Therapeutic Mating: A Case Study

McDaniel (43) postulated that a market signal indicate the change in characteristics of the composite breed. Although, the initial selection of a breed depends on the desirable trait for a particular environment and target market. The percentage of heterosis increases as more breeds contribute in the initial mating program (17). While the heterosis will not be as high as that achieved with a rotational crossbreeding program with the same number of breeds, the management requirements will be reduced. For instance, if there are three breeds in a given composite, the amount of retained heterosis would be expected to be (3-1) ÷ 3 or two-thirds or 67%. Inbreeding is not usually significant when numbers are greater than 200 or 300 breeders. In the
Those individual giving the best results in crosses from the two strains are inter- se mated to continue their respective strains. The crosses are made reciprocally like M1×F2 & M2×F1 & the parents are evaluated on the basis of cross progeny performance. The best performer male and female lines are selected. The selected lines are mated to member of their own line to produce the next generation of individuals which are to be tested by mating them with individuals of the second line produced in the same way. Thus RRS is the kind of progeny selection. This cycle is repeated over & over. The initial difference of gene frequency between the two lines is a pre-requisite of this
Studies from cattle tend to suggest that there is a positive relationship between the genetic distance between the lines, as estimated from AFLP markers, and the amount of heterosis in the crossbred progeny (2). Genetic distance between two populations is a measure of the number of generation they have diverged from a presumed common ancestor, and is usually calculated from differences in allele frequencies between the populations at a number of loci (8). Although the correlation between genetic distance and heterosis is not always large, this method might target specific line combinations which are more likely to produce fit, high performing progeny (2). If this technique of combining lines for crossbreeding works, it should also be possible to use the same principles for mate selection. To achieve a better commercial animal, the exploitation of non-additive variation in purebred lines should aim to produce the favourable allelic combinations in the crossbred generation. Selection methods such as Recurrent Reciprocal Selection (RRS) may be implemented to select the purebred lines at the nucleus level, with or without the use of molecular markers (23). RSS is a traditional selection method in which purebred parents are selected for purebred breeding on the basis of their predicted crossbred performance. RRS is theoretically efficient in the presence of over dominance,