It is now known that there are various types of dominance. The most popular in Mendelian genetics is complete dominance, but others also exist. Each of these determines the phenotypic manifestation of a single gene pair in the absence of epistasis or interallelic gene interaction.
These dominance relations involve intraallelic rather than “interallelic” gene interaction. The Latin prefix “intra” means within or inside; “inter” means between or among (Borror 1988).
Intraallelic interaction refers to the interaction of alternative alleles within the same gene pair (located in the same locus in the chromosome) as if other gene pairs are absent. As to the genotype TtGg, this means that the interaction involves that of T with t and of G with g, but not between either T or t with either G or g, or between separate allelic combinations.
Intraallelic interaction, therefore, occurs in the monohybrid which has a single gene pair or is presumed in a multi-hybrid in which gene pairs are treated separately with the assumption that interloci gene to gene interaction does not exist even if it does. This is consistent with Mendel’s Law of Independent Assortment which provides that each gene pair acts independently of other gene pairs.
Distinguishing Various Types of Dominance
Aside from complete dominance which Mendel (1865) discovered, other types of dominance relationships are incomplete dominance, overdominance, and codominance. These are distinguished here below and in the component pages.
Consider only the heterozygote (with heterozygous genotype) for the length of stem or height and, separately, for pod color in garden pea. Assuming that we are still ignorant as to the dominance relation governing these characters, let us denote by the symbol s1 the allele for the tall stem, s2 for the short stem, p1 for the green pod, and p2 for the yellow pod. Therefore, our heterozygotes have the heterozygous genotype s1s2 for stem length and p1p2 for pod color.
In Mendel’s study, s1s2 is the hybrid (F1) of the cross between pure-breeding parents which possess homozygous genotypes (s1s1 x s2s2) and p1p2 is from p1p1 x p2p2. Consequently, selfing of the F1 will yield F2 progenies consisting of the genotypes s1s1, s1s2, and s2s2 for stem length and p1p1, p1p2, and p2p2 for pod color both in the proportion of 1:2:1.
For the succeeding comparison of types of dominance, let us presume also that there is no epistasis, that gene pair consists of two alleles, and that expressivity is full.
1. Complete Dominance. This is a type of dominance in which the heterozygote exhibits the character carried by one of the two alleles making up the heterozygous genotype. In this case, one allele expresses itself, completely masking (or hides or conceals) the phenotypic expression of a contrasting allele in the heterozygote.
With reference to the parents of a cross that possesses homozygous genotypes (s1s1 and s2s2; p1p1 and p2p2), it is the dominance relation in which the heterozygote (s1s2 and p1p2) exhibits one of the two parental characters. Among the types of dominance here compared, it is only complete dominance that does not produce a new, non-parental phenotype.
Here are hypothetical heterozygote s1s2 will be either tall or short and p1p2 will be either green or yellow pod, depending on which is dominant. However, it is already well established that the tall character is dominant over short and the green pod is dominant over yellow, hence the genotypes are commonly denoted by the letter symbols Tt and Gg. The capital letters (T and G) denote the dominant characters (tall and green, respectively) while the small letters (t and g) denote the recessive characters (short and yellow, respectively).
As to the F2 progeny, there are two phenotypes, that is, tall and short for stem length, and green and yellow for pod color. In both characters the F2 pnenotypic ratio will be 3:1 as shown below:
• F1 genotype: s1s2 or Tt; p1p2 or Gg
• F1 phenotype: tall; green
• F2 genotypes and ratio: 1:2:1 for both stem length and pod color (1s1s1:2s1s2:1s2s2 or 1TT:2Tt:1tt; 1p1p1:2p1p2:1p2p2 or 1GG:2Gg:1gg)
• F2 phenotypes and ratio: 3:1 tall and short (3T_ [1TT + 2Tt = 3 tall]: 1tt [short]); 3:1 green and yellow pod (3G_ [1GG + 2Gg = 3 green]: 1gg [yellow])