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Mendelian inheritance is a type of
biological Biology is the natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowl ...

biological
inheritance Inheritance is the practice of passing on private property Private property is a legal designation for the ownership of property by non-governmental legal entities. Private property is distinguishable from public property Public property i ...
that follows the principles originally proposed 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 th ...

Gregor Mendel
in 1865 and 1866, re-discovered in 1900 by
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 of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scienti ...

Hugo de Vries
and
Carl Correns Carl Erich Correns (19 September 1864 – 14 February 1933) was a German botanist Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a sc ...

Carl Correns
, and popularized by
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
. These principles were initially controversial. When Mendel's theories were integrated with the Boveri–Sutton chromosome theory of inheritance by
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
in 1915, they became the core of
classical genetics Classical genetics is the branch of 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, Molecula ...
.
Ronald Fisher Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a subs ...
combined these ideas with the theory of
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
in his 1930 book ''
The Genetical Theory of Natural Selection ''The Genetical Theory of Natural Selection'' is a book by Ronald Fisher Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British statistician, geneticist, and academic. For his work in statistics, he has been describ ...
'', putting
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
onto a
mathematical Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and their changes (cal ...
footing and forming the basis for
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 ...
within the modern evolutionary synthesis.


History

The principles of Mendelian inheritance were named for and first derived by
Gregor Johann 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 th ...
, a nineteenth-century
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 ...
monk A monk (, from el, μοναχός, ''monachos'', "single, solitary" via Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (f ...

monk
who formulated his ideas after conducting simple hybridisation experiments with pea plants (''
Pisum sativum The pea is most commonly the small spherical seed A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band' ...

Pisum sativum
'') he had planted in the garden of his monastery. Between 1856 and 1863, Mendel cultivated and tested some 5,000 pea plants. From these experiments, he induced two generalizations which later became known as ''Mendel's Principles of Heredity'' or ''Mendelian inheritance''. He described his experiments in a two-part paper, ''Versuche über Pflanzen-Hybriden'' (''
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 ...
''), that he presented to the Natural History Society of
Brno Brno ( , ; german: Brünn, ) is a city A city is a large human settlement.Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. and Kuper, J., eds (1996) ''The Social Science Encyclopedia''. 2nd edition. ...

Brno
on 8 February and 8 March 1865, and which was published in 1866. Mendel's results were largely ignored by the vast majority. Although they were not completely unknown to biologists of the time, they were not seen as generally applicable, even by Mendel himself, who thought they only applied to certain categories of species or traits. A major block to understanding their significance was the importance attached by 19th-century biologists to the
apparent blending
apparent blending
of many inherited traits in the overall appearance of the progeny, now known to be due to multi-gene interactions, in contrast to the organ-specific binary characters studied by Mendel. In 1900, however, his work was "re-discovered" by three European scientists,
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 of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scienti ...

Hugo de Vries
,
Carl Correns Carl Erich Correns (19 September 1864 – 14 February 1933) was a German botanist Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a sc ...

Carl Correns
, and
Erich von Tschermak Erich Tschermak, Edler von Seysenegg (15 November 1871 – 11 October 1962) was an Austria Austria (, ; german: Österreich ), officially the Republic of Austria (german: Republik Österreich, links=no, ), is a landlocked Eastern Al ...
. The exact nature of the "re-discovery" has been debated: De Vries published first on the subject, mentioning Mendel in a footnote, while Correns pointed out Mendel's priority after having read De Vries' paper and realizing that he himself did not have priority. De Vries may not have acknowledged truthfully how much of his knowledge of the laws came from his own work and how much came only after reading Mendel's paper. Later scholars have accused Von Tschermak of not truly understanding the results at all. Regardless, the "re-discovery" made Mendelism an important but controversial theory. Its most vigorous promoter in Europe was
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
, who coined the terms "
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 interactions, Physiology, ...

genetics
" and "
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 ...
" to describe many of its tenets. The model of
heredity Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cell (biology), cells or or ...

