited aspects) and
Francis Galton's reformulation of pangenesis as both particulate and inherited.
Mendelian genetics
![Sexlinked inheritance white](https://upload.wikimedia.org/wikipedia/commons/4/49/Sexlinked_inheritance_white.jpg)
Modern genetics started with Mendel's studies of the nature of inheritance in plants. In his paper "''Versuche über Pflanzenhybriden''" ("
Experiments on Plant Hybridization
"Experiments on Plant Hybridization" (German: "Versuche über Pflanzen-Hybriden") is a seminal paper written in 1865 and published in 1866 by Gregor Mendel, an Augustinian friar considered to be the founder of modern genetics. The paper was the r ...
"), presented in 1865 to the ''Naturforschender Verein'' (Society for Research in Nature) in
Brünn
Brno ( , ; german: Brünn ) is a city in the South Moravian Region of the Czech Republic. Located at the confluence of the Svitava and Svratka rivers, Brno has about 380,000 inhabitants, making it the second-largest city in the Czech Republic ...
, Mendel traced the inheritance patterns of certain traits in pea plants and described them mathematically. Although this pattern of inheritance could only be observed for a few traits, Mendel's work suggested that heredity was particulate, not acquired, and that the inheritance patterns of many traits could be explained through simple rules and ratios.
The importance of Mendel's work did not gain wide understanding until 1900, after his death, when
Hugo de Vries
Hugo Marie de Vries () (16 February 1848 – 21 May 1935) was a Dutch botanist and one of the first geneticists. He is known chiefly for suggesting the concept of genes, rediscovering the laws of heredity in the 1890s while apparently unaware o ...
and other scientists rediscovered his research.
William Bateson
William Bateson (8 August 1861 – 8 February 1926) was an English biologist who was the first person to use the term genetics to describe the study of heredity, and the chief populariser of the ideas of Gregor Mendel following their rediscove ...
, a proponent of Mendel's work, coined the word ''genetics'' in 1905. (The adjective ''genetic'', derived from the Greek word ''genesis''—γένεσις, "origin", predates the noun and was first used in a biological sense in 1860.) Bateson both acted as a mentor and was aided significantly by the work of other scientists from Newnham College at Cambridge, specifically the work of
Becky Saunders,
Nora Darwin Barlow, and
Muriel Wheldale Onslow
Muriel Wheldale Onslow (31 March 1880 – 19 May 1932) was a British biochemist, born in Birmingham, England. She studied the inheritance of flower colour in the common snapdragon Antirrhinum and the biochemistry of anthocyanin pigment molecules ...
. Bateson popularized the usage of the word ''genetics'' to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in
London
London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
in 1906.
[ :Initially titled the "International Conference on Hybridisation and Plant Breeding", the title was changed as a result of Bateson's speech. See: ]
After the rediscovery of Mendel's work, scientists tried to determine which molecules in the cell were responsible for inheritance. In 1900, Nettie Stevens began studying the mealworm.
Over the next 11 years, she discovered that females only had the X chromosome and males had both X and Y chromosomes.
She was able to conclude that sex is a chromosomal factor and is determined by the male.
In 1911,
Thomas Hunt Morgan
Thomas Hunt Morgan (September 25, 1866 – December 4, 1945) was an American evolutionary biologist, geneticist, embryologist, and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating the role that ...
argued that genes are on
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s, based on observations of a sex-linked
white eye mutation in
fruit flies
Fruit fly may refer to:
Organisms
* Drosophilidae, a family of small flies, including:
** ''Drosophila'', the genus of small fruit flies and vinegar flies
** ''Drosophila melanogaster'' or common fruit fly
** '' Drosophila suzukii'' or Asian frui ...
. In 1913, his student
Alfred Sturtevant
Alfred Henry Sturtevant (November 21, 1891 – April 5, 1970) was an American geneticist. Sturtevant constructed the first genetic map of a chromosome in 1911. Throughout his career he worked on the organism ''Drosophila melanogaster'' with ...
used the phenomenon of
genetic linkage to show that genes are arranged linearly on the chromosome.
Molecular genetics
![DNA Overview2](https://upload.wikimedia.org/wikipedia/commons/e/e4/DNA_Overview2.png)
Although genes were known to exist on chromosomes, chromosomes are composed of both
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
and DNA, and scientists did not know which of the two is responsible for inheritance.
In 1928,
Frederick Griffith
Frederick Griffith (1877–1941) was a British bacteriologist whose focus was the epidemiology and pathology of bacterial pneumonia. In January 1928 he reported what is now known as Griffith's Experiment, the first widely accepted demonstrati ...
discovered the phenomenon of
transformation
Transformation may refer to:
Science and mathematics
In biology and medicine
* Metamorphosis, the biological process of changing physical form after birth or hatching
* Malignant transformation, the process of cells becoming cancerous
* Tran ...
: dead bacteria could transfer
genetic material
Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main clas ...
to "transform" other still-living bacteria. Sixteen years later, in 1944, the
Avery–MacLeod–McCarty experiment
The Avery–MacLeod–McCarty experiment was an experimental demonstration, reported in 1944 by Oswald Avery, Colin MacLeod, and Maclyn McCarty, that DNA is the substance that causes bacterial transformation, in an era when it had been widely b ...
identified DNA as the molecule responsible for transformation.
[ Reprint: ] The role of the nucleus as the repository of genetic information in eukaryotes had been established by
Hämmerling in 1943 in his work on the single celled alga ''
Acetabularia''. The
Hershey–Chase experiment in 1952 confirmed that DNA (rather than protein) is the genetic material of the viruses that infect bacteria, providing further evidence that DNA is the molecule responsible for inheritance.
James Watson and
Francis Crick determined the structure of DNA in 1953, using the
X-ray crystallography
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
work of
Rosalind Franklin and
Maurice Wilkins that indicated DNA has a
helical
Helical may refer to:
* Helix, the mathematical concept for the shape
* Helical engine, a proposed spacecraft propulsion drive
* Helical spring, a coilspring
* Helical plc, a British property company, once a maker of steel bar stock
* Helicoil
A t ...
structure (i.e., shaped like a corkscrew).
Their double-helix model had two strands of DNA with the nucleotides pointing inward, each matching a complementary nucleotide on the other strand to form what look like rungs on a twisted ladder.
