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Ingenetics,dominance is the phenomenon of one variant (allele) of agene on achromosome masking or overriding theeffect of a different variant of the same gene onthe other copy of the chromosome.[1][2] The first variant is termeddominant and the second is calledrecessive. This state of havingtwo different variants of the same gene on each chromosome is originally caused by amutation in one of the genes, either new (de novo) orinherited. The termsautosomal dominant orautosomal recessive are used to describe gene variants on non-sex chromosomes (autosomes) and their associated traits, while those onsex chromosomes (allosomes) are termedX-linked dominant,X-linked recessive orY-linked; these have an inheritance and presentation pattern that depends on the sex of both the parent and the child (seeSex linkage). Since there is only oneY chromosome, Y-linked traits cannot be dominant or recessive.[3] Additionally, there are other forms of dominance, such asincomplete dominance, in which a gene variant has a partial effect compared to when it is present on both chromosomes, andco-dominance, in which different variants on each chromosome both show their associated traits.
Dominance is a key concept inMendelian inheritance andclassical genetics. Letters andPunnett squares are used to demonstrate the principles of dominance in teaching, and the upper-case letters are used to denote dominant alleles and lower-case letters are used for recessive alleles. An often quoted example of dominance is the inheritance ofseed shape inpeas. Peas may be round, associated with alleleR, or wrinkled, associated with alleler. In this case, three combinations of alleles (genotypes) are possible:RR,Rr, andrr. TheRR (homozygous) individuals have round peas, and therr (homozygous) individuals have wrinkled peas. InRr (heterozygous) individuals, theR allele masks the presence of ther allele, so these individuals also have round peas. Thus, alleleR is dominant over alleler, and alleler is recessive to alleleR.[4]
Dominance is not inherent to an allele or its traits (phenotype). It is a strictly relative effect between two alleles of a given gene of any function; one allele can be dominant over a second allele of the same gene, recessive to a third, andco-dominant with a fourth. Additionally, one allele may be dominant for one trait but not others.[5] Dominance differs fromepistasis, the phenomenon of an allele of one gene masking the effect of alleles of adifferent gene.[6]
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Gregor Johann Mendel, "The Father of Genetics", promulgated the idea of dominance in the 1860s. However, it was not widely known until the early twentieth century. Mendel observed that, for a variety of traits of garden peas having to do with the appearance of seeds, seed pods, and plants, there were two discrete phenotypes, such as round versus wrinkled seeds, yellow versus green seeds, red versus white flowers or tall versus short plants. When bred separately, the plants always produced the same phenotypes, generation after generation. However, when lines with different phenotypes were crossed (interbred), one and only one of the parental phenotypes showed up in the offspring (green, round, red, or tall). However, when thesehybrid plants were crossed, the offspring plants showed the two original phenotypes, in a characteristic 3:1 ratio, the more common phenotype being that of the parental hybrid plants. Mendel reasoned that each parent in the first cross was a homozygote for different alleles (one parent AA and the other parent aa), that each contributed one allele to the offspring, with the result that all of these hybrids were heterozygotes (Aa), and that one of the two alleles in the hybrid cross dominated expression of the other: A masked a. The final cross between two heterozygotes (Aa X Aa) would produce AA, Aa, and aa offspring in a 1:2:1 genotype ratio with the first two classes showing the (A) phenotype, and the last showing the (a) phenotype, thereby producing the 3:1 phenotype ratio.
Mendel did not use the terms gene, allele, phenotype, genotype, homozygote, and heterozygote, all of which were introduced later. He did introduce the notation of capital and lowercase letters for dominant and recessive alleles, respectively, still in use today.
In 1928, British population geneticistRonald Fisher proposed that dominance acted based on natural selection through the contribution ofmodifier genes. In 1929, American geneticistSewall Wright responded by stating that dominance is simply a physiological consequence of metabolic pathways and the relative necessity of the gene involved.[7][8][9][5]
In complete dominance, the effect of one allele in a heterozygous genotype completely masks the effect of the other. The allele that masks are considereddominant to the other allele, and the masked allele is consideredrecessive.[10]
When we only look at one trait determined by one pair of genes, we call itmonohybrid inheritance. If the crossing is done between parents (P-generation, F0-generation) who are homozygote dominant and homozygote recessive, the offspring (F1-generation) will always have the heterozygote genotype and always present the phenotype associated with the dominant gene.
