In a flock of highly prolific Romney ewes obtained from industry flocks, one ewe (A281), with a production record of 33 lambs born in 11 lambings, produced a number of female descendants with high ovulation rates. The mode of inheritance of this trait was determined in a series of four progeny tests of male descendants of this ewe. The first progeny test produced strong evidence for a new major gene affecting ovulation rate in this family line; this finding was supported by two subsequent progeny tests. The fourth progeny test was designed to test the hypothesis that this gene is carried on the X chromosome. The results showed that six sons of a carrier ram did not inherit the gene, but it was passed on to three of his five maternal grandsons. This finding, together with evidence of genetic segregation in the progeny of carrier females, demonstrates for the first time the presence of a major gene for prolificacy specifically located on the X chromosome. The effect of the gene is to increase ovulation rate by about one additional egg per ewe.
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