Modelling breeding strategies against genetic defects in a vulnerable cattle breed

This paper investigates different selection scenarios to reduce the prevalence of a genetic defect in a vulnerable breed, using simulations of selection against gonadal hypoplasia in Swedish Mountain Cattle as a case study. Gonadal hypoplasia (reduced size of ovaries/testes) that leads to reduced fertility has long been a problem in Swedish Mountain cattle (Fjällko). Despite breeding efforts that have reduced the frequency of the defect, it is still present in the breed. There are two translocations involving chromosomes 6 and 29 associated with desired colour phenotypes that segregates in several Swedish local cattle breeds. One of these is associated with gonadal hypoplasia although there is incomplete penetrance. With modern DNA methods it is possible to detect the carriers of this defect and tackle this old problem, but the key issue is to do this without harming genetic diversity. We performed simulations with allele frequencies and penetrance that have been estimated previously in the breed and different scenarios of marker-assisted selection. The simulated scenarios were: genetic testing of potential bulls prior to selection to exclude carriers or homozygotes for the deleterious allele; genetic testing of bulls used for artificial insemination to exclude carriers or homozygotes for the deleterious allele; and genetic testing of bulls used for artificial insemination but where 1 or 5 carrier sires were allowed to be used. Simulations showed that it is possible to reduce the frequency of the allele causing the genetic defect while maintaining the effective population size. This however comes with a cost of requiring many genetic tests. Based on these results, the most promising approach would be to pre-screen bull calves before selection of sires, maintaining the number of bulls that are used. These results suggest that efforts to genotype and phenotype cattle for validation and testing of the marker in Swedish Mountain cattle is warranted.