heredity
was contested by other biologists because it implied that heredity was discontinuous, in opposition to the apparently continuous variation observable for many traits. Many biologists also dismissed the theory because they were not sure it would apply to all species. However, later work by biologists and statisticians such as
Ronald Fisher Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a subs ...
showed that if multiple Mendelian factors were involved in the expression of an individual trait, they could produce the diverse results observed, and thus showed that Mendelian genetics is compatible with
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
.
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
and his assistants later integrated Mendel's theoretical model with the
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
theory of inheritance, in which the chromosomes of cells were thought to hold the actual hereditary material, and created what is now known as
classical genetics Classical genetics is the branch of 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, Molecula ...
, a highly successful foundation which eventually cemented Mendel's place in history. Mendel's findings allowed scientists such as Fisher and J.B.S. Haldane to predict the expression of traits on the basis of mathematical probabilities. An important aspect of Mendel's success can be traced to his decision to start his crosses only with plants he demonstrated were true-breeding. He only measured discrete (binary) characteristics, such as color, shape, and position of the seeds, rather than quantitatively variable characteristics. He expressed his results numerically and subjected them to
statistical analysis Statistical inference is the process of using data analysis to infer properties of an underlying probability distribution, distribution of probability.Upton, G., Cook, I. (2008) ''Oxford Dictionary of Statistics'', OUP. . Inferential statistical ...
. His method of data analysis and his large
sample size Sample size determination is the act of choosing the number of observations or replicates to include in a statistical sample In statistics and quantitative research methodology, a sample is a set of individuals or objects collected or selected ...
gave credibility to his data. He had the foresight to follow several successive generations (P, F1, F2, F3) of pea plants and record their variations. Finally, he performed "test crosses" (
backcrossing Backcrossing is a crossing of a hybrid Hybrid may refer to: Economics and finance * Hybrid market, a system allowing stock trades to be completed either electronically or manually * Hybrid security, a type of economic instrument Technology Elect ...

backcrossing
descendants of the initial hybridization to the initial true-breeding lines) to reveal the presence and proportions of
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 ...
characters.


Mendel's genetic discoveries

Five parts of Mendel's discoveries were an important divergence from the common theories at the time and were the prerequisite for the establishment of his rules. #Characters are unitary, that is, they are discrete e.g: purple ''vs''. white, tall ''vs''. dwarf. There is no medium sized plant or light purple flower. #Genetic characteristics have alternate forms, each inherited from one of two parents. Today, we call these
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. #One allele is dominant over the other. The phenotype reflects the dominant allele. #Gametes are created by random segregation. Heterozygotic individuals produce gametes with an equal frequency of the two alleles. #Different traits have independent assortment. In modern terms, genes are unlinked. According to customary terminology we refer here to the principles of inheritance discovered by Gregor Mendel as Mendelian laws, although today's geneticists also speak of ''Mendelian rules'' or ''Mendelian principles'', as there are many exceptions summarized under the collective term
Non-Mendelian inheritance Non-Mendelian inheritance is any pattern of inheritance in which traits do not segregate in accordance with Mendel's laws Mendelian inheritance is a type of biology, biological inheritance (biology), inheritance that follows the principles o ...
. Mendel selected for the experiment the following characters of pea plants: * Form of the ripe seeds (round or roundish, surface shallow or wrinkled) * Colour of the seed–coat (white, gray, or brown, with or without violet spotting) * Colour of the
seeds A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, it was released to generally positi ...

seeds
and cotyledons (yellow or green) * Flower colour (white or yellow) * Form of the ripe pods (simply inflated, not contracted, or constricted between the seeds and wrinkled) * Colour of the unripe pods (yellow or green) * Position of the flowers (axial or terminal) * Length of the stem When he crossed purebred white flower and purple flower pea plants (the parental or P generation) by
artificial Artificiality (the state of being artificial or man-made) is the state of being the product of intentional human manufacture, rather than occurring nature, naturally through processes not involving or requiring human activity. Connotations Artific ...
pollination, the resulting flower colour was not a blend. Rather than being a mix of the two, the offspring in the first generation ( F1-generation) were all purple-flowered. Therefore, he called this
biological trait A phenotypic trait, simply trait, or character state is a distinct variant of a phenotypic In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their ...
dominant. When he allowed
self-fertilization Autogamy, or self-fertilization, refers to the fusion of two gamete A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''K ...
in the uniform looking F1-generation, he obtained both colours in the F2 generation with a purple flower to white flower ratio of 3 : 1. In some of the other characters also one of the traits was dominant. He then conceived the idea of heredity units, which he called hereditary "factors". Mendel found that there are alternative forms of factors—now called
gene 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 mecha ...

gene
s—that account for variations in inherited characteristics. For example, the gene for flower color in pea plants exists in two forms, one for purple and the other for white. The alternative "forms" are now 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. For each trait, an organism inherits two alleles, one from each parent. These alleles may be the same or different. An organism that has two identical alleles for a gene is said to be
homozygous 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 or ...
for that gene (and is called a homozygote). An organism that has two different alleles for a gene is said be
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 or ...

