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Dominance in genetics is a relationship between alleles of one gene, in which the effect on phenotype of one allele masks the contribution of a second allele at the same locus. The first allele is dominant and the second allele is recessive. For genes on an autosome (any chromosome other than a sex chromosome), the alleles and their associated traits are autosomal dominant or autosomal recessive. Dominance is a key concept in Mendelian inheritance and classical genetics. Often the dominant allele codes for a functional protein whereas the recessive allele does not. A classic example of dominance is the inheritance of seed shape, for example a pea shape in peas. Peas may be round, associated with allele ''R'' or wrinkled, associated with allele ''r''. In this case, three combinations of alleles (genotypes) are possible: ''RR'', ''Rr'', and ''rr''. The ''RR'' individuals have round peas and the ''rr'' individuals have wrinkled peas. In ''Rr'' individuals the ''R'' allele masks the presence of the ''r'' allele, so these individuals also have round peas. Thus, allele ''R'' is dominant to allele ''r'', and allele ''r'' is recessive to allele ''R''. This use of upper case letters for dominant alleles and lower case ones for recessive alleles is a widely followed convention. More generally, where a gene exists in two allelic versions (designated ''A'' and ''a''), three combinations of alleles are possible: ''AA'', ''Aa'', and ''aa''. If ''AA'' and ''aa'' individuals (homozygotes) show different forms of some trait (phenotypes), and ''Aa'' individuals (heterozygotes) show the same phenotype as ''AA'' individuals, then allele ''A'' is said to ''dominate'' or ''be dominant to'' or ''show dominance to'' allele ''a'', and ''a'' is said to ''be recessive to'' ''A''. Dominance is not inherent to an allele. It is a relationship between alleles; one allele can be dominant over a second allele, recessive to a third allele, and codominant to a fourth. Also, an allele may be dominant for a particular aspect of phenotype but not for other aspects influenced by the same gene. Dominance differs from epistasis, a relationship in which an allele of one gene affects the expression of another allele at a different gene. ==Background== The concept of dominance was introduced by Gregor Mendel. Though Mendel, "The Father of Genetics", first used the term in the 1860s, 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, or round, or red, or tall). However, when these hybrid 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 (A), 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dominance (genetics)」の詳細全文を読む スポンサード リンク
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