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Genetics is a branch of
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology
concerned with the study of
gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language, Greek) meaning ''generation'' or ''birth'' ) is a basic unit of her ...

gene
s,
genetic variation thumb File:Genetic Variation and Inheritance.svg, Parents have similar gene coding in this specific situation where they reproduce and variation in the offspring is seen. Offspring containing the variation also reproduce and passes down traits t ...

genetic variation
, and
heredity Heredity, also called inheritance or biological inheritance, is the passing on of traits Trait may refer to: * Phenotypic trait in biology, which involve genes and characteristics of organisms * Trait (computer programming), a model for str ...

heredity
in
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological me ...

organism
s.Hartl D, Jones E (2005) Though heredity had been observed for millennia,
Gregor Mendel Gregor Johann Mendel (; cs, Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was a meteorologist, mathematician, biologist, AugustinianAugustinian may refer to: *Augustinians Augustinians are members of Christian religious orders tha ...

Gregor Mendel
,
Moravian Moravian is the adjective form of the Czech Republic region of Moravia, and refers to people of ancestry from Moravia. Moravian may also refer to: * Moravia, the region * Moravians, people from Moravia * Moravian dialects, dialects of Czech spoken ...

Moravian
scientist and
AugustinianAugustinian may refer to: *Augustinians Augustinians are members of Christian religious orders that follow the Rule of Saint Augustine, written in about 400 AD by Augustine of Hippo. There are two distinct types of Augustinians in Catholic relig ...
friar A friar is a brother A brother is a man A man is an adult male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger female gamete, or ovum, in the process of fertil ...
working in the 19th century in
Brno Brno ( , ; german: Brünn, ) is a in the of the Czech Republic. Located at the confluence of the and rivers, Brno has about 380,000 inhabitants, making it the second-largest city in the Czech Republic after the capital, , and one of the . Th ...

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 Trait may refer to: * Phenotypic trait in biology, which involve genes and characteristics of organisms * Trait (computer programming), a model for structuring object-oriented programs (a template class in the C++ programming language) * Trait the ...
inheritance and
molecular A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atom An atom is the smallest unit of ordinary matter In ...
inheritance mechanisms of genes are still primary principles of genetics in the 21st century, but modern genetics has expanded beyond inheritance to studying the function and behavior of genes. Gene structure and function, variation, and distribution are studied within the context of the cell, 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 heritability, heritable phenotype changes that do not involve alterations in the DNA sequence. The Ancient Greek, Greek prefix ''wikt:epi-, epi-'' ( "over, outside of, around") in ''epigenetics'' implies f ...
and
population genetics Population genetics is a subfield of that deals with genetic differences within and between s, and is a part of . Studies in this branch of examine such phenomena as , , and . Population genetics was a vital ingredient in the of the . Its pri ...
. Organisms studied within the broad field span the domains of life (
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaea
,
bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sm ...

bacteria
, and
eukarya Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), tax ...
). Genetic processes work in combination with an organism's environment and experiences to influence development and
behavior Behavior (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American English ...
, often referred to as
nature versus nurture The nature versus nurture debate involves whether human behavior is determined by the environment, either prenatal or during a person's life, or by a person's genes. The alliterative expression "nature and nurture" in English has been in use sinc ...
. The
intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemic ...
or
extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemic ...
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 climate which there is an excess of evaporation over precipitation. The typically bald, rocky, or sandy surfaces in desert climates hold little ...
only grows to half the height of the one in the temperate climate due to lack of water and nutrients in its environment.


Etymology

The word ''genetics'' stems from the
ancient Greek Ancient Greek includes the forms of the used in and the from around 1500 BC to 300 BC. It is often roughly divided into the following periods: (), Dark Ages (), the period (), and the period (). Ancient Greek was the language of an ...
' meaning "genitive"/"generative", which in turn derives from ' meaning "origin".


History

The observation that living things inherit
traits Trait may refer to: * Phenotypic trait in biology, which involve genes and characteristics of organisms * Trait (computer programming), a model for structuring object-oriented programs (a template class in the C++ programming language) * Trait the ...
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 Animal breeding is a branch of animal science Animal science (also bioscience) is described as "studying the biology Biology i ...
. The modern science of genetics, seeking to understand this process, began with the work of the
AugustinianAugustinian may refer to: *Augustinians Augustinians are members of Christian religious orders that follow the Rule of Saint Augustine, written in about 400 AD by Augustine of Hippo. There are two distinct types of Augustinians in Catholic relig ...
friar A friar is a brother A brother is a man A man is an adult male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger female gamete, or ovum, in the process of fertil ...
Gregor Mendel Gregor Johann Mendel (; cs, Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was a meteorologist, mathematician, biologist, AugustinianAugustinian may refer to: *Augustinians Augustinians are members of Christian religious orders tha ...

