This review concerns genetic aspects of the more common (dizygotic) form of twinning. Breed differences in twin calving rate are discussed, although apparent differences often are -confounded with country, dairy or beef systems (milked vs suckled) and management practice. A possible positive relationship between ‘postpartum interval to conception and twin pregnancy rate also is noted. In spite of low estimates of repeatability and heritability for twin calving rate, four screening and selection experiments have been established. Foundation cows provided subsequent twin calving rates of .14 and daughters' performance is still under study. The possibility of a major gene segregating in some populations cannot be ignored. The potential for increasing twin calving rates through genetic means is discussed by considering variation in component traits. Ovulation rate differences have been found. The contrast between uterine capacity and embryo survival is drawn; genetic and phenotypic variation havebeen demonstrated in other species. Other possible sources of variation, are the levels of steroid hormones and sensitivity to controlling hormones, e.g. greater FSH effects on ovulation rate in twin strain than control cows. Variation in other factors could be worth studying, e.g. GnRH at day 11 to 13 postmating, which is known to increase conception rate. Possible indicator traits in either males or females also are discussed.

C. A Morris, A. M Day

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XI. Genetics of reproduction, lactation, growth, adaptation, disease, and parasite resistance., , 14–29, 1986
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