calf weight as a result of increased reproductive performance and maternal ability. Crossbred cow longevity is also increased by up to two years when compared with straightbred cows. However, the benefits of hybrid vigour in a crossbred cow decline in subsequent generations if progeny are mated to cattle of parentage similar to the cow, and increase if a new breed is introduced. Although studies acknowledge that the major production benefits of hybrid vigour occur in crossbred cow herds, the main use of Limousins outside of Europe continues to be as terminal sires in purebred British breed cow herds. Genetic basis for crossbreeding Progeny of two parents of different breeds are termed F1 hybrids, F1 crosses or first crosses. F1 hybrids generally have an extremely uniform phenotype and benefit from hybrid vigour. These advantages are observed in the breeding of a wide variety of animals and plants, and arise because progeny inherit one of each paired gene from each parent. When both parents are homozygous for different variants of genes (known as alleles), which is likely to be the case when a breed has been developed and selected over several generations, progeny will inherit both gene variants present in the parents. The F1 hybrid progeny will then be heterozygous for each gene variant, which in turn increases the likelihood that the genes will code for an optimal protein or enzyme. This is the genetic basis of hybrid vigour. While many gene variants have effects that are of little consequence to beef production, a few, such as the myostatin variants found in different cattle breeds, have a major effect. Loss of hybrid vigour occurs and phenotype varies greatly in subsequent generations if F1 hybrids are interbred or backcrossed with animals genetically similar to the F1 parent. Interbred F1 hybrids produce progeny that can be either heterozygous for each gene variant, homozygous for one gene variant, or homozygous for the other gene variant. When one of the variants has a large effect on a trait, for example the effect of myostatin variants on muscularity, larger phenotypic variation will occur among the progeny. Backcross progeny have less phenotypic variation and comprise animals that are either heterozygous for each gene variant or homozygous for the variant found in the original F1 backcross parent. A third form of progeny arises when F1 hybrids are bred with animals genetically dissimilar to their parents. If heterozygosity is maintained