heterozygous
for that gene (and is called a heterozygote). Mendel hypothesized that allele pairs separate randomly, or segregate, from each other during the production of the
gametes A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the foll ...
in the seed plant (
egg cell The egg cell, or ovum (plural ova), is the female reproductive Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduct ...
) and the pollen plant (
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
). Because allele pairs separate during gamete production, a
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
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 not. eg ...

egg
carries only one allele for each inherited trait. When sperm and egg unite at
fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise.2C -ize .28-isation.2C -ization.29, spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes ...

fertilization
, each contributes its allele, restoring the paired condition in the offspring. Mendel also found that each pair of alleles segregates independently of the other pairs of alleles during gamete formation. The
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 ...
of an individual is made up of the many alleles it possesses. The
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
is the result of the expression of all characteristics that are genetically determined by its alleles as well as by its environment. The presence of an allele does not mean that the trait will be expressed in the individual that possesses it. If the two alleles of an inherited pair differ (the heterozygous condition), then one determines the organism's appearance and is called the
dominant allele 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 scientis ...
; the other has no noticeable effect on the organism's appearance and is called the
recessive allele 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 scientis ...
.


Law of Dominance and Uniformity

If two parents are mated with each other who differ in one genetic characteristic for which they are both
homozygous 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 or ...
(each pure-bred), all offspring in the first generation (F1) are equal to the examined characteristic in
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 ...
and
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
showing the dominant trait. This ''uniformity rule'' or ''reciprocity rule'' applies to all individuals of the F1-generation. The principle of dominant inheritance discovered by Mendel states that in a heterozygote the dominant allele will cause the recessive allele to be "masked": that is, not expressed in the phenotype. Only if an individual is homozygous with respect to the recessive allele will the recessive trait be expressed. Therefore, a cross between a homozygous dominant and a homozygous recessive organism yields a heterozygous organism whose phenotype displays only the dominant trait. The F1 offspring of Mendel's pea crosses always looked like one of the two parental varieties. In this situation of "complete dominance," the dominant allele had the same phenotypic effect whether present in one or two copies. But for some characteristics, the F1 hybrids have an appearance ''in between'' the phenotypes of the two parental varieties. A cross between two four o'clock (''
Mirabilis jalapa ''Mirabilis jalapa'', the marvel of Peru or four o'clock flower, is the most commonly grown ornamental species of ''Mirabilis'' plant Plants are mainly multicellular organisms, predominantly photosynthetic Photosynthesis is a process ...

Mirabilis jalapa
'') plants shows an exception to Mendel's principle, called ''incomplete dominance''. Flowers of heterozygous plants have a phenotype somewhere between the two homozygous genotypes. In cases of intermediate inheritance (incomplete dominance) in the F1-generation Mendel's principle of uniformity in genotype and phenotype applies as well. Research about intermediate inheritance was done by other scientists. The first was
Carl Correns Carl Erich Correns (19 September 1864 – 14 February 1933) was a German botanist Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a sc ...

Carl Correns
with his studies about Mirabilis jalapa.


Law of Segregation of genes

The Law of Segregation of genes applies when two individuals, both heterozygous for a certain trait are crossed, for example hybrids of the F1-generation. The offspring in the F2-generation differ in genotype and phenotype, so that the characteristics of the grandparents (P-generation) regularly occur again. In a dominant-recessive inheritance an average of 25% are homozygous with the dominant trait, 50% are heterozygous showing the dominant trait in the phenotype (
genetic carrier Genetic may refer to: *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, mo ...
s), 25% are homozygous with the recessive trait and therefore
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'' ...

express
the recessive trait in the phenotype. The genotypic ratio is 1 : 2 : 1, the phenotypic ratio is 3 : 1. In the pea plant example, the capital "B" represents the dominant allele for purple blossom and lowercase "b" represents the recessive allele for white blossom. The
pistil Gynoecium (; ) is most commonly used as a collective term for the parts of a flower A flower, sometimes known as a bloom or blossom Image:Cerisier du Japon Prunus serrulata.jpg, Cherry blossoms in Paris in full bloom. In botany, blossoms ...

pistil
plant and the
pollen Pollen is a powdery substance consisting of pollen grains which are microsporophytes of seed plants The spermatophytes (; ), also known as phanerogams (taxon Phanerogamae) or phaenogams (taxon Phaenogamae), comprise those plant Plant ...

