Principles of Genetics and Breeding

11.1.14 Breeding Value

In selecting for polygenic traits, breeders try to choose as parents those individuals with the best sets of genes - those individuals with the best breeding values.

• Breeding value is defined as parental value- the value of an individual as a contributor of genes to the next generation.
• Genotypic value represents the overall effect of an individual’s genes, breeding value represents only that part of genotypic value that can be transmitted from parent to offspring.
• Breeding value can be computed in two ways, i.e. in terms of average effect of genes and the mean value of the progeny.
• The breeding value of the three genotypes in terms of the average effect are: A₁A₁=2p α, A₁A₂ = (q-p) α and A₂A₂ =-2p α.
• The breeding value calculated in this way is more hypothetical, because it is not possible to calculate the average effect of a gene in quantitative traits thus is polygenic in nature.
• The effect of individual gene is not recognizable in quantitative traits.
• Breeding value calculated based on progeny mean is practical in nature and can be measured.
• If an individual is mated to a number of individuals taken at random from the population, the breeding value is twice the mean deviation of its progeny from the population mean.
• The breeding value is a property of the individual as well as the population from which its mates are drawn.
• The breeding value A = 2 (P-H),

where P is the progeny average of the individual and

H is the population average.

‘Population’ is defined as an intraspecific (within a species) group of randomly mating individuals which exists (and can therefore be sampled) in a defined geographic position or at a defined time.

A key point to note here is that (assumed) all individuals comprising a population are able to mate randomly with one another and therefore there can be no further genetic subdivision within a population.

To summarize,

• genotypic value represents the overall effect of an individual’s genes (singly and in combination) on that individual’s own performance for a trait.
• Not all of genotypic value is heritable, however, breeding value is the part of an individual’s genotypic value that is due to independent gene effects that can be transmitted from parent to offspring.
• It can also be understood as the genotypic vale as the value of an individual’s genes to its own performance,and breeding value as the value of an individual’s genes to its progeny’s performance.
• An offspring’s breeding value for a trait will be, on average, the average of its parent’s breeding values for the trait.