The a-helix is a secondary structure and the twisting in the a-helix is caused by hydrogen bonds between the carboxyl (C=O) and the amine H (N-H) constituents of the polypeptide backbone. This structure showed that genetic information exists in the sequence of nucleotides on each strand of DNA. The structure also suggested a simple method for
replication: if the strands are separated, new partner strands can be reconstructed for each based on the sequence of the old strand. This property is what gives DNA its semi-conservative nature where one strand of new DNA is from an original parent strand.
Although the structure of DNA showed how inheritance works, it was still not known how DNA influences the behavior of cells. In the following years, scientists tried to understand how DNA controls the process of
protein production
Protein production is the biotechnological process of generating a specific protein. It is typically achieved by the manipulation of gene expression in an organism such that it expresses large amounts of a recombinant gene. This includes the t ...
.
It was discovered that the cell uses DNA as a template to create matching
messenger RNA, molecules with
nucleotide
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecule ...
s very similar to DNA. The nucleotide sequence of a messenger RNA is used to create an
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
sequence in protein; this translation between nucleotide sequences and amino acid sequences is known as the
genetic code
The genetic code is the set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets, or codons) into proteins. Translation is accomplished by the ribosome, which links ...
.
With the newfound molecular understanding of inheritance came an explosion of research.
A notable theory arose from
Tomoko Ohta
Tomoko (ともこ, トモコ) is a female Japanese given name.
Like many Japanese names, Tomoko can be written using different kanji characters and can mean:
* 友子 - "friendly child"
* 知子 - "knowing child"
* 智子 - "wise child"
* 朋 ...
in 1973 with her amendment to the
neutral theory of molecular evolution
The neutral theory of molecular evolution holds that most evolutionary changes occur at the molecular level, and most of the variation within and between species are due to random genetic drift of mutant alleles that are selectively neutral. The ...
through publishing the
nearly neutral theory of molecular evolution
The nearly neutral theory of molecular evolution is a modification of the neutral theory of molecular evolution that accounts for the fact that not all mutations are either so deleterious such that they can be ignored, or else neutral. Slightly del ...
. In this theory, Ohta stressed the importance of natural selection and the environment to the rate at which genetic
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
occurs. One important development was chain-termination
DNA sequencing in 1977 by
Frederick Sanger. This technology allows scientists to read the nucleotide sequence of a DNA molecule.
In 1983,
Kary Banks Mullis
Kary Banks Mullis (December 28, 1944August 7, 2019) was an American biochemist. In recognition of his role in the invention of the polymerase chain reaction (PCR) technique, he shared the 1993 Nobel Prize in Chemistry with Michael Smith and was ...
developed the
polymerase chain reaction
The polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete or partial) of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it (or a part of it) ...
, providing a quick way to isolate and amplify a specific section of DNA from a mixture.
The efforts of the
Human Genome Project, Department of Energy, NIH, and parallel private efforts by
Celera Genomics
Celera is a subsidiary of Quest Diagnostics which focuses on genetic sequencing and related technologies. It was founded in 1998 as a business unit of Applera, spun off into an independent company in 2008, and finally acquired by Quest Diagnost ...
led to the sequencing of the
human genome
The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the ...
in 2003.
Features of inheritance
Discrete inheritance and Mendel's laws
![Punnett square mendel flowers](https://upload.wikimedia.org/wikipedia/commons/1/17/Punnett_square_mendel_flowers.svg)
At its most fundamental level, inheritance in organisms occurs by passing discrete heritable units, called
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s, from parents to offspring.
This property was first observed by Gregor Mendel, who studied the segregation of heritable traits in
pea plants, showing for example that flowers on a single plant were either purple or white—but never an intermediate between the two colors. The discrete versions of the same gene controlling the inherited appearance (phenotypes) are called
allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
s.
In the case of the pea, which is a
diploid species, each individual plant has two copies of each gene, one copy inherited from each parent.
Many species, including humans, have this pattern of inheritance. Diploid organisms with two copies of the same allele of a given gene are called
homozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mo ...
at that
gene locus
In genetics, a locus (plural loci) is a specific, fixed position on a chromosome where a particular gene or genetic marker is located. Each chromosome carries many genes, with each gene occupying a different position or locus; in humans, the total ...
, while organisms with two different alleles of a given gene are called
heterozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mo ...
. The set of alleles for a given organism is called its
genotype, while the observable traits of the organism are called its
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological pr ...
. When organisms are heterozygous at a gene, often one allele is called
dominant as its qualities dominate the phenotype of the organism, while the other allele is called
recessive
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
as its qualities recede and are not observed. Some alleles do not have complete dominance and instead have
incomplete dominance
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and ...
by expressing an intermediate phenotype, or
codominance
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
by expressing both alleles at once.
When a pair of organisms
reproduce sexually, their offspring randomly inherit one of the two alleles from each parent. These observations of discrete inheritance and the segregation of alleles are collectively known as
Mendel's first law or the Law of Segregation. However, the probability of getting one gene over the other can change due to dominant, recessive, homozygous, or heterozygous genes. For example, Mendel found that if you cross homozygous dominate trait and homozygous recessive trait your odds of getting the dominant trait is 3:1. Real geneticist study and calculate probabilities by using theoretical probabilities, empirical probabilities, the product rule, the sum rule, and more.
Notation and diagrams
![Pedigree-chart-example](https://upload.wikimedia.org/wikipedia/commons/9/9c/Pedigree-chart-example.svg)
Geneticists use diagrams and symbols to describe inheritance. A gene is represented by one or a few letters. Often a "+" symbol is used to mark the usual,
non-mutant allele for a gene.
In fertilization and breeding experiments (and especially when discussing Mendel's laws) the parents are referred to as the "P" generation and the offspring as the "F1" (first filial) generation. When the F1 offspring mate with each other, the offspring are called the "F2" (second filial) generation. One of the common diagrams used to predict the result of cross-breeding is the
Punnett square.
When studying human genetic diseases, geneticists often use
pedigree chart
A pedigree chart is a diagram that shows the occurrence and appearance of phenotypes of a particular gene or organism and its ancestors from one generation to the next, most commonly humans, show dogs, and race horses.
Definition
The word pedigre ...
s to represent the inheritance of traits.