Gregor Mendel
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 num ...
, a
HungarianHungarian may refer to: * Hungary, a country in Central Europe * Kingdom of Hungary, state of Hungary, existing between 1000 and 1946 * Hungarians, ethnic groups in Hungary * Hungarian algorithm, a polynomial time algorithm for solving the assignmen ...

Hungarian
noble, who lived in Kőszeg before Mendel, was the first who used the word "genetics." He described several rules of genetic inheritance in his work ''The genetic law of the Nature'' (Die genetische Gesätze 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 Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterpris ...

Hugo de Vries
). 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 , and , best known for his contributions to the science of . His proposition that all species of life have descended from is now widely accepted and cons ...

Charles Darwin
'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 inheritance is an Superseded scientific theories, obsolete theory in biology from the 19th century. The theory is that the progeny inheritance (biology), inherits any characteristic as the average of the parents' values of that characteris ...

blending inheritance
: 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 Inheritance is the practice of passing on private property, Title (property), titles, debts, entitlements, Privilege (law), privileges, rights, and Law of obligations, obligations upon the death of an individual. The rules of inheritance differ ...
: 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 Natural history is a domain of inquiry involving organisms, including animals, fu ...

Jean-Baptiste Lamarck
) is now known to be wrong—the experiences of individuals do not affect the genes they pass to their children, Other theories included the
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 heritable ...

pangenesis
of
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English , and , best known for his contributions to the science of . His proposition that all species of life have descended from is now widely accepted and cons ...

Charles Darwin
(which had both acquired and inherited aspects) and
Francis Galton Sir Francis Galton, FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Re ...

Francis Galton
's reformulation of pangenesis as both particulate and inherited.


Mendelian and classical genetics

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 German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany, see also German nationa ...
"), presented in 1865 to the ''Naturforschender Verein'' (Society for Research in Nature) in
Brünn
Brünn
, 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 Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterpris ...

Hugo de Vries
and other scientists rediscovered his research.
William Bateson William Bateson (8 August 1861 – 8 February 1926) was an English biology, 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 red ...

William Bateson
, 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 Biochemists are scientists who are trained in biochemistry Biochemistry or biological chemistry, is the study of chemical processes within and relating to livi ...
. 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 and of and the . It stands on the in south-east England at the head of a down to the , and has been a major settlement for two millennia. The , its ancient core and financial centre, was founded by the as ' and retains b ...

London
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 t ...

Thomas Hunt Morgan
argued that genes are on
chromosome A chromosome is a long DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A mole ...

chromosome
s, based on observations of a sex-linked white eye mutation in
fruit flies Fruit fly may refer to: Organisms * Drosophilidae, a family of smaller flies, including: ** ''Drosophila'', the genus of small fruit flies and vinegar flies ** ''Drosophila melanogaster'' or common fruit fly, an important model organism in modern b ...

fruit flies
. 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 T ...
used the phenomenon of
genetic linkage Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction. Two genetic markers that are physically near to each other are unlikely to be separa ...
to show that genes are arranged linearly on the chromosome.


Molecular genetics

Although genes were known to exist on chromosomes, chromosomes are composed of both protein 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 A bacteriologist is a microbiologist or a professional trained in bacteriology, a subdivision of microbiology. The duties of a bacteriologist include prevention, diagnosis and prog ...
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 * Transf ...
(see
Griffith's experiment Griffith's experiment, reported in 1928 by Frederick Griffith Frederick Griffith (1877–1941) was a British bacteriologist A bacteriologist is a microbiologist or a professional trained in bacteriology, a subdivision of microbiology. The d ...
): dead bacteria could transfer genetic material to "transform" other still-living bacteria. Sixteen years later, in 1944, the
Avery–MacLeod–McCarty experiment such as this, precipitated from solutions of cell components, to perform bacterial transformations The Avery–MacLeod–McCarty experiment was an experimental demonstration, reported in 1944 by Oswald Avery, Colin Munro MacLeod, Colin MacLeod, and ...
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 ''Acetabularia'' is a genus of green algae in the family Polyphysaceae, Typically found in subtropical waters, ''Acetabularia'' is a single-celled organism, but gigantic in size and complex in form, making it an excellent model organism for stud ...

