AGB 211: Livestock and Poultry Breeding (2+1)

Combining ability

COMBINIG ABILITY

• At the present state of knowledge, performance of two or more breeds or lines in crosses is somewhat unpredictable. Some lines or breeds appear to "combine well" whereas others do not. This can be determined only by test crosses. There are two types of combining abilities viz., general combining ability (GCA) and specific combining ability (SCA). GCA is the mean performance of the F₁ offspring of a line with other lines and it is due to additive genetic variance. SCA is the superiority of a particular cross over the average GCA of the two lines and it is due to non-additive genetic variance. GCA and SCA are expressed as variance and not as values.
• To estimate the combining ability of two or more lines, “diallel mating system” is followed. In this system of crossing, all possible combinations of the lines are produced. This mating scheme allows estimating the performance of the individual combinations. The diagram below explains the diallel mating system and the combining abilities of four lines, x₁, x₂, x₃ and x₄.

Line	x1	x2	x3	x4	GCA
x1	x1x1	x1x2	x1x3	x1x4	x1
x2	x2x1	x2x2	x2x3	x2x4	x2
x3	x3x1	x3x2	x3x3	x3x4	x3
x4	x4x1	x4x2	x4x3	x4x4	x4

SCA: The diagonals elements

In symbols, the performance of a combination of lines is composed as follows:

G_(x1x2) = GCA_(x1) + GCA_(x2) + SCA_(x1x2)

where,

G_(x1x2) denotes the genotypic value of the cross “x₁x₂”.