These charts map the inheritance of a trait in a family tree.
Multiple gene interactions
![Galton-height-regress](https://upload.wikimedia.org/wikipedia/commons/6/62/Galton-height-regress.png)
Organisms have thousands of genes, and in sexually reproducing organisms these genes generally assort independently of each other. This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. This phenomenon, known as "
Mendel's second law" or the "law of independent assortment," means that the alleles of different genes get shuffled between parents to form offspring with many different combinations. Different genes often interact to influence the same trait. In the
Blue-eyed Mary (''Omphalodes verna''), for example, there exists a gene with alleles that determine the color of flowers: blue or magenta. Another gene, however, controls whether the flowers have color at all or are white. When a plant has two copies of this white allele, its flowers are white—regardless of whether the first gene has blue or magenta alleles. This interaction between genes is called
epistasis, with the second gene epistatic to the first.
Many traits are not discrete features (e.g. purple or white flowers) but are instead continuous features (e.g. human height and
skin color
Human skin color ranges from the darkest brown to the lightest hues. Differences in skin color among individuals is caused by variation in pigmentation, which is the result of genetics (inherited from one's biological parents and or individu ...
). These
complex traits
Complex traits, also known as quantitative traits, are traits that do not behave according to simple Mendelian inheritance laws. More specifically, their inheritance cannot be explained by the genetic segregation of a single gene. Such traits show ...
are products of many genes. The influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. The degree to which an organism's genes contribute to a complex trait is called
heritability.
Measurement of the heritability of a trait is relative—in a more variable environment, the environment has a bigger influence on the total variation of the trait. For example, human height is a trait with complex causes. It has a heritability of 89% in the United States. In Nigeria, however, where people experience a more variable access to good nutrition and
health care, height has a heritability of only 62%.
Molecular basis for inheritance
DNA and chromosomes
![DNA sequence, sequences](https://upload.wikimedia.org/wikipedia/commons/a/ac/DNA_sequence%2C_sequences.gif)
The
molecular basis for genes is
deoxyribonucleic acid (DNA). DNA is composed of
deoxyribose
Deoxyribose, or more precisely 2-deoxyribose, is a monosaccharide with idealized formula H−(C=O)−(CH2)−(CHOH)3−H. Its name indicates that it is a deoxy sugar, meaning that it is derived from the sugar ribose by loss of a hydroxy group. D ...
(sugar molecule), a phosphate group, and a base (amine group). There are four types of bases:
adenine
Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its deri ...
(A),
cytosine
Cytosine () ( symbol C or Cyt) is one of the four nucleobases found in DNA and RNA, along with adenine, guanine, and thymine (uracil in RNA). It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached (an ...
(C),
guanine
Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is c ...
(G), and
thymine
Thymine () ( symbol T or Thy) is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidi ...
(T). The phosphates make hydrogen bonds with the sugars to make long phosphate-sugar backbones. Bases specifically pair together (T&A, C&G) between two backbones and make like rungs on a ladder. The bases, phosphates, and sugars together make a
nucleotide
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecule ...
that connects to make long chains of DNA. Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain.
These chains coil into a double a-helix structure and wrap around proteins called
Histones
In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosomes in turn ar ...
which provide the structural support. DNA wrapped around these histones are called chromosomes.
Virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsk ...
es sometimes use the similar molecule
RNA instead of DNA as their genetic material.
DNA normally exists as a double-stranded molecule, coiled into the shape of a
double helix
A double is a look-alike or doppelgänger; one person or being that resembles another.
Double, The Double or Dubble may also refer to:
Film and television
* Double (filmmaking), someone who substitutes for the credited actor of a character
* ...
. Each nucleotide in DNA preferentially pairs with its partner nucleotide on the opposite strand: A pairs with T, and C pairs with G. Thus, in its two-stranded form, each strand effectively contains all necessary information, redundant with its partner strand. This structure of DNA is the physical basis for inheritance: DNA replication duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand.
![Human karyotype with bands and sub-bands](https://upload.wikimedia.org/wikipedia/commons/b/b1/Human_karyotype_with_bands_and_sub-bands.png)
Genes are arranged linearly along long chains of DNA base-pair sequences. In
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
, each cell usually contains a single circular
genophore
The nucleoid (meaning ''nucleus-like'') is an irregularly shaped region within the prokaryotic cell that contains all or most of the genetic material. The chromosome of a prokaryote is circular, and its length is very large compared to the cell d ...
, while
eukaryotic
Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
organisms (such as plants and animals) have their DNA arranged in multiple linear chromosomes. These DNA strands are often extremely long; the largest human chromosome, for example, is about 247 million
base pairs in length. The DNA of a chromosome is associated with structural proteins that organize, compact, and control access to the DNA, forming a material called
chromatin
Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in r ...
; in eukaryotes, chromatin is usually composed of
nucleosomes, segments of DNA wound around cores of
histone
In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosomes in turn a ...
proteins. The full set of hereditary material in an organism (usually the combined DNA sequences of all chromosomes) is called the
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
.
DNA is most often found in the nucleus of cells, but Ruth Sager helped in the discovery of nonchromosomal genes found outside of the nucleus.
In plants, these are often found in the chloroplasts and in other organisms, in the mitochondria.
These nonchromosomal genes can still be passed on by either partner in sexual reproduction and they control a variety of hereditary characteristics that replicate and remain active throughout generations.
While
haploid organisms have only one copy of each chromosome, most animals and many plants are
diploid, containing two of each chromosome and thus two copies of every gene. The two alleles for a gene are located on identical
loci of the two
homologous chromosomes
A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points alon ...
, each allele inherited from a different parent.
![Zellsubstanz-Kern-Kerntheilung](https://upload.wikimedia.org/wikipedia/commons/6/6d/Zellsubstanz-Kern-Kerntheilung.jpg)
Many species have so-called
sex chromosome
A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical ...
s that determine the sex of each organism.
In humans and many other animals, the
Y chromosome
The Y chromosome is one of two sex chromosomes (allosomes) in therian mammals, including humans, and many other animals. The other is the X chromosome. Y is normally the sex-determining chromosome in many species, since it is the presence or abse ...
contains the gene that triggers the development of the specifically male characteristics. In evolution, this chromosome has lost most of its content and also most of its genes, while the
X chromosome
The X chromosome is one of the two sex-determining chromosomes (allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO sex ...
is similar to the other chromosomes and contains many genes. This being said, Mary Frances Lyon discovered that there is X-chromosome inactivation during reproduction to avoid passing on twice as many genes to the offspring.