pollen
plant are both F1-hybrids with genotype "B b". Each has one allele for purple and one allele for white. In the offspring, in the F2-plants in the Punnett-square, three combinations are possible. The genotypic ratio is 1 ''BB'' : 2 ''Bb'' : 1 ''bb''. But the phenotypic ratio of plants with purple blossoms to those with white blossoms is 3 : 1 due to the dominance of the allele for purple. Plants with homozygous "b b" are white flowered like one of the grandparents in the P-generation. In cases of
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 ...

incomplete dominance
the same segregation of alleles takes place in the F2-generation, but here also the phenotypes show a ratio of 1 : 2 : 1, as the heterozygous are different in phenotype from the homozygous because the
genetic expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. These ...
of one allele compensates the missing expression of the other allele only partially. This results in an intermediate inheritance which was later described by other scientists. In some literature sources the principle of segregation is cited as "first law". Nevertheless, Mendel did his crossing experiments with heterozygous plants after obtaining these hybrids by crossing two purebred plants, discovering the principle of dominance and uniformity at first. Neil A. Campbell, Jane B. Reece: Biologie. Spektrum-Verlag 2003, page 293-315. Molecular proof of segregation of genes was subsequently found through observation of
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
by two scientists independently, the German botanist
Oscar Hertwig Illustration from O. Hertwig's book ''Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbeltiere'' (Textbook of developmental history of humans and vertebrates), 1906. Oscar Hertwig (21 April 1849 in Friedberg – 25 October 1922 in B ...
in 1876, and the Belgian zoologist
Edouard Van Beneden Édouard Joseph Louis Marie Van Beneden (5 March 1846 in Leuven – 28 April 1910 in Liège), son of Pierre-Joseph Van Beneden, was a Belgian embryologist, cytologist and marine biologist. He was professor of zoology Zoology ()The pronunciat ...
in 1883. Most alleles are located in
chromosomes 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 ...
in the
cell nucleus In cell biology, the nucleus (pl. ''nuclei''; from Latin or , meaning ''kernel'' or ''seed'') is a biological membrane#Function, membrane-bound organelle found in eukaryote, eukaryotic cell (biology), cells. Eukaryotes usually have a single n ...

cell nucleus
. Paternal and maternal chromosomes get separated in meiosis, because during
spermatogenesis Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the Mitosis, mitotic division of the stem cells located close to the basement membrane of the ...

spermatogenesis
the chromosomes are segregated on the four sperm cells that arise from one mother sperm cell, and during
oogenesis Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum The egg cell, or ovum (plural ova), is the female reproductive Reproduction (or procreation or breeding) is the biological process by which new individual organisms ...

oogenesis
the chromosomes are distributed between the polar bodies and the
egg cell The egg cell, or ovum (plural ova), is the female reproductive Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduct ...
. Every individual organism contains two alleles for each trait. They segregate (separate) during meiosis such that each
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 ...
contains only one of the alleles. When the gametes unite in the
zygote A zygote (, ) is a eukaryotic 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 ...

zygote
the alleles—one from the mother one from the father—get passed on to the offspring. An offspring thus receives a pair of alleles for a trait by inheriting
homologous chromosome A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome A chromosome is a long DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule File:Pen ...
s from the parent organisms: one allele for each trait from each parent. Heterozygous individuals with the dominant trait in the phenotype are
genetic carrier Genetic may refer to: *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, mo ...
s of the recessive trait.


Law of Independent Assortment

The Law of Independent Assortment states that alleles for separate traits are passed independently of one another. That is, the biological selection of an allele for one trait has nothing to do with the selection of an allele for any other trait. Mendel found support for this law in his dihybrid cross experiments. In his monohybrid crosses, an idealized 3:1 ratio between dominant and recessive phenotypes resulted. In dihybrid crosses, however, he found a 9:3:3:1 ratios. This shows that each of the two alleles is inherited independently from the other, with a 3:1 phenotypic ratio for each. Independent assortment occurs in
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 ...
organisms during meiotic metaphase I, and produces a gamete with a mixture of the organism's chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent chromosome along the metaphase plate with respect to the other bivalent chromosomes. Along with crossing over, independent assortment increases genetic diversity by producing novel genetic combinations. There are many deviations from the principle of independent assortment due to
genetic linkage Genetic linkage is the tendency of DNA sequences 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 molecules , CH4; is ...
. Of the 46 chromosomes in a normal
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 ...
human cell, half are maternally derived (from the mother's
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 not. eg ...

