Principles of Genetics and Breeding

13.1.12.4. Diallel Mating

Diallel mating is a variation of the factorial mating scheme and is most useful when developing captive broodstock from an imperiled donor population whose numbers have dropped drastically low. In that case, all fish are used to make all possible combinations of matings to maximize genotypic combinations according to a matrix format.

Each male is bred with each female. Although not increasing N_e above that obtained in simple factorial matings, it does maximize genotypic diversity in a population that has lost some allelic diversity and likely to lose more shortly.

This has the effect of producing new genotypes, some of which may confer high fitness in the progeny. Factorial mating schemes, and in extreme cases, diallel matings, are recommended for breeding some endangered species and are currently being employed in various captive propagation fishes.

The single matings may be between two pure breeding lines, P₁ and P₂.

The multiple mating programmes includes the diallel analysis (full and partial). In a full diallel, all parents are crossed to make hybrids in all possible combinations. The set of crosses so produced is called the diallel cross and its analysis is called diallel analysis.

The diallel may be complete (full diallel) or incomplete (partial diallel). Diallel involves the data from all the lines crossed together and their F₁ hybrids including the reciprocals. This is complete or full diallel.

The diallel analysis helps to estimate the genetic parameters and the combining ability viz., general combining ability (GCA) of the parents (parental lines) and the specific combing ability (SCA) of crosses.

The diallel analysis has been proposed by Hayman (1954) and Griffing (1956).

Hayman pointed out that diallel analysis is based on certain assumptions, viz., random mating, normal segregation, heterozygosity of parents, lack of maternal effects, no linkage among genes, no epistasis and no multiple allelism.

Diallel crossing in Rohu (6x6 diallel)

Females

	Fa.S	Ga.S	Go.S	Y.S	Su.S	Br.S
Fa.S	O	X	X	X	X	X
Ga.S	X	O	X	X	X	X
Go.S	X	X	O	X	X	X
Y.S	X	X	X	O	X	X
Su.S	X	X	X	X	O	X
Br.S	X	X	X	X	X	O

5 Riverine stock O- Crosses within stock

1 Farm stock X- Crosses between the stock

A complete diallel cross involving four strains of red tilapia (Oreochromis spp.) from Malaysia, Stirling, Taiwan and Thailand was performed with the aim to establish a foundation population for genetic improvement.

The mating involved 16 parental breeders per sex per strain, producing 64 full-sib families in total.

Malaysian strain exhibited the highest additive genetic performance for harvest weight.