When different purebred breeds are mated together a phenomenon occurs called heterosis or hybrid vigor. Their offspring's outcomes are greater than their parts, meaning performance metrics are greater than the simple average of the parents metrics. Purebreds often share common ancestors, which increases the probability that they have more homozygous gene pairs for some traits. This can have a negative effect of reduction in some reproduction and fitness traits, which is called inbreeding depression. When different purebred breeds are mated together, the crossbred progeny are less inbred than their parents as a result of increased heterozygosity and a reduction in some of the effects of accumulated inbreeding. As a result of increased heterozygosity of some gene pairs, the calves perform at a level above the average of their parents in some traits, which is called heterosis or hybrid vigor and is the result of the recovery of accumulated inbreeding depression in pure breeds.
