Segregation of a major gene influencing fecundity in progeny of Booroola sheep

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Abstract
Ovulation rate records from 283 Booroola × Merino (F1), 176 Booroola parent backcross (¾ Booroola), and 152 Merino parent backcross (¼ Booroola) ewes have been analysed. The results support the hypothesis that the exceptional fecundity of the Booroola arises because, at one of the relevant loci, the majority of Booroolas carry a gene(s) with a major effect on ovulation rate. Carriers of the putative gene have at least one observation where their ovulation rate is ≥ 3. The overall mean ovulation rate of F I carriers with from 3 to 6 records was 1.24 higher than non-carriers (2.78 v. 1.54). The mean ovulation rate of non-carriers was only 0.15 higher than control Merinos (1.39). A further segregation criterion of at least one observation where a ewe's ovulation rate is ≥ 5 is postulated to distinguish between heterozygous and homozygous carriers of the gene. The effect of the gene appears to be additive for ovulation rate and partially dominant for litter size when segregation criteria of at least one record of ovulation rate ≥ 3 and ≥ 5 are applied to identify heterozygous and homozygous carriers in ¾ Booroola ewes. The suggested segregation criteria are known to be in error in a small proportion of Merino-cross ewes. They may require further modification where Booroolas are crossed with flocks with ovulation rates differing from those of the Merinos used in this study.

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