200 Peculiar Pigmentation of the Sillcy Fowl 



Exceptions. 



In our account we have mentioned certain exceptions which occurred 

 in several of the various matings. These are : 



Table II, p. 192 . ex Br. L. ? x Silky i 1 $ partially pigmented 



„„ . ( 2 ? ? partially pigmented 



P-1" • exBr.L.,xPP,».,F, { i , Jy pign'ented 



Table VII, p. 196 . ex Fj (PpJi) ? x Silky j 4 ? f partially pigmented 



S partially pigmented 

 L i fully pigmented 



Table XI, p. 198 . ex Fj (Fpli) ? x ;);;/( <? j " 



In all these cases the % % should have been fully pigmented and 

 the (/'(/' should have been partially pigmented on our hypothesis. It 

 will be noticed that wherever these e.xceptions occuiTed the mother 

 was a bird heterozygous for both F and /. These cases raise the 

 question whether the normal repulsion between F and / in such birds 

 may not occasionally break down, and whether in addition to Fi andfl 

 gametes they may not produce FI and the complementary fi gametes. 

 This appears the more likely as in two out of the four cases a fully 

 pigmented </ also appeared as an excejition ; and in Table VII even 

 if such birds appeared they would not be noticed, since fully pigmented 

 </(/ are one of the classes normally produced from the mating of 

 slightly pigmented F^ $ and the Silky J'. We incline therefore to 

 think that upon occasion the repulsion between factors may be im- 

 perfect, though whether this imperfection is sporadic, or whether it 

 can be conceived as part of some orderly scheme we do not yet know 

 enough to say. 



The Grades of Pigmentation. 



The dependence of pigmentation upon the presence or absence of 

 two factors (P and /), as well as upon the heterozygous or homozygous 

 condition of the individual with regard to either or both of them, 

 would naturally lead the observer to look for a considerable range of 

 variation in the pigmented condition. For in the full zygotic series 

 are the nine possible combinations, PPii, Ppii, PPII, PPIi, PpTI, 

 Ppli, ppll, ppIi, ppii. The great majority of the chicks with which 

 we dealt in these experiments were killed and recorded on hatching, 

 and our practice was to refer them in so far as pigmentation was 

 concerned to one of the following grades, viz. none, faint, slight, some, 

 moderate, much, full, very full. Though not corresponding accurately 

 to the various zygotic constitutions, these empirical grades nevertheless 



