After reading this article you will learn about the genetic components of variation from a diallel analysis.
The methods of estimating genetic components of variation and their proportions were given by B.I. Hayman in 1954.
These components and proportions are as follows:
D = Additive variation.
F = Measures the relative frequency of dominant and recessive alleles in the parents. This is positive, if there is an excess of dominant alleles and negative with an excess of recessive alleles.
H1 = Dominance variation.
H2 = Dominance indicating asymmetry of positive and negative effects of genes, H1 [1 – (u – v)2].
h2 = Dominance effect – (as algebraic sum over all loci in heterozygous phase in all crosses).
(H1/D)1/2 = This is the weighted measure of the average degree of dominance at each locus. It is equal to 1 in case of full dominance, more than one in case of over dominance and less than 1 in case of partial dominance.
H2/4H2 = Provides an estimate of the value of uv, where u is the proportion of positive alleles and v the proportion of negative alleles, and u + v = 1. The maximum value of H2/4 H1 will be 1/4 when u = v = 1/2. If the value is close to 1/4, there is symmetry at loci exhibiting dominance otherwise genes are considered to be asymmetrically distributed.
(4DH1)1/2 + F/(4DH)1/2 – F or KD/KR = This is the ratio of total number of dominant to recessive genes in all parents. If it is near unity, it implies equality between the number of dominant and recessive alleles in the parents. This is, of course, a necessary consequence of u = v = 1/2
h2/H2 or K = Number of effective factors which control the character and exhibit dominance.
The proportions are evaluated and interpreted, when the relevant components are significant.