Further breeding experiments with New Zealand Romney N-type sheep to 1956

Abstract

Another NS-grade sheep, a horned ewe from an ordinary Romney flock, is judged on progeny to have been N/+. Further breeding results are consistent with the simple genetic basis of horns in N/+ ewes previously postulated, namely, a fortifier gene F. Additional evidence is given for a genetic basis of the comparative scarcity of halo-hairs on the main area of the body of low-N N/+ lambs. It is a reasonable guess that selection would fairly quickly build up a stock in which the gene N could be called recessive, in that its expression in heterozygotes fell short of VI. In work with low-N sheep two horned rams, twin sons of a low-N ram with very large horns, have proved not to possess N. Horns in these rams clearly have a simple genetic basis. One N-grade ram has been shown to be +/nr, and it is likely that his twin brother, which closely resembled him, was also +/nr. The N-type characterization of these twin rams was feeble in several respects. There is evidence that the nr gene is helped to N-grade expression by multiple factors which are thus dominigenes. This is a third possible way, in addition to picking up N from a low-N or nr from a carrier, in which an nr/nr sheep mated with non-N may have an N-grade lamb. The original hypothesis of a main dominigene remains unproved. Better evidence is given that the nr gene helps horns to grow in N/+ . +/nr ewes. After repeating the back-cross, N/ +. +/nr x + / + .nr/nr, the conclusion is adopted that N and nr are independent, residing on different chromosomes. Results from that back-cross have strengthened the belief that at least the great majority of sheep of both sexes of the high-dosage genotype N/+ .nrlnr have the complete halo-coverage and hornedness of N/N. +/ +