Lyon's discovery led to the discovery of X-linked diseases.
Reproduction
When cells divide, their full genome is copied and each
daughter cell
Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...
inherits one copy. This process, called
mitosis, is the simplest form of reproduction and is the basis for asexual reproduction. Asexual reproduction can also occur in multicellular organisms, producing offspring that inherit their genome from a single parent. Offspring that are genetically identical to their parents are called
clones
Clone or Clones or Cloning or Cloned or The Clone may refer to:
Places
* Clones, County Fermanagh
* Clones, County Monaghan, a town in Ireland
Biology
* Clone (B-cell), a lymphocyte clone, the massive presence of which may indicate a pathologi ...
.
Eukaryotic
Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
organisms often use sexual reproduction to generate offspring that contain a mixture of genetic material inherited from two different parents. The process of sexual reproduction alternates between forms that contain single copies of the genome (
haploid) and double copies (
diploid).
Haploid cells fuse and combine genetic material to create a diploid cell with paired chromosomes. Diploid organisms form haploids by dividing, without replicating their DNA, to create daughter cells that randomly inherit one of each pair of chromosomes. Most animals and many plants are diploid for most of their lifespan, with the haploid form reduced to single cell
gamete
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
s such as
sperm or
eggs
Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...
.
Although they do not use the haploid/diploid method of sexual reproduction,
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
have many methods of acquiring new genetic information. Some bacteria can undergo
conjugation
Conjugation or conjugate may refer to:
Linguistics
* Grammatical conjugation, the modification of a verb from its basic form
* Emotive conjugation or Russell's conjugation, the use of loaded language
Mathematics
* Complex conjugation, the chang ...
, transferring a small circular piece of DNA to another bacterium.
Bacteria can also take up raw DNA fragments found in the environment and integrate them into their genomes, a phenomenon known as
transformation
Transformation may refer to:
Science and mathematics
In biology and medicine
* Metamorphosis, the biological process of changing physical form after birth or hatching
* Malignant transformation, the process of cells becoming cancerous
* Tran ...
.
These processes result in
horizontal gene transfer
Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...
, transmitting fragments of genetic information between organisms that would be otherwise unrelated.
Natural bacterial transformation occurs in many
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
l species, and can be regarded as a
sexual process for transferring DNA from one cell to another cell (usually of the same species).
Transformation requires the action of numerous bacterial
gene product
A gene product is the biochemical material, either RNA or protein, resulting from expression of a gene. A measurement of the amount of gene product is sometimes used to infer how active a gene is. Abnormal amounts of gene product can be correlate ...
s, and its primary adaptive function appears to be
repair
The technical meaning of maintenance involves functional checks, servicing, repairing or replacing of necessary devices, equipment, machinery, building infrastructure, and supporting utilities in industrial, business, and residential installa ...
of
DNA damages in the recipient cell.
Recombination and genetic linkage
![Morgan crossover 2 cropped](https://upload.wikimedia.org/wikipedia/commons/2/2c/Morgan_crossover_2_cropped.png)
The diploid nature of chromosomes allows for genes on different chromosomes to
assort independently or be separated from their homologous pair during sexual reproduction wherein haploid gametes are formed. In this way new combinations of genes can occur in the offspring of a mating pair. Genes on the same chromosome would theoretically never recombine. However, they do, via the cellular process of
chromosomal crossover. During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes.
This process of chromosomal crossover generally occurs during
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
, a series of cell divisions that creates haploid cells.
Meiotic recombination
Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryo ...
, particularly in microbial
eukaryotes, appears to serve the adaptive function of repair of DNA damages.
[
The first cytological demonstration of crossing over was performed by Harriet Creighton and Barbara McClintock in 1931. Their research and experiments on corn provided cytological evidence for the genetic theory that linked genes on paired chromosomes do in fact exchange places from one homolog to the other.
The probability of chromosomal crossover occurring between two given points on the chromosome is related to the distance between the points. For an arbitrarily long distance, the probability of crossover is high enough that the inheritance of the genes is effectively uncorrelated.] For genes that are closer together, however, the lower probability of crossover means that the genes demonstrate genetic linkage; alleles for the two genes tend to be inherited together. The amounts of linkage between a series of genes can be combined to form a linear linkage map that roughly describes the arrangement of the genes along the chromosome.
Gene expression
Genetic code
Genes generally express their functional effect through the production of proteins, molecules responsible for most functions in the cell. Proteins are made up of one or more polypeptide chains, each composed of a sequence of amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
s. The DNA sequence of a gene is used to produce a specific amino acid sequence
Protein primary structure is the linear sequence of amino acids in a peptide or protein. By convention, the primary structure of a protein is reported starting from the amino-terminal (N) end to the carboxyl-terminal (C) end. Protein biosynthe ...
. This process begins with the production of an RNA molecule with a sequence matching the gene's DNA sequence, a process called transcription
Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including:
Genetics
* Transcription (biology), the copying of DNA into RNA, the fir ...
.
This messenger RNA molecule then serves to produce a corresponding amino acid sequence through a process called translation
Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text. The English language draws a terminological distinction (which does not exist in every language) between ''transla ...
. Each group of three nucleotides in the sequence, called a codon, corresponds either to one of the twenty possible amino acids in a protein or an instruction to end the amino acid sequence; this correspondence is called the genetic code
The genetic code is the set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets, or codons) into proteins. Translation is accomplished by the ribosome, which links ...
. The flow of information is unidirectional: information is transferred from nucleotide sequences into the amino acid sequence of proteins, but it never transfers from protein back into the sequence of DNA—a phenomenon Francis Crick called the central dogma of molecular biology.
The specific sequence of amino acids results
A result is the outcome of an event.
Result or Results may also refer to: Music
* Results (album), ''Results'' (album), a 1989 album by Liza Minnelli
* ''Results'', a 2012 album by Murder Construct
* "The Result", a single by The Upsetters
* "The ...
in a unique three-dimensional structure for that protein, and the three-dimensional structures of proteins are related to their functions. Some are simple structural molecules, like the fibers formed by the protein collagen. Proteins can bind to other proteins and simple molecules, sometimes acting as enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s by facilitating chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
s within the bound molecules (without changing the structure of the protein itself). Protein structure is dynamic; the protein hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyt ...
bends into slightly different forms as it facilitates the capture, transport, and release of oxygen molecules within mammalian blood.