Acetabularia
''. The
Hershey–Chase experiment Overview of experiment and observations, 357x357px The Hershey–Chase experiments were a series of experiment An experiment is a procedure carried out to support, refute, or validate a hypothesis. Experiments provide insight into Causality, ...
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 James Dewey Watson (born April 6, 1928) is an American molecular biologist, geneticist A geneticist is a biologist Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all time A biologist is a ...

James Watson
and
Francis Crick Francis Harry Compton Crick (8 June 1916 – 28 July 2004) was a British molecular biologist, biophysicist, and neuroscientist A neuroscientist (or neurobiologist) is a scientist A scientist is a person who conducts Scientific method, scie ...

Francis Crick
determined the structure of DNA in 1953, using the
X-ray crystallography X-ray crystallography (XRC) is the experimental science determining the atomic and molecular structure of a crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a ...

X-ray crystallography
work of
Rosalind Franklin Rosalind Elsie Franklin (25 July 192016 April 1958) was an English chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) is a classica ...

Rosalind Franklin
and
Maurice Wilkins Maurice Hugh Frederick Wilkins (15 December 1916 – 5 October 2004) was a New Zealand-born and whose research spanned multiple areas of physics and biophysics, contributing to the scientific understanding of , , and , and to the developme ...
that indicated DNA has a
helical Helical may refer to: *Helix, the mathematical concept for the shape * Helical spring, a coilspring *Helical plc, a British property company, once a maker of steel bar stock * Helicoil, a mechanical thread repairing insert * H-el-ical//, stage name ...

helical
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. 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 : 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 Biotechnology is a broad area of biology, involving the use of living systems and organisms to develop or make products. Depending on the tools and applications, it often overlaps with related scienti ...
. It was discovered that the cell uses DNA as a template to create matching
messenger RNA In molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...
, molecules with
nucleotide Nucleotides are organic molecules , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, ...

nucleotide
s very similar to DNA. The nucleotide sequence of a messenger RNA is used to create an
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
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 cell (biology), cells to Translation (biology), translate information encoded within genetic material (DNA or Messenger RNA, mRNA sequences of nucleotide triplets, or codons) into proteins. Tran ...

genetic code
. With the newfound molecular understanding of inheritance came an explosion of research. A notable theory arose from Tomoko Ohta 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 Genetic drift (allelic drift or the Sewall Wright ...
through publishing the nearly neutral theory of molecular evolution. In this theory, Ohta stressed the importance of natural selection and the environment to the rate at which genetic evolution occurs. One important development was chain-termination
DNA sequencing DNA sequencing is the process of determining the nucleic acid sequence A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides Nucleotides are organic ...

DNA sequencing
in 1977 by
Frederick Sanger Frederick Sanger (; 13 August 1918 – 19 November 2013) was a British biochemist Biochemists are scientists who are trained in biochemistry Biochemistry or biological chemistry, is the study of chemical processes within and relatin ...

Frederick Sanger
. This technology allows scientists to read the nucleotide sequence of a DNA molecule. In 1983, Kary Banks Mullis developed the
polymerase chain reaction Polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete copies or partial copies) of a specific DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM. ...

polymerase chain reaction
, providing a quick way to isolate and amplify a specific section of DNA from a mixture. The efforts of the
Human Genome Project The Human Genome Project (''HGP'') was an international scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge that has characterized the development of science since at least the 17th cen ...
, Department of Energy, NIH, and parallel private efforts by
Celera Genomics Celera is a subsidiary of Quest Diagnostics Quest Diagnostics is an American clinical laboratory. A Fortune 500 company, Quest operates in the United States The United States of America (USA), commonly known as the United States (U.S. ...
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 Mitochondrial DNA, mitochondria. These are usually treated se ...

human genome
in 2003.


Features of inheritance


Discrete inheritance and Mendel's laws

At its most fundamental level, inheritance in organisms occurs by passing discrete heritable units, called
gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language, Greek) meaning ''generation'' or ''birth'' ) is a basic unit of her ...

gene
s, from parents to offspring. This property was first observed by
Gregor Mendel Gregor Johann Mendel (; cs, Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was a meteorologist, mathematician, biologist, AugustinianAugustinian may refer to: *Augustinians Augustinians are members of Christian religious orders tha ...

