Controlled heterozygosity in livestock

Abstract

The word heterozygosity appears in the title of this paper in preference to heterosis for three reasons. First, heterosis has different usages and might be misleading. A simple illustration is provided by the colours of Shorthorn cattle which are determined by a pair of alleles. The heterozygotes, which are roan, may fairly be described as intermediate between the red and the white of the two homozygotes, and thus show no dominance and therefore no heterosis. Since, however, breeders prefer roans, they are maintained in breeding herds at a frequency greater than 0·5 (Rendel 1952).* On Dobzhansky’s (1952) definition of heterosis, this would be an example of single-locus heterosis and over-dominance. A similar situation obtains in respect of the white saddle of Wessex Saddleback pigs (Donald 1951). for neither the cattle nor the pigs is there evidence that breeders’ preferences for heterozygotes lead to higher production of meat or milk. Second, too little is known about metrical bias and scaling (Mather 1946) which are basic to a proper onception of the dominance relations involved; and third, heterosis in any current sense is not sufficiently comprehensive to describe the uses which controlled heterozygosity has or might have in livestock breeding. One simple and common method of increasing heterozygosity is to mate animals rawn from two distinct breeds. As may be judged from their appearance and their ross-bred progeny, many of these breeds differ substantially in genotype. The ncrease in heterozygosity consequent on crossing them has the assured merit of Adapting the progeny to market demands and the probable merit of improving Performance through heterosis.