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In these crossbreeding systems heterosis percentages vary as can be seen in the table below (the crossbreeding systems will be outlined in the subchapters):
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In the pure-breed cross the heterosis in the F1 is 100 %. In the F1 population the difference in allele frequencies between the F1 and one of the parent breeds is half the difference between the two parent breeds. This fact makes that e.g. when the F1 is mated with one of the parent breeds in a backcross the heterosis in the F2 is 50 % compared to the original two way cross. The % of heterosis depends of the difference in allele frequencies between the dam and the sire population. As stated before, the heterosis is most pronounced and most valuable for the improvement of traits with a low heritability like health and fertility traits.
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