egg
) and half are paternally derived (from the father's
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 ...
). This occurs as
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, ...
involves the fusion of two
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 ...
gametes (the egg and sperm) to produce a zygote and a new organism, in which every cell has two sets of chromosomes (diploid). During
gametogenesis Gametogenesis is a biological Biology is the natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Ta ...
the normal complement of 46 chromosomes needs to be halved to 23 to ensure that the resulting haploid gamete can join with another haploid gamete to produce a diploid organism. In independent assortment, the chromosomes that result are randomly sorted from all possible maternal and paternal chromosomes. Because zygotes end up with a mix instead of a pre-defined "set" from either parent, chromosomes are therefore considered assorted independently. As such, the zygote can end up with any combination of paternal or maternal chromosomes. For human gametes, with 23 chromosomes, the number of possibilities is 223 or 8,388,608 possible combinations. This contributes to the genetic variability of progeny. Generally, the recombination of genes has important implications for many evolutionary processes.


Mendelian trait

A Mendelian trait is one that is controlled by a single
locus 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 ...
in an inheritance pattern. In such cases, a mutation in a single gene can cause a disease that is inherited according to Mendel's principles. Dominant diseases manifest in heterozygous individuals. Recessive ones are sometimes inherited unnoticeably by
genetic carrier Genetic may refer to: *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, mo ...
s. Examples include
sickle-cell anemia Sickle cell disease (SCD) is a group of blood disorders typically Heredity, 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 i ...
,
Tay–Sachs disease Tay–Sachs disease is a 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 In biology, a gene (from ''genos'' "...Wilhelm Johannse ...
,
cystic fibrosis Cystic fibrosis (CF) is a genetic disorder A genetic disorder is a health problem caused by one or more abnormalities in the genome In the fields of molecular biology and genetics, a genome is all genetic information of an organis ...
and xeroderma pigmentosa. A disease controlled by a single gene contrasts with a multi-factorial disease, like heart disease, which is affected by several loci (and the environment) as well as those diseases inherited in a
non-Mendelian'' Image:Carl Correns.jpg, Carl Correns Non-Mendelian inheritance is any pattern of inheritance in which traits do not segregate in accordance with Mendelian inheritance#Mendel.27s laws, Mendel's laws. These laws describe the inheritance of traits l ...
fashion.


Non-Mendelian inheritance

After Mendel's studies and discoveries more and more new discoveries about genetics were made. Mendel himself has said that the regularities he discovered apply only to the organisms and characteristics he consciously chose for his experiments. Mendel explained inheritance in terms of discrete factors—genes—that are passed along from generation to generation according to the rules of probability. Mendel's laws are valid for all sexually reproducing organisms, including garden peas and human beings. However, Mendel's laws stop short of explaining some patterns of genetic inheritance. For most sexually reproducing organisms, cases where Mendel's laws can strictly account for all patterns of inheritance are relatively rare. Often the inheritance patterns are more complex.Khan Academy
Variations on Mendel's laws (overview)
/ref> In cases of codominance the phenotypes produced by both alleles are clearly expressed. Mendel chose genetic traits in plants that are determined by only two alleles, such as "A" and "a". In nature, genes often exist in several different forms with
multiple alleles 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 ...
. Furthermore, many traits are produced by the interaction of several genes. Traits controlled by two or more genes are said to be polygenic traits.


See also

* List of Mendelian traits in humans *
Mendelian disease A genetic disorder is a health problem caused by one or more abnormalities in the genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and li ...
s (monogenic disease) *
Mendelian error A Mendelian error in the genetic analysis of a species, describes an allele in an individual which could not have been received from either of its biological parents by Mendelian inheritance. Inheritance is defined by a set of related individuals ...
*
Particulate inheritance Particulate inheritance is a pattern of inheritance discovered by Mendelian genetics theorists, such as William Bateson William Bateson (8 August 1861 – 8 February 1926) was an English biology, biologist who was the first person to use the ...
*
Punnett square The Punnett square is a square diagram that is used to predict the genotypes of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach in 1905. The diagram is used by biologist A biologi ...


References


Notes

* * *


External links


Khan Academy, video lecture



Mendel's principles of Inheritance


{{DEFAULTSORT:Mendelian Inheritance Classical genetics
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 interactions, Physiology, ...
it:Gregor Mendel#Le leggi di Mendel