Quantitative Variation Quantitative (metric or polygenic) characters of Productive traits Examples : milk production, egg production, meat production, body weight etc

Quantitative characters typically have continuous, approximately normal distributions and include characters such as reproductive fitness, longevity, height, weight, disease resistance, etc.

A major challenge in the study of quantitative genetics is to determine how much of the observed variation is due to genetics and how much is due to environment. One of the central concepts of quantitative genetics is heritability. Heritability is the proportion of the total phenotypic variance in a population due to genetic differences among individuals.

Algebraically, we can define the phenotypic value Of an individual as the consequence of the alleles It inherits together with environmental influences As: P = G + E Where P = phenotype, G = Genotype, and E = Environment.

The genetic component can be partitioned from the environmental component as: VP = VG + VE + 2CovGE Where, CovGE is the covariance between genetic and environmental effects. The covariance for this component is expected to be 0 if conditions for different genotypes are equalized by randomly allocating individuals across the range of environment, which is difficult to achieve in wild populations.

For example, in territorial species of birds and mammals, the genetically fittest parents may obtain the best territories. Offspring inheriting the best fitness genotypes also inherit the best environments. This results in a genotype X environment correlation that increases phenotypic resemblance among relatives.

Differences in performance of genotypes in different environments is referred to as Genotype X Environment Interactions. These develop when populations adapt to particular environmental conditions, and survive and reproduce better in their native conditions than in other environments. Genotype X Environment Interactions are of major significance to the genetic management of endangered species as follows:

Quantitative genetic variation has contributions from the average effects of loci VA, from their dominance deviations VD, and from interactions (epistatic) deviations among gene loci VI as: VG = VA + VD + VI These are referred to as additive genetic variance (VA), dominance variance (VD), and interaction variance (VI). Each of these has major conservation implications as follows:

VA and especially the ratio VA/VP (heritability) reflect the adaptive evolutionary potential of the population for the character under study. VD reflects susceptibility to inbreeding depression. VI influences the effects of outbreeding, whether beneficial or deleterious.

Heritabilities range from 0 to 1. Heritabilities of 0 are found in highly inbred populations with no genetic variation. Heritabilities of 1 are expected for characters with no environmental variance in an outbred population if all genetic variance is additive. Heritabilities are specific to particular populations living under specific environmental conditions.

Heritability and VA are fundamentally measures of how well quantitative traits are transmitted from one generation to the next. Unfortunately, very few heritability estimates exist for endangered species and there clearly is need for many more estimates of heritability in threatened and endangered species.

THE USE OF HERITABILITY : SELECTION BASIS FOR ALL ORGANISM THE PRINCIPLE TO DO ANIMA;\L BREEDING PROGRAM THE BASIC EQUATION : - BREEDING VALUE _ _ BV = h2 ( Y – H)