A single nucleotide difference within DNA can cause a change in the amino acid sequence of a protein. Because protein structures are the result of their amino acid sequences, some changes can dramatically change the properties of a protein by destabilizing the structure or changing the surface of the protein in a way that changes its interaction with other proteins and molecules. For example, sickle-cell anemia
Sickle cell disease (SCD) is a group of blood disorders typically inherited from a person's parents. The most common type is known as sickle cell anaemia. It results in an abnormality in the oxygen-carrying protein haemoglobin found in red bl ...
is a human genetic disease
A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...
that results from a single base difference within the coding region for the β-globin section of hemoglobin, causing a single amino acid change that changes hemoglobin's physical properties.
Sickle-cell versions of hemoglobin stick to themselves, stacking to form fibers that distort the shape of red blood cell
Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
s carrying the protein. These sickle-shaped cells no longer flow smoothly through blood vessel
The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away ...
s, having a tendency to clog or degrade, causing the medical problems associated with this disease.
Some DNA sequences are transcribed into RNA but are not translated into protein products—such RNA molecules are called non-coding RNA
A non-coding RNA (ncRNA) is a functional RNA molecule that is not Translation (genetics), translated into a protein. The DNA sequence from which a functional non-coding RNA is transcribed is often called an RNA gene. Abundant and functionally im ...
. In some cases, these products fold into structures which are involved in critical cell functions (e.g. ribosomal RNA
Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosom ...
and transfer RNA). RNA can also have regulatory effects through hybridization interactions with other RNA molecules (such as microRNA
MicroRNA (miRNA) are small, single-stranded, non-coding RNA molecules containing 21 to 23 nucleotides. Found in plants, animals and some viruses, miRNAs are involved in RNA silencing and post-transcriptional regulation of gene expression. mi ...
).
The genetic code is a dictionary that matches together the amino acid and nucleotide sequences called add-ons. 64 genetic codons are there in which every codon has 3 bases. In 64 codons,20 amino acids are coded by 61 codons which are found in proteins and 3 codons don’t code for any amino acid. There are different types of codons. The codons that code for amino acids are called Sense codons and the codons that code for protein synthesis are called Signal codons. Signal codons are of two types which are Terminating codons and Initiating codons. UAA UAG UGA is termed as termination codons or also called nonsense codons. AUG is called an initiation codon used to code for the first amino acids in every protein. During the translation process the t-RNA base sequence pairs with the codon of m RNA which is known as an Anticodon. The difference between codon and anticodon is that codon is present not only in DNA but in RNA, whereas anticodon is present only in RNA but not in DNA. Codons will be directed from 5’ end to 3’ end in the same way anticodons are directed in the opposite way i.e., 3’ end to 5’ end. In some t RNA molecules, anticodons must pair with more than one codon. The arrangement of codons is sequence manner based while an arrangement of anticodons is discretely present in cells with amino acids.
Nature and nurture
Although genes contain all the information an organism uses to function, the environment plays an important role in determining the ultimate phenotypes an organism displays. The phrase " nature and nurture" refers to this complementary relationship. The phenotype of an organism depends on the interaction of genes and the environment. An interesting example is the coat coloration of the Siamese cat
The Siamese cat ( th, แมวไทย, Maeo Thai; แมวสยาม, Maeo Seeaam) is one of the first distinctly recognized breeds of Asian cat. Derived from the Wichianmat landrace, one of several varieties of cat native to Thailand ( ...
. In this case, the body temperature of the cat plays the role of the environment. The cat's genes code for dark hair, thus the hair-producing cells in the cat make cellular proteins resulting in dark hair. But these dark hair-producing proteins are sensitive to temperature (i.e. have a mutation causing temperature-sensitivity) and denature in higher-temperature environments, failing to produce dark-hair pigment in areas where the cat has a higher body temperature. In a low-temperature environment, however, the protein's structure is stable and produces dark-hair pigment normally. The protein remains functional in areas of skin that are colder—such as its legs, ears, tail, and faceso the cat has dark hair at its extremities.
Environment plays a major role in effects of the human genetic disease phenylketonuria
Phenylketonuria (PKU) is an inborn error of metabolism that results in decreased metabolism of the amino acid phenylalanine. Untreated PKU can lead to intellectual disability, seizures, behavioral problems, and mental disorders. It may also re ...
. The mutation that causes phenylketonuria disrupts the ability of the body to break down the amino acid phenylalanine, causing a toxic build-up of an intermediate molecule that, in turn, causes severe symptoms of progressive intellectual disability and seizures. However, if someone with the phenylketonuria mutation follows a strict diet that avoids this amino acid, they remain normal and healthy.
A common method for determining how genes and environment ("nature and nurture") contribute to a phenotype involves studying identical and fraternal twins, or other siblings of multiple birth
A multiple birth is the culmination of one multiple pregnancy, wherein the mother gives birth to two or more babies. A term most applicable to vertebrate species, multiple births occur in most kinds of mammals, with varying frequencies. Such bir ...
s. Identical siblings are genetically the same since they come from the same zygote. Meanwhile, fraternal twins are as genetically different from one another as normal siblings. By comparing how often a certain disorder occurs in a pair of identical twins to how often it occurs in a pair of fraternal twins, scientists can determine whether that disorder is caused by genetic or postnatal environmental factors. One famous example involved the study of the Genain quadruplets The Genain quadruplets (born in 1930) are a set of identical quadruplet sisters. All four developed schizophrenia, suggesting a large genetic component to the cause of the disease. The pseudonym ''Genain'', used to protect the identity of the fa ...
, who were identical quadruplets all diagnosed with schizophrenia
Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social wit ...
.
Gene regulation
The genome of a given organism contains thousands of genes, but not all these genes need to be active at any given moment. A gene is expressed when it is being transcribed into mRNA and there exist many cellular methods of controlling the expression of genes such that proteins are produced only when needed by the cell. Transcription factor
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The f ...
s are regulatory proteins that bind to DNA, either promoting or inhibiting the transcription of a gene. Within the genome of ''Escherichia coli
''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...
