Training Manual Application of Genetics and Biotechnology in ...

of genetic differences among the individuals in a population, it has got very little
predictive power. It cannot be used to predict the values of a quantitative trait in the
offspring of specific matings, nor can it be used to study the nature of the genes
affecting the trait. For theSe purposes, we need a more refined analysis.
In order to predict an offspring's phenotype, it is necessary to subdivide the
genotypic variance in the model : Vp = Vg + Ve. The variance due to genotype (Vg)
encompasses all the genetic factors affecting a trait including interaction among
genes. Such interactions have little or no predictive value because Mendelian
segregation breaks up gene combinations. Consequently, the methods for predicting
phenotypes depend primarily on the effects of individual alleles.
Let us represent the cumulative sum of the effects by A (for allelic effects)
and the sum of the interactions by the letter I.
ThusG=A+IandP=A+I+E
In this model, the phenotype of an individual is determined by three separate
components. In a population, this model implies that the phenotypic variance will
also be determined by three components, provided that the terms A, I and E are
unconelated. If this is assumed, then:
oZp = o2 A + o2 I + o2 E
where o2 A is called the additive genetic variance because it accounts for the
variability caused by the additive effects of alleles. o2 I is the interaction genetic
variance, a term that measures the impact of genetic combinations. Sometimes this
term is subdivided into components for dominance and epistasis. Dominance
involves combination of alleles at the same locus and epistasis involves combinations
at different loci. Since neither o2 I nor part of o2 I has much predictive power,
quantitative geneticists tend to focus their attention on additive genetic variance. This
leads us to the narrow sense heritability which is defined as the ratio o2 A/ oZP i.e. the
proportion of the total phenotypic variance that is due to the additive effect of alleles.
Quantitative geneticists estimate heritability from correlation between relatives.
This method involves computing the correlation coefficient and then dividing it by
the fraction of genes that the relatives share by virtue of their common ancestory.
Half-sibs reared in different environments are useful as they share one-fourth of their
genes but none of their environmental effects.