The exploitation of heterosis in animal production has tended to be an empirical procedure. It was recognised that heterosis tended to be most pronounced for traits of low heritability (Sang 1956) and that its level of expression tended to increase as the degree of relationship between the parental lines decreased. It was further recognised that commercially important heterosis was most likely to be found in crosses between populations that had already been selected in the desired direction (Bowman 1959). As heterosis is normally measured relative to the mid-parent mean (Shull 1914 as cited by Shull 1948) the economic value of heterosis present in crosses between improved lines will be enhanced by this higher mid-parent value. Although heterosis is usually explained in terms of dominant, overdominant and/or epistatic gene action (Bowman 19 59) , it has generally tended to be assumed that epistasis is of minor importance as a source of heterosis in crosses between breeds of domestic animals (Falconer 1960) . In the absence of both epistasis and genotype by environment interactions (Cunningham 1982), the amount of heterosis expected to be found in a particular crossbred population relative to that present in the FI population(s) from which it is descended can be conveniently predicted from the comparison of their proportional heterozygosities. However, results from animal crossbreeding experiments (Sheridan 1981, Hickman 1982) indicate that these assumptions do not necessarily hold true and that lower than expected levels of residual heterosis can occur in secondary crossbred populations,
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 6. Round tables, , 185-189, 1982
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