Gregor Mendel
, who studied the segregation of heritable traits in
pea The pea is most commonly the small spherical seed A seed is an embryonic plant enclosed in a protective outer covering. The formation of the seed is part of the process of reproduction Reproduction (or procreation or breeding) is ...

pea
plants. In his experiments studying the trait for flower color, Mendel observed that the flowers of each pea plant were either purple or white—but never an intermediate between the two colors. These different, discrete versions of the same gene are called
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...
s. In the case of the pea, which is a
diploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mate ...
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 A zygote (from Greek ζυγωτός ''zygōtos'' "joined" or "yoked", from ζυγοῦν ''zygoun'' "to join" or "to yoke") is a eukaryotic cell formed by a fertilization event between two gamete A gamete ( /ˈ ...
at that gene locus, while organisms with two different alleles of a given gene are called
heterozygous Zygosity (the noun, zygote, is from the Greek zygotos "yoked," from zygon "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 org ...

heterozygous
. The set of alleles for a given organism is called its
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a specific ...
, while the observable traits of the organism are called its
phenotype In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inter ...

phenotype
. When organisms are heterozygous at a gene, often one allele is called
dominant Domination or dominant may refer to: Society * World domination, which is mainly a conspiracy theory * Colonialism in which one group (usually a nation) invades another region for material gain or to eliminate competition * Chauvinism in which a p ...
as its qualities dominate the phenotype of the organism, while the other allele is called
recessive In genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientist ...
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 In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Gr ...
by expressing an intermediate phenotype, or codominance 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.


Notation and diagrams

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 phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. The letters B ...
s to represent the inheritance of traits. These charts map the inheritance of a trait in a family tree.


Multiple gene interactions

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. (Some genes do not assort independently, demonstrating
genetic linkage Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction. Two genetic markers that are physically near to each other are unlikely to be separa ...
, a topic discussed later in this article.) Often different genes can interact in a way that influences the same trait. In the (''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 File:Epistasis.png, Example of epistasis in coat colour genetics: If no pigments can be produced the other coat colour genes have no effect on the phenotype, no matter if they are dominant or if the individual is homozygous. Here the genotype "c ...

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 Afghan children with fair skin Human skin color ranges from the darkest brown to the lightest hues. Differences in skin color among individuals is caused by variation in pigment A pigment is a colored material that is completely or nearly i ...
). 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 Heritability is a used in the fields of and that estimates the degree of ''variation'' in a in a that is due to between individuals in that population. It measures how much of the variation of a trait can be attributed to variation of geneti ...

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 Healthcare is the maintenance or improvement of health Health, according to the , is "a state of complete physical, and social and not merely the absence of and ".. (2006)''Constitution of the World Health Organization''– ''Basic Docume ...

health care
, height has a heritability of only 62%.


Molecular basis for inheritance


DNA and chromosomes

The
molecular A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atom An atom is the smallest unit of ordinary matter In ...

molecular
basis for genes is
deoxyribonucleic acid File:DNA animation.gif, The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule composed of two polynucleotide chains that coil around each other to form a Nucleic acid double helix, double helix carrying genet ...
(DNA). DNA is composed of a chain of
nucleotide Nucleotides are organic molecules , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, ...

nucleotide
s, of which there are four types:
adenine Adenine (A, Ade) is a nucleobase 230px, Pyrimidine nucleobases are simple ring molecules. Nucleobases, also known as ''nitrogenous bases'' or often simply ''bases'', are nitrogen-containing biological compounds that form nucleosides Nucleos ...

adenine
(A),
cytosine Cytosine () (symbol A symbol is a mark, sign, or that indicates, signifies, or is understood as representing an , , or . Symbols allow people to go beyond what is n or seen by creating linkages between otherwise very different s and s. Al ...

cytosine
(C),
guanine Guanine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical meaning ...

guanine
(G), and
thymine Thymine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical m ...

thymine
(T). Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain.
Virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecu ...

Virus
es are the only exception to this rule—sometimes viruses use the very similar molecule
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...

