The diploid chromosome number in the goat is 60. A gene is the unit of heredity occupying a particular position (locus) on a chromosome. Alleles are different forms of a gene occupying a given locus. When alleles are the same, they are homozygous, and if different, they are heterozygous for the trait. There can be several alleles for any locus, but only two alleles can be present in any one individual (zygote). Most characters of economic importance in goats (growth rate, milk production, prolificacy, fertility) are influenced by large numbers of genes (quantitative) instead of single genes (qualitative). These characters are measurable and vary among breeds and among individuals within breeds. For such traits it is appropriate to consider the sum of the average effect of all genes (additive genetic effect) influencing the trait.

A gene does not have the same effect (value) in all individuals in which it occurs. The effect of a particular allele may differ according to the identity of the other allele at the same locus (known as dominance) and also depends on which alleles are found at other loci (epistasis). Dominance and epistasis are nonlinear genetic effects that mask the expression of the additive genetic variation. The variation within a herd can be partitioned into environmental and additive genetic variance. An animal's breeding value is its additive genetic value, and its variance constitutes the additive genetic variance. The heritability (h2) of a trait is that proportion of the total variance that is due to additive genetic variance, and is used to predict the probable response to selection for quantitative characters.[1]

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