Published online by Cambridge University Press: 27 February 2018
Molecular markers can be used to explore the diversity present in livestock populations. In cattle the diversity among breeds, revealed using molecular markers, is greater than the within breed diversity. Therefore both at the phenotypic and genetic level breeds form discrete populations, which could be used to conserve maximum diversity in the species. The best way to conserve breeds is to ensure their commercial utility; therefore selection of breeds for commercially advantageous phenotypes should be encouraged. Gene mapping studies suggest that, even for complex traits, there may be very few genes involved in controlling variation in the phenotype. Therefore selection for a particular trait does not necessarily affect the genetic base of the population, other than at the genes under selection. This seems to be the situation in the Hereford breed, where the phenotype has changed considerably over the past 50 years, while blood group data suggests that the genetic base of the population has not been greatly affected. Genome mapping approaches allow first the chromosomal location and ultimately the genes controlling traits to be identified. This information provides DNA markers for favourable alleles at trait genes that can be used in selective breeding programmes to improve breeds for a range of traits. Work on double muscling in Belgian Blue cattle has shown that a single gene, myostatin, can be responsible for an extreme phenotype, so selection for double muscling potentially only affects diversity around this gene. However, the examination of the phenotypes associated with this gene in other breeds suggests that genes in addition to myostatin are involved in the development of the extreme phenotype. Thus information on single genes is too narrow to be of value in making conservation decisions. With the current state of knowledge genetic information can aid the choice of individuals to use for breeding and for conservation of diversity, but the information should be used with caution and in conjunction with additional information, such as pedigree, phenotype or function data.