On the varj-ing dominance of certain white breeds of the silk-worm, etc. 261 



heterozygous white (W'Y) mated with recessive white (ww) would pro- 

 duce a mixed brood (iW:iy) in which both white and yellow are 

 heterozygous. 



This may illustrate the result obtained in the white mating. 



From the facts and considerations above referred to, we are led 

 to the conclusion that there are two sorts of white breeds in silk- 

 worms, one behaving as recessive and the other dominant towards 

 the yellow. 



Parallel cases have already been observed in other animals and 

 plants. Bateson says that "white birds arisen from the cross between 

 Wliite Dorking and Indian Game is recessive to the coloured birds. The 

 same holds good in the white of the silky fowl, that of the Rose- 

 comb Bantams, while the normal white is dominant over the co- 

 loured". Gregory ('n), Keeble and Pellew's ('10) observations made 

 on Primula sinensis add other examples of similar nature. 



Taking it for granted that our explanation is correct the possi- 

 bility naturally arises that there may be some white breeds in which 

 both the antagonistic whites (dominant and recessive) are mingled 

 together. When inbred, they will remain true to parents and behave 

 like a pure white, but once crossed with yellow what results would 

 be produced? 



Suppose there is a heterozj'gous white represented by a zygotic 

 formula, Ww. When inbred, it will remain as a white breed but the 

 zygotic constitutions of its offspring will not remain the same, some 

 of them being ww, some w«- and the rest ww. If matings take place 

 between this white and yellow, we should expect to have three ga- 

 metic combinations in F^. 



1. WW X YY = WY = all white. 



2. WW X YY = wY = all yellow. 



3. Ww ■ YY = WY -\- wY = one white : one yellow. 



The first or dominant white mating produces all white F^ (^^Y), 

 the second or recessive white all yellow Fi (wY) and the third or 

 heterozygous white a mixed Fi (WY -f- wY). 



Both Fj white and Fj yellow paired inter sc will gi\-e a mixed 

 offspring, as shown below: 



1. F, white ■ F, white = WY X WY = WW -f 2 WY -|- YY = ? white : i yellow. 



2. F, yellow x Fi yellow = wY x wY = ww -|- 2 wY + Y^Y x i white : 3 yellows. 



In the dominant white mating, the proportion of white and yellow 

 is 3 : I which in the recessive white mating or Fi yellow is 1:3. 