'' bacteria, for example, there exists a series of genes necessary for the synthesis of the amino acid tryptophan
Tryptophan (symbol Trp or W)
is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic ...
. However, when tryptophan is already available to the cell, these genes for tryptophan synthesis are no longer needed. The presence of tryptophan directly affects the activity of the genes—tryptophan molecules bind to the tryptophan repressor (a transcription factor), changing the repressor's structure such that the repressor binds to the genes. The tryptophan repressor blocks the transcription and expression of the genes, thereby creating negative feedback regulation of the tryptophan synthesis process.
Differences in gene expression are especially clear within multicellular organisms, where cells all contain the same genome but have very different structures and behaviors due to the expression of different sets of genes. All the cells in a multicellular organism derive from a single cell, differentiating into variant cell types in response to external and intercellular signals and gradually establishing different patterns of gene expression to create different behaviors. As no single gene is responsible for the development
Development or developing may refer to:
Arts
*Development hell, when a project is stuck in development
*Filmmaking, development phase, including finance and budgeting
*Development (music), the process thematic material is reshaped
* Photograph ...
of structures within multicellular organisms, these patterns arise from the complex interactions between many cells.
Within eukaryotes, there exist structural features of chromatin
Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in r ...
that influence the transcription of genes, often in the form of modifications to DNA and chromatin that are stably inherited by daughter cells. These features are called " epigenetic" because they exist "on top" of the DNA sequence and retain inheritance from one cell generation to the next. Because of epigenetic features, different cell types grown within the same medium can retain very different properties. Although epigenetic features are generally dynamic over the course of development, some, like the phenomenon of paramutation
In epigenetics, a paramutation is an interaction between two alleles at a single locus, whereby one allele induces a heritable change in the other allele. The change may be in the pattern of DNA methylation or histone modifications. The allele indu ...
, have multigenerational inheritance and exist as rare exceptions to the general rule of DNA as the basis for inheritance.
Genetic change
Mutations
During the process of DNA replication, errors occasionally occur in the polymerization of the second strand. These errors, called mutations, can affect the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. Error rates are usually very low—1 error in every 10–100 million bases—due to the "proofreading" ability of DNA polymerase
A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create ...
s. Processes that increase the rate of changes in DNA are called mutagenic
In genetics, a mutagen is a physical or chemical agent that permanently changes genetic material, usually DNA, in an organism and thus increases the frequency of mutations above the natural background level. As many mutations can cause cancer i ...
: mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing, while UV radiation
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
induces mutations by causing damage to the DNA structure. Chemical damage to DNA occurs naturally as well and cells use DNA repair
DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA da ...
mechanisms to repair mismatches and breaks. The repair does not, however, always restore the original sequence. A particularly important source of DNA damages appears to be reactive oxygen species
In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen.
The reduction of molecular oxygen () p ...
produced by cellular aerobic respiration, and these can lead to mutations.
In organisms that use chromosomal crossover to exchange DNA and recombine genes, errors in alignment during meiosis can also cause mutations. Errors in crossover are especially likely when similar sequences cause partner chromosomes to adopt a mistaken alignment; this makes some regions in genomes more prone to mutating in this way. These errors create large structural changes in DNA sequence— duplications, inversions, deletions of entire regions—or the accidental exchange of whole parts of sequences between different chromosomes, chromosomal translocation
In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes balanced and unbalanced translocation, with two main types: reciprocal-, and Robertsonian translocation. Reciprocal translo ...
.
GENETIC MUTATION
A highly pathogenic, more or less permanent alteration to the genetic code (genome) of a virus or a cell in a living creature that can be handed down to the progeny of the original cell or virus. A somatic mutation is a change in a multicellular organism's DNA that can progress to progeny by DNA replication. Alterations can happen as a result of subjection to electromagnetic spectrum with high intensity (such as X-rays, ultraviolet light), mishaps that can place during the normal chemical transactions of DNA, most frequently during replication. The bulk of variations are expected to be harmful because they are random changes, however, certain mutations might be helpful in certain circumstances.
Types of mutations
Mutations and perhaps other gene changes can be inherited or acquired. An inherited gene mutation is one that, as its name suggests, is passed down from one parent to the next. As a result, it can be found in the very first cell that develops into a human after the egg cell and sperm cell have mated. Because it began in the first cell, which gave rise to all the other cells in the body, this modification is present in every cell in the body and can be passed on to the next generation. Because the cells that give rise to eggs and sperm are known as germ cells, also known as a hereditary alteration. A gene mutation that is acquired does not come from the parent. As opposed to that, it emerges at some point in a person's life. Acquired mutations start in one cell and spread to any subsequent cells that develop from that cell. Because this mutation does not affect sperm or egg cells, it cannot be passed down to a person's offspring. Somatic mutation or spontaneous mutation are other names for this kind of mutation.
Natural selection and evolution
Mutations alter an organism's genotype and occasionally this causes different phenotypes to appear. Most mutations have little effect on an organism's phenotype, health, or reproductive fitness. Mutations that do have an effect are usually detrimental, but occasionally some can be beneficial. Studies in the fly ''Drosophila melanogaster
''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the " vinegar fly" or "pomace fly". Starting with ...
'' suggest that if a mutation changes a protein produced by a gene, about 70 percent of these mutations are harmful with the remainder being either neutral or weakly beneficial.
Population genetics
Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as Adaptation (biology), adaptation, ...
studies the distribution of genetic differences within populations and how these distributions change over time. Changes in the frequency of an allele in a population are mainly influenced by natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
, where a given allele provides a selective or reproductive advantage to the organism, as well as other factors such as mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
, genetic drift
Genetic drift, also known as allelic drift or the Wright effect, is the change in the frequency of an existing gene variant (allele) in a population due to random chance.
Genetic drift may cause gene variants to disappear completely and there ...
, genetic hitchhiking
Genetic may refer to:
*Genetics, in biology, the science of genes, heredity, and the variation of organisms
**Genetic, used as an adjective, refers to genes
***Genetic disorder, any disorder caused by a genetic mutation, whether inherited or de nov ...
, artificial selection and migration
Migration, migratory, or migrate may refer to: Human migration
* Human migration, physical movement by humans from one region to another
** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...
.