RNA
instead of DNA as their genetic material. Viruses cannot reproduce without a
host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places *Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host (19 ...
and are unaffected by many genetic processes, so tend not to be considered living organisms. DNA normally exists as a double-stranded molecule, coiled into the shape of a
double helix In molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, mech ...
. 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 In , DNA replication is the of producing two identical replicas of DNA from one original molecule. DNA replication occurs in all acting as the most essential part for . This is essential for cell division during growth and repair of damaged tis ...

DNA replication
duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand. Genes are arranged linearly along long chains of DNA base-pair sequences. In
bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sm ...

bacteria
, 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 A chromosome is a long DNA molecule with part or all of the genetic mate ...
, while
eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryotic
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 pair A base pair (bp) is a fundamental unit of double-stranded nucleic acids Nucleic acids are biopolymer Biopolymers are natural polymers produced by the cells of Organism, living organisms. Biopolymers consist of monomeric units that are Covalent_ ...
s 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 Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecu ...
; in eukaryotes, chromatin is usually composed of
nucleosome A nucleosome is the basic structural unit of packaging in . The structure of a nucleosome consists of a segment of DNA wound around eight   and resembles thread wrapped around a spool. The nucleosome is the fundamental subunit of . Each nucleo ...

nucleosome
s, segments of DNA wound around cores of
histone In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
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 genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The genome includes both the genes (the coding regions) and the noncodin ...

genome
. 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 Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by ...
organisms have only one copy of each chromosome, most animals and many plants are
diploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mate ...
, containing two of each chromosome and thus two copies of every gene. The two alleles for a gene are located on identical
loci Locus (plural loci) is Latin for "place". It may refer to: Entertainment * Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front * Locus (magazine), ''Locus'' (magazine), science fiction and fantasy magazine ...
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 gene In biology, a gene (from ''genos'' "...Wilh ...
, each allele inherited from a different parent. 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 p ...
s that determine the gender 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 chromosome#Sex determination, sex-determining chromosome in many species, s ...
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 chromosome A chromosome is a long DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecul ...

X chromosome
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 other things including X-linked diseases. The X and Y chromosomes form a strongly heterogeneous pair.


Reproduction

When cells divide, their full genome is copied and each
daughter cell Cell division is the process by which a parent cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious ...

daughter cell
inherits one copy. This process, called
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
, is the simplest form of reproduction and is the basis for
asexual reproduction Asexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Gree ...
. 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 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 pathological conditio ...

clones
.
Eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

Eukaryotic
organisms often use
sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, ...
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 Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by ...
) and double copies (
diploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mate ...
). 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 ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply ...
s such as
sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as ...

sperm
or
eggs Egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excluding s), and lay eggs, although some, such as s, do ...

eggs
. Although they do not use the haploid/diploid method of sexual reproduction,
bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sm ...

bacteria
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 change ...
, 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 * Transf ...
. These processes result in
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient G ...
, transmitting fragments of genetic information between organisms that would be otherwise unrelated. Natural bacterial transformation occurs in many
bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sm ...

bacteria
l species, and can be regarded as a sexual process for transferring
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

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 Ribonucleic acid (RNA) is a polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecule File:Pentacene on Ni( ...
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 installati ...

repair
of
DNA damage DNA repair is a collection of processes by which a identifies and corrects damage to the molecules that encode its . In human cells, both normal activities and environmental factors such as can cause DNA damage, resulting in tens of thousan ...
s in the recipient cell.


Recombination and genetic linkage

The diploid nature of chromosomes allows for genes on different chromosomes to 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 Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' sister chromatids, non-sister chromatids that results in recombinant chromosomes. It is one of the final ...
. 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 (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
, a series of cell divisions that creates haploid cells. Meiotic recombination, particularly in microbial
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
s, 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 Barbara McClintock (June 16, 1902 – September 2, 1992) was an American scientist and cytogeneticist Cytogenetics is essentially a branch of genetics, but is also a part of cell biology/cytology (a subdivision of human anatomy), that is conce ...
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 Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction. Two genetic markers that are physically near to each other are unlikely to be separa ...
; 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 Express or EXPRESS may refer to: Arts, entertainment, and media Films * ''Express: Aisle to Glory'', a 1998 comedy short film featuring Kal Penn * ''The Express: The Ernie Davis Story'', a 2008 film starring Dennis Quaid Music * Express (album), ...

express
their functional effect through the production of
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s, which are complex molecules responsible for most functions in the cell. Proteins are made up of one or more polypeptide chains, each of which is composed of a sequence of
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
s, and the DNA sequence of a gene (through an RNA intermediate) is used to produce a specific
amino acid sequence Protein primary structure is the linear sequence of amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen ...
. This process begins with the production of an
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Re ...

