U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


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Published in Beef Research Program Progress Report, No. 4, Part 2 (May 1993)


Two genetic mechanisms have been described as potential explanations for heterosis. The first mechanism is dominance. Dominance occurs when there are two differing forms of a gene (alleles) at a given position (locus) on a pair of chromosomes and where one of the pair of alleles masks or over powers the effect of the second. Having two different alleles at a locus is referred to as heterozygosity and the affected individual is heterozygous. Whether an individual has one or two copies of a dominant allele makes little difference in its superiority over others having two copies of the recessive allele. A higher degree of heterozygosity is expected when one population carrying the dominant allele in high frequency is crossed with a second population carrying the recessive allele in high frequency. Alternatively, heterosis may result from joint effects of genes at several loci. This alternative mechanism is called epistasis. Previous research documents reduced performance resulting from the mating of closely related individuals (inbreeding). Inbreeding generally reduces growth and reproductive rates and delays maturity. This inbreeding depression arises from increasing the frequency with which two alleles at a locus are identical (homozygous) and again coupled with dominant gene action. Thus, effects of inbreeding and heterosis are of similar size but opposite in direction, if dominance at individual loci causes both. In this study, we used inbreeding and linecrossing of Hereford cattle in an attempt to distinguish between these two explanations for heterosis influencing maternal traits. Answering this question sheds light on the amount of heterosis to be expected in composite breeding schemes.