11.3 The genetic background of heterosis

To start with a simplified example: suppose (not valid in the real world) that in chicken a single gene with two different alleles A and a determines the amount of eggs produced in a year.

Breed 1 is homozygous (fixed) for A: all animals have the genotype AA.

Breed 2 is homozygous (fixed) for a: all animals have the genotype aa.

Breed 1 is producing 96 eggs per year and breed 2 94 eggs per year. Cocks of breed 1 are mated to hens of breed 2.

Their offspring with genotype Aa is expected to produce 95 eggs per year, the average of the two parent breeds, but they do produce 100 eggs per year. This is the effect of heterosis: the performance of a crossbred (Aa) is better than the average performance of the two parental breeds (AA and aa). The heterosis is 5 eggs per year of expressed as a percentage 5/95 = 5.2 %. Heterosis is based on the phenomenon of dominance: the genotype Aa has a value that is higher than the average of the genotype AA and aa.

Definition

Dominance is when the alleles of a locus are non-additive. When a locus shows dominance, the genotypic value of the heterozygote on a trait is not the average of the two homozygotes. An extreme type is overdominance. It occurs when the heterozygote has a genotypic value more extreme than either parent

Below you will find another example as an illustration of heterosis due to a dominance effect on a single locus. The genotypic value of the homozygote BB = 125. The genotypic value of the homozygote bb = 115. The heterozygote Bb has a genotypic value of 122. The additive effect of B over b = 125-115/2 = 5. The dominance effect of Bb is 122-120 (120 = the average value of BB and bb) = 2.