However, if the F1-generation is further crossed with the F1-generation (heterozygote crossed with heterozygote) the offspring (F2-generation) will present the phenotype associated with the dominant gene ¾ times. Although heterozygote monohybrid crossing can result in two phenotype variants, it can result in three genotype variants - homozygote dominant, heterozygote and homozygote recessive, respectively.[11]
Indihybrid inheritance we look at the inheritance of two pairs of genes simultaneous. Assuming here that the two pairs of genes are located at non-homologous chromosomes, such that they are not coupled genes (seegenetic linkage) but instead inherited independently. Consider now the cross between parents (P-generation) of genotypes homozygote dominant and recessive, respectively. The offspring (F1-generation) will always heterozygous and present the phenotype associated with the dominant allele variant. However, when crossing the F1-generation there are four possible phenotypic possibilities and the phenotypicalratio for the F2-generation will always be 9:3:3:1.[12]
Incomplete dominance (also calledpartial dominance,semi-dominance,intermediate inheritance, or occasionally incorrectlyco-dominance in reptile genetics[13]) occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes. The phenotypic result often appears as a blended form of characteristics in the heterozygous state. For example, thesnapdragon flower color is homozygous for either red or white. When the red homozygous flower is paired with the white homozygous flower, the result yields a pink snapdragon flower. The pink snapdragon is the result of incomplete dominance. A similar type of incomplete dominance is found in thefour o'clock plant wherein pink color is produced when true-bred parents of white and red flowers are crossed. Inquantitative genetics, where phenotypes are measured and treated numerically, if a heterozygote's phenotype is exactly between (numerically) that of the two homozygotes, the phenotype is said to exhibitno dominance at all, i.e. dominance exists only when the heterozygote's phenotype measure lies closer to one homozygote than the other.
When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Pink:White).[14]
Co-dominance occurs when the contributions of both alleles are visible in the phenotype and neither allele masks another.
For example, in theABO blood group system, chemical modifications to aglycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other (IA,IB) and dominant over the recessivei at theABO locus. TheIA andIB alleles produce different modifications. The enzyme coded for byIA adds an N-acetylgalactosamine to a membrane-bound H antigen. TheIB enzyme adds a galactose. Thei allele produces no modification. Thus theIA andIB alleles are each dominant toi (IAIA andIAi individuals both have type A blood, andIBIB andIBi individuals both have type B blood), butIAIB individuals have both modifications on their blood cells and thus have type AB blood, so theIA andIB alleles are said to be co-dominant.[14]
Another example occurs at the locus for thebeta-globin component ofhemoglobin, where the three molecular phenotypes ofHbA/HbA,HbA/HbS, andHbS/HbS are all distinguishable byprotein electrophoresis. (The medical condition produced by the heterozygous genotype is calledsickle-cell trait and is a milder condition distinguishable fromsickle-cell anemia, thus the alleles showincomplete dominance concerning anemia, see above). For most gene loci at the molecular level, both alleles are expressed co-dominantly, because both aretranscribed intoRNA.[14]
Co-dominance, where allelic products co-exist in the phenotype, is different from incomplete dominance, where the quantitative interaction of allele products produces an intermediate phenotype. For example, in co-dominance, a red homozygous flower and a white homozygous flower will produce offspring that have red and white spots. When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Spotted:White). These ratios are the same as those for incomplete dominance. Again, this classical terminology is inappropriate – in reality, such cases should not be said to exhibit dominance at all.[14]
Dominance can be influenced by various genetic interactions and it is essential to evaluate them when determining phenotypic outcomes.Multiple alleles,epistasis andpleiotropic genes are some factors that might influence the phenotypic outcome.[15]
Although any individual of a diploid organism has at most two different alleles at a given locus, most genes exist in a large number of allelic versions in the population as a whole. This is calledpolymorphism, and is caused by mutations. Polymorphism can have an effect on the dominance relationship and phenotype, which is observed in theABO blood group system. The gene responsible for human blood type have three alleles; A, B, and O, and their interactions result in different blood types based on the level of dominance the alleles expresses towards each other.[15][16]
Pleiotropicgenes are genes where one single gene affects two or more characters (phenotype). This means that a gene can have a dominant effect on one trait, but a more recessive effect on another trait.[17]
Epistasis is interactions between multiple alleles at different loci. More specifically, epistasis is when one gene can mask the phenotype of a gene at a completely different locus.[18] Therefore, several genes can influence the phenotype expressed. Epistasis is slightly different from dominance in the fact that dominance is an allele-to-allele interaction at one locus while epistasis is a gene-to-gene interaction at different loci.[19] The dominance relationship between alleles involved in epistatic interactions can influence the observed phenotypic ratios in offspring.[20]
An example of epistasis can be seen in Labrador retriever coat colors. One gene at one locus codes for the color of hair but another gene at a different locus determines if the color is even deposited in the hair.[19][18] Recessive epistasis is seen in this example due to recessive alleles for color desposition masking both the dominant black (B) allele and recessive brown (b) allele at the first locus to express a yellow coat in the Labrador retriever.[19][18] The yellow color comes from no pigment being deposited in the hair shaft.[19]
Other examples of epistasis interactions are dominant epistasis and duplicate recessive epistasis.[19] Each type of epistasis is a modification of the dihyrbid ratio of 9:3:3:1.[18]
The term co-dominant is often used interchangeably with incomplete dominant, but the two terms have different meanings.