Over many generations, the genomes of organisms can change significantly, resulting in evolution. In the process called adaptation, selection for beneficial mutations can cause a species to evolve into forms better able to survive in their environment.
Earlier related ideas were acknowledged in New species are formed through the process of speciation, often caused by geographical separations that prevent populations from exchanging genes with each other.
By comparing the homology between different species' genomes, it is possible to calculate the evolutionary distance between them and when they may have diverged. Genetic comparisons are generally considered a more accurate method of characterizing the relatedness between species than the comparison of phenotypic characteristics. The evolutionary distances between species can be used to form evolutionary tree
A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...
s; these trees represent the common descent and divergence of species over time, although they do not show the transfer of genetic material between unrelated species (known as horizontal gene transfer
Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...
and most common in bacteria).
Model organisms
Although geneticists originally studied inheritance in a wide variety of organisms, the range of species studied has narrowed. One reason is that when significant research already exists for a given organism, new researchers are more likely to choose it for further study, and so eventually a few model organisms became the basis for most genetics research. Common research topics in model organism genetics include the study of gene regulation
Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products (protein or RNA). Sophisticated programs of gene expression are wi ...
and the involvement of genes in development
Development or developing may refer to:
Arts
*Development hell, when a project is stuck in development
*Filmmaking, development phase, including finance and budgeting
*Development (music), the process thematic material is reshaped
* Photograph ...
and cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
. Organisms were chosen, in part, for convenience—short generation times and easy genetic manipulation
Genetic engineering, also called genetic modification or genetic manipulation, is the modification and manipulation of an organism's genes using technology. It is a set of technologies used to change the genetic makeup of cells, including ...
made some organisms popular genetics research tools. Widely used model organisms include the gut bacterium ''Escherichia coli
''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...
'', the plant '' Arabidopsis thaliana'', baker's yeast (''Saccharomyces cerevisiae
''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have b ...
''), the nematode '' Caenorhabditis elegans'', the common fruit fly (''Drosophila melanogaster
''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the " vinegar fly" or "pomace fly". Starting with ...
''), the zebrafish (''Danio rerio
The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family (Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ca ...
''), and the common house mouse (''Mus musculus
Mus or MUS may refer to:
Abbreviations
* MUS, the NATO country code for Mauritius
* MUS, the IATA airport code for Minami Torishima Airport
* MUS, abbreviation for the Centre for Modern Urban Studies on Campus The Hague, Leiden University, Net ...
'').
Medicine
Medical genetics
Medical genetics is the branch
tics in that human genetics is a field of scientific research that may or may not apply to medicine, while medical genetics refers to the application of genetics to medical care. For example, research on the caus ...
seeks to understand how genetic variation relates to human health and disease. When searching for an unknown gene that may be involved in a disease, researchers commonly use genetic linkage and genetic pedigree chart
A pedigree chart is a diagram that shows the occurrence and appearance of phenotypes of a particular gene or organism and its ancestors from one generation to the next, most commonly humans, show dogs, and race horses.
Definition
The word pedigre ...
s to find the location on the genome associated with the disease. At the population level, researchers take advantage of Mendelian randomization
In epidemiology, Mendelian randomization (commonly abbreviated to MR) is a method using measured variation in genes to interrogate the causal effect of an exposure on an outcome. Under key assumptions (see below), the design reduces both reverse ca ...
to look for locations in the genome that are associated with diseases, a method especially useful for multigenic traits not clearly defined by a single gene. Once a candidate gene is found, further research is often done on the corresponding (or homologous) genes of model organisms. In addition to studying genetic diseases, the increased availability of genotyping methods has led to the field of pharmacogenetics
Pharmacogenomics is the study of the role of the genome in drug response. Its name ('' pharmaco-'' + ''genomics'') reflects its combining of pharmacology and genomics. Pharmacogenomics analyzes how the genetic makeup of an individual affects the ...
: the study of how genotype can affect drug responses.
Individuals differ in their inherited tendency to develop cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
, and cancer is a genetic disease. The process of cancer development in the body is a combination of events. Mutations occasionally occur within cells in the body as they divide. Although these mutations will not be inherited by any offspring, they can affect the behavior of cells, sometimes causing them to grow and divide more frequently. There are biological mechanisms that attempt to stop this process; signals are given to inappropriately dividing cells that should trigger cell death
Cell death is the event of a biological cell ceasing to carry out its functions. This may be the result of the natural process of old cells dying and being replaced by new ones, as in programmed cell death, or may result from factors such as d ...
, but sometimes additional mutations occur that cause cells to ignore these messages. An internal process of natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
occurs within the body and eventually mutations accumulate within cells to promote their own growth, creating a cancerous tumor
A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
that grows and invades various tissues of the body. Normally, a cell divides only in response to signals called growth factor
A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for regul ...
s and stops growing once in contact with surrounding cells and in response to growth-inhibitory signals. It usually then divides a limited number of times and dies, staying within the epithelium where it is unable to migrate to other organs. To become a cancer cell, a cell has to accumulate mutations in a number of genes (three to seven). A cancer cell can divide without growth factor and ignores inhibitory signals. Also, it is immortal and can grow indefinitely, even after it makes contact with neighboring cells. It may escape from the epithelium and ultimately from the primary tumor A primary tumor is a tumor growing at the anatomical site where tumor progression began and proceeded to yield a cancerous mass. Most cancers develop at their primary site but then go on to metastasize or spread to other parts of the body. These fur ...
. Then, the escaped cell can cross the endothelium of a blood vessel and get transported by the bloodstream to colonize a new organ, forming deadly metastasis
Metastasis is a pathogenic agent's spread from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, then ...
. Although there are some genetic predispositions in a small fraction of cancers, the major fraction is due to a set of new genetic mutations that originally appear and accumulate in one or a small number of cells that will divide to form the tumor and are not transmitted to the progeny (somatic mutation
A somatic mutation is a change in the DNA sequence of a somatic cell of a multicellular organism with dedicated reproductive cells; that is, any mutation that occurs in a cell other than a gamete, germ cell, or gametocyte. Unlike germline mutatio ...
s). The most frequent mutations are a loss of function of p53 protein, a tumor suppressor
A tumor suppressor gene (TSG), or anti-oncogene, is a gene that regulates a cell during cell division and replication. If the cell grows uncontrollably, it will result in cancer. When a tumor suppressor gene is mutated, it results in a loss or re ...