RNA
molecule with a sequence matching the gene's DNA sequence, a process called transcription. This
messenger RNA In molecular biology Molecular biology is the branch of biology that seeks to understand the molecule, molecular basis of biological activity in and between Cell (biology), cells, including biomolecule, molecular synthesis, modification, m ...
molecule then serves to produce a corresponding amino acid sequence through a process called . Each group of three nucleotides in the sequence, called a
codon The genetic code is the set of rules used by living cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or reli ...

codon
, corresponds either to one of the twenty possible amino acids in a protein or an stop codon, 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 cell (biology), cells to Translation (biology), translate information encoded within genetic material (DNA or Messenger RNA, mRNA sequences of nucleotide triplets, or codons) into proteins. Tran ...

genetic code
. 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 Francis Harry Compton Crick (8 June 1916 – 28 July 2004) was a British molecular biologist, biophysicist, and neuroscientist A neuroscientist (or neurobiologist) is a scientist A scientist is a person who conducts Scientific method, scie ...

Francis Crick
called the central dogma of molecular biology. The specific sequence of amino acids protein folding, results 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 enzymes by facilitating chemical reactions within the bound molecules (without changing the structure of the protein itself). Protein structure is dynamic; the protein hemoglobin bends into slightly different forms as it facilitates the capture, transport, and release of oxygen molecules within mammalian blood. A Single-nucleotide polymorphism, 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 is a human Genetic disorder, genetic disease 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 cells carrying the protein. These sickle-shaped cells no longer flow smoothly through blood vessels, 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. In some cases, these products fold into structures which are involved in critical cell functions (e.g. ribosomal RNA and transfer RNA). RNA can also have regulatory effects through hybridization interactions with other RNA molecules (such as microRNA).


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), Siamese cat. 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 Denaturation (biochemistry), 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 coldersuch 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. 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 twin study, studying identical and fraternal twins, or other siblings of multiple births. 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, who were Multiple birth, identical quadruplets all diagnosed with schizophrenia. However, such tests cannot separate genetic factors from environmental factors affecting fetal development.


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 Gene expression, 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 factors are regulatory proteins that bind to DNA, either promoting or inhibiting the transcription of a gene. Within the genome of ''Escherichia coli'' bacteria, for example, there exists a series of genes necessary for the synthesis of the amino acid tryptophan. 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 Trp repressor, 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 Cell signaling, intercellular signals and gradually establishing different patterns of gene expression to create different behaviors. As no single gene is responsible for the Development (biology), development of structures within multicellular organisms, these patterns arise from the complex interactions between many cells. Within
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
s, there exist structural features of
chromatin Chromatin is a complex of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecu ...
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 cell culture, 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, 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 In , DNA replication is the of producing two identical replicas of DNA from one original molecule. DNA replication occurs in all acting as the most essential part for . This is essential for cell division during growth and repair of damaged tis ...

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 polymerases. Processes that increase the rate of changes in DNA are called mutagenic: mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing, while UV radiation induces mutations by causing damage to the DNA structure. Chemical damage to DNA occurs naturally as well and cells use DNA repair 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 produced by cellular respiration, cellular aerobic respiration, and these can lead to mutations. In organisms that use
chromosomal crossover Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' sister chromatids, non-sister chromatids that results in recombinant chromosomes. It is one of the final ...
to exchange DNA and recombine genes, errors in alignment during
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

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—Gene duplication, duplications, Chromosomal inversion, inversions, Gene deletion, deletions of entire regions—or the accidental exchange of whole parts of sequences between different chromosomes (chromosomal translocation).


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 (biology), fitness. Mutations that do have an effect are usually detrimental, but occasionally some can be beneficial. Studies in the fly ''Drosophila melanogaster'' suggest that if a mutation changes a protein produced by a gene, about 70 percent of these mutations will be harmful with the remainder being either neutral or weakly beneficial. Population genetics studies the distribution of genetic differences within populations and how these distributions change over time. Changes in the Allele frequency, frequency of an allele in a population are mainly influenced by natural selection, where a given allele provides a selective or reproductive advantage to the organism, as well as other factors such as mutation, genetic drift, genetic hitchhiking, artificial selection and Gene flow, migration. 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 Sequence homology, homology between different species' genomes, it is possible to calculate the evolutionary distance between them and Molecular clock, 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 trees; 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 A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient G ...
and most common in bacteria).


