IX 



SEX 95 



tion consists of pigmented and unpigmented in the 

 ratio 3:1. Now a cock of such a strain crossed 

 with a Brown Leghorn hen should give only com- 

 pletely unpigmented birds. But if, as we have 

 supposed, the Brown Leghorn hen is producing 

 gametes Fpi and fpl, the male birds produced by 

 such a cross should be heterozygous for /, i.e. in 

 constitution ffPpU, while the hen birds, though 

 identical in appearance so far as absence of pig- 

 mentation goes, should not contain this factor but 

 should be constitutionally Ffppii. Crossed with the 

 pure Silky, the Fj birds of opposite sexes should 

 give an entirely different result For while the hens 

 should give only deeply pigmented birds of both 

 sexes, the cocks should give equal numbers oi deeply 

 pigmented and slightly pigmented birds (cf. Fig. 

 25). These were the results which the experiment 

 actually gave, thus affording strong confirmation of 

 the view which we have been led to take of the 

 Brown Leghorn hen. 



Essentially the Brown Leghorn case is that of 

 the Sebrights and the Plymouth Rocks. It differs 

 in that the factor which repels femalcness produces 

 no visible effect, and its presence or absence can only 

 be determined by the introduction of a third factor, 

 that for pigmentation. 



^ex-limited inheritance of the type we have be en 

 discussing has been demonstrated in other birds 

 besides poultry. Th ere is a white variety of the 

 ring-dove which is_ r^rp<;qivp to the well-known fawn 

 type. Fawn hens are always heterozyij^ous. and 

 *when crossed with^hite cocks give only fawn cocks 

 arTd white hens. In canaries, again, there are ruby- 



