Association of genetic defects with yield and type traits: the weaver locus effect on yield.

The association of recessive genetic disorders with yield and type traits was investigated. The frequency of a defective gene could be increased by selection if it is positively associated with selected traits, despite efforts to reduce it. Genetic defects considered were weaver in Brown Swiss and rectovaginal constriction and limber leg in Jerseys. Data sets for linkage analysis consisted of 245 sons of 9 carrier sires, 1036 sons of 16 carrier sires, and 557 sons of 10 carrier sires, respectively. Weaver carrier sons had higher producing daughters than noncarrier sons within all 9 sire families. Weaver carrier cows have an advantage of 673.6 kg milk and 26.0 kg fat and a disadvantage in rear legs score, indicating that the condition may not be completely recessive. Carriers of the other defect genes have no advantage for milk production, are scored lower for pelvic angle, and limber leg carriers have more desirable udders. Estimates of defect gene frequencies in 264,000 Jersey cows show a decrease over time for rectovaginal constriction and limber leg; in 97,723 Brown Swiss cows, frequency of the weaver gene increased over time. Gene frequencies in daughters of the youngest sires were 5.48, 2.13, and 8.89%, respectively. Consistently higher yield evaluations of weaver carrier sons within each sire family, large advantage in production of weaver carrier cows, and increasing gene frequency over time indicate that a chromosome segment with major effect on yield is tightly linked to weaver in Brown Swiss.