, or in the p53 pathway, and gain of function mutations in the Ras proteins, or in other oncogene
An oncogene is a gene that has the potential to cause cancer. In tumor cells, these genes are often mutated, or expressed at high levels. s.
Research methods
DNA can be manipulated in the laboratory. Restriction enzymes
A restriction enzyme, restriction endonuclease, REase, ENase or'' restrictase '' is an enzyme that cleaves DNA into fragments at or near specific recognition sites within molecules known as restriction sites. Restriction enzymes are one class o ...
are commonly used enzymes that cut DNA at specific sequences, producing predictable fragments of DNA. DNA fragments can be visualized through use of gel electrophoresis, which separates fragments according to their length.
The use of ligation enzymes allows DNA fragments to be connected. By binding ("ligating") fragments of DNA together from different sources, researchers can create recombinant DNA
Recombinant DNA (rDNA) molecules are DNA molecules formed by laboratory methods of genetic recombination (such as molecular cloning) that bring together genetic material from multiple sources, creating sequences that would not otherwise be fo ...
, the DNA often associated with genetically modified organisms. Recombinant DNA is commonly used in the context of plasmids: short circular DNA molecules with a few genes on them. In the process known as molecular cloning
Molecular cloning is a set of experimental methods in molecular biology that are used to assemble recombinant DNA molecules and to direct their DNA replication, replication within Host (biology), host organisms. The use of the word ''cloning'' re ...
, researchers can amplify the DNA fragments by inserting plasmids into bacteria and then culturing them on plates of agar (to isolate clones of bacteria cells). "Cloning" can also refer to the various means of creating cloned ("clonal") organisms.
DNA can also be amplified using a procedure called the polymerase chain reaction
The polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete or partial) of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it (or a part of it) ...
(PCR). By using specific short sequences of DNA, PCR can isolate and exponentially amplify a targeted region of DNA. Because it can amplify from extremely small amounts of DNA, PCR is also often used to detect the presence of specific DNA sequences.
DNA sequencing and genomics
DNA sequencing, one of the most fundamental technologies developed to study genetics, allows researchers to determine the sequence of nucleotides in DNA fragments. The technique of chain-termination sequencing, developed in 1977 by a team led by Frederick Sanger, is still routinely used to sequence DNA fragments. Using this technology, researchers have been able to study the molecular sequences associated with many human diseases.
As sequencing has become less expensive, researchers have sequenced the genomes of many organisms using a process called genome assembly
In bioinformatics, sequence assembly refers to aligning and merging fragments from a longer DNA sequence in order to reconstruct the original sequence. This is needed as DNA sequencing technology might not be able to 'read' whole genomes in one ...
, which uses computational tools to stitch together sequences from many different fragments. These technologies were used to sequence the human genome in the Human Genome Project completed in 2003. New high-throughput sequencing
DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. Th ...
technologies are dramatically lowering the cost of DNA sequencing, with many researchers hoping to bring the cost of resequencing a human genome down to a thousand dollars.
Next-generation sequencing Massive parallel sequencing or massively parallel sequencing is any of several high-throughput approaches to DNA sequencing using the concept of massively parallel processing; it is also called next-generation sequencing (NGS) or second-generation ...
(or high-throughput sequencing) came about due to the ever-increasing demand for low-cost sequencing. These sequencing technologies allow the production of potentially millions of sequences concurrently. The large amount of sequence data available has created the subfield of genomics, research that uses computational tools to search for and analyze patterns in the full genomes of organisms. Genomics can also be considered a subfield of bioinformatics, which uses computational approaches to analyze large sets of biological data Biological data refers to a compound or information derived from living organisms and their products. A medicinal compound made from living organisms, such as a serum or a vaccine, could be characterized as biological data. Biological data is highly ...
. A common problem to these fields of research is how to manage and share data that deals with human subject and personally identifiable information.
Society and culture
On 19 March 2015, a group of leading biologists urged a worldwide ban on clinical use of methods, particularly the use of CRISPR and zinc finger
A zinc finger is a small protein structural motif that is characterized by the coordination of one or more zinc ions (Zn2+) in order to stabilize the fold. It was originally coined to describe the finger-like appearance of a hypothesized struct ...
, to edit the human genome in a way that can be inherited. In April 2015, Chinese researchers reported results of basic research
Basic research, also called pure research or fundamental research, is a type of scientific research with the aim of improving scientific theories for better understanding and prediction of natural or other phenomena. In contrast, applied rese ...
to edit the DNA of non-viable human embryos using CRISPR.
See also
* Bacterial genome size
* Cryoconservation of animal genetic resources
Cryoconservation of animal genetic resources is a strategy wherein samples of animal genetic materials are preserved cryogenically."Cryoconservation of Animal Genetic Resources", Rep. Rome: Food and Agriculture Organization of the United Nations, ...
* Eugenics
Eugenics ( ; ) is a fringe set of beliefs and practices that aim to improve the genetic quality of a human population. Historically, eugenicists have attempted to alter human gene pools by excluding people and groups judged to be inferior o ...
* Embryology
Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''-logia'') is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos ...
* Genetic disorder
A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...
* Genetic diversity
* Genetic engineering
* Genetic enhancement
* Glossary of genetics (M−Z)
* Index of genetics articles
* Medical genetics
Medical genetics is the branch
tics in that human genetics is a field of scientific research that may or may not apply to medicine, while medical genetics refers to the application of genetics to medical care. For example, research on the caus ...
* Molecular tools for gene study
Nucleic acid methods are the techniques used to study nucleic acids: DNA and RNA.
Purification
*DNA extraction
*Phenol–chloroform extraction
*Minicolumn purification
*RNA extraction
* Boom method
* Synchronous coefficient of drag alteration ( ...
* Neuroepigenetics
* Outline of genetics
* Timeline of the history of genetics
The history of genetics can be represented on a timeline of events from the earliest work in the 1850s, to the DNA era starting in the 1940s, and the genomics era beginning in the 1970s.
Early timeline
* 1856–1863: Mendel studied the inheritan ...
* Plant genetic resources
References
Further reading
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External links
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{{Authority control
Genetics
Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar wor ...