150 MUTATION. 



by mating the hybrids inter se, it was found that of the rams, 

 three times as many were horned as horn-less, and that among 

 the ewes there were three horn-less to one horned. Woods offer- 

 ed the following explanation. He assumed the existence of 

 one definite horn-determining gene in the Dorset breed. All 

 the animals of the Dorset breed are homozygous for this gene, 

 and all the Suffolk sheep lack it. The hybrids are heterozygous 

 for it, and the gene is fully dominant in males, even if present 

 in single dose, but a single dose is without effect on females, 

 and they have to be homozygous for it to have horns. In other 

 words, presence of this gene is dominant in males, but recessive 

 in females. Now, so far as the facts have been stated, they are 

 fitted by the hypothesis. But this hypothesis implies among 

 other things, that a male can only be horn-less if it lacks the 

 gene, so that even with heterozygous horn-less females, a horn- 

 less father can never have a daughter with horns, as presence 

 of horns in a female pre-supposes that she has inherited the 

 gene from both parents. And Woods records two instances in 

 which horned ewes have been produced from two horn-less 

 parents. 



The main objection to Woods hypothesis, and the one which 

 will show us the way to another interpretation of the facts, 

 is this, that according to it a pure breed of sheep could, in res- 

 pect to horns, be only in two different states, either horned or 

 hornless. In reality several breeds of sheep exist in which the 

 males are always horned and the females horn-less. If we ex- 

 amine a great number of different species of sheep, we see that 

 the existence and the size of the horns must depend upon sev- 

 eral different factors. There exist breeds in which both sexes 

 are truly horn-less, as the Suffolk, the Texel, and the Lincoln. 

 Next come the breeds in which the males have small scurs or 

 very small horns occasionally, but the females never. The 

 Cheviot breed is a fair example. In the breeds in which the 

 females generally have small horny scurs, the horns of the 

 males are much larger than in the breeds where the females are 

 truly horn-less. In those species where both sexes are horned 



