He did introduce the notation of capital and lowercase letters for dominant and recessive alleles, respectively, still in use today. Mendel did not use the terms gene, allele, phenotype, genotype, homozygote, and heterozygote, all of which were introduced later. 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 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. 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. 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). When bred separately, the plants always produced the same phenotypes, generation after generation. 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. Though Mendel, 'The Father of Genetics', first used the term in the 1860s, it was not widely known until the early twentieth century. The concept of dominance was introduced by Gregor Johann Mendel. Demonstrating the heights of plants from the two parent variations and their F1 heterozygous hybrid (centre) Thus, allele R is dominant over allele r, and allele r is recessive to allele R.ĭominance differs from epistasis, the phenomenon of an allele of one gene masking the effect of alleles of a different gene. In Rr ( heterozygous) individuals, the R allele masks the presence of the r allele, so these individuals also have round peas. The RR ( homozygous) individuals have round peas, and the rr (homozygous) individuals have wrinkled peas. In this case, three combinations of alleles (genotypes) are possible: RR, Rr, and rr.

Peas may be round, associated with allele R, or wrinkled, associated with allele r. A classic example of dominance is the inheritance of seed shape in peas. Letters and Punnett squares are used to demonstrate the principles of dominance in teaching, and the use of upper case letters for dominant alleles and lower case letters for recessive alleles is a widely followed convention. Additionally, one allele may be dominant for one trait but not others.ĭominance is a key concept in Mendelian inheritance and classical genetics. 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 and co-dominant with a fourth. Additionally, there are other forms of dominance such as incomplete dominance, in which a gene variant has a partial effect compared to when it is present on both chromosomes, and co-dominance, in which different variants on each chromosome both show their associated traits.ĭominance is not inherent to an allele or its traits ( phenotype). Since there is only one copy of the Y chromosome, Y-linked traits cannot be dominant nor recessive. The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes ( autosomes) and their associated traits, while those on sex chromosomes (allosomes) are termed X-linked dominant, X-linked recessive or Y-linked these have an inheritance and presentation pattern that depends on the sex of both the parent and the child (see Sex linkage). This state of having two different variants of the same gene on each chromosome is originally caused by a mutation in one of the genes, either new ( de novo) or inherited. The first variant is termed dominant and the second recessive. In genetics, dominance is the phenomenon of one variant ( allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. An autosome is any chromosome other than a sex chromosome. Autosomal dominant and autosomal recessive inheritance, the two most common Mendelian inheritance patterns.

Meaning of dominant traits