Model organisms

Although geneticists originally studied inheritance in a wide range of organisms, researchers began to specialize in studying the genetics of a particular subset of organisms. The fact that significant research already existed for a given organism would encourage new researchers 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 and the involvement of genes in morphogenesis, development and cancer. Organisms were chosen, in part, for convenience—short generation times and easy genetic engineering, genetic manipulation made some organisms popular genetics research tools. Widely used model organisms include the gut bacterium ''Escherichia coli'', the plant ''Arabidopsis thaliana'', baker's yeast (''Saccharomyces cerevisiae''), the nematode ''Caenorhabditis elegans'', the common fruit fly (''Drosophila melanogaster''), and the common house mouse (''Mus musculus'').


Medicine

Medical genetics 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 Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction. Two genetic markers that are physically near to each other are unlikely to be separa ...
and genetic
pedigree chart A pedigree chart is a diagram that shows the occurrence and appearance of phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. The letters B ...
s to find the location on the genome associated with the disease. At the population level, researchers take advantage of Mendelian randomization to look for locations in the genome that are associated with diseases, a method especially useful for Quantitative trait locus, multigenic traits not clearly defined by a single gene. Once a candidate gene is found, further research is often done on the corresponding (or Homology (biology), homologous) genes of model organisms. In addition to studying genetic diseases, the increased availability of genotyping methods has led to the field of pharmacogenetics: the study of how genotype can affect drug responses. Individuals differ in their inherited tendency to develop cancer, 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 Apoptosis, cell death, but sometimes additional mutations occur that cause cells to ignore these messages. An internal process of natural selection occurs within the body and eventually mutations accumulate within cells to promote their own growth, creating a cancerous Tumour heterogeneity, tumor that grows and invades various tissues of the body. Normally, a cell divides only in response to signals called growth factors and Contact inhibition, 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. 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. 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 mutations). The most frequent mutations are a loss of function of p53 protein, a tumor suppressor, or in the p53 pathway, and gain of function mutations in the Ras proteins, or in other oncogenes.


Research methods

DNA can be manipulated in the laboratory. Restriction enzymes 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 DNA ligase, ligation enzymes allows DNA fragments to be connected. By binding ("ligating") fragments of DNA together from different sources, researchers can create recombinant DNA, 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, researchers can amplify the DNA fragments by inserting plasmids into bacteria and then culturing them on plates of agar (to isolate Cloning#Unicellular organisms, 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 Polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete copies or partial copies) of a specific DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM. ...

polymerase chain reaction
(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 DNA sequencing is the process of determining the nucleic acid sequence A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides Nucleotides are organic ...

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 Sanger sequencing, chain-termination sequencing, developed in 1977 by a team led by
Frederick Sanger Frederick Sanger (; 13 August 1918 – 19 November 2013) was a British biochemist Biochemists are scientists who are trained in biochemistry Biochemistry or biological chemistry, is the study of chemical processes within and relatin ...

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 Genome project, sequenced the genomes of many organisms using a process called genome assembly, which utilizes computational tools to stitch together sequences from many different fragments. These technologies were used to sequence 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 Mitochondrial DNA, mitochondria. These are usually treated se ...

human genome
in the
Human Genome Project The Human Genome Project (''HGP'') was an international scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge that has characterized the development of science since at least the 17th cen ...
completed in 2003. New DNA sequencing#New sequencing methods, high-throughput sequencing 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 (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. 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, to edit 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 Mitochondrial DNA, mitochondria. These are usually treated se ...

human genome
in a way that can be inherited. In April 2015, Chinese researchers CRISPR#Society and culture, reported results of basic research to edit the
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
of non-viable human embryos using CRISPR.


See also

* Bacterial genome size * Cryoconservation of animal genetic resources * Eugenics * Embryology * Evolution * Genetic disorder * Genetic diversity * Genetic engineering * Genetic enhancement * Index of genetics articles * Medical genetics * Molecular tools for gene study * Mutation * Neuroepigenetics * Outline of genetics * Timeline of the history of genetics * Plant genetic resources


References


Further reading

* * * * *


External links

* * {{Authority control Genetics,