On the varying dominanct- of certain white breeds of the silk-worm, etc. 275 



1:3m the latter, provided that nearly the same number of sper- 

 matozoa from both males would enter into the spermatheca and 

 would act with the same force, but practically this is not the case, 

 since as we have already shown in our former paper ('06. 2) the 

 proportion depends much upon the duration of copulation and other 

 mechanical conditions of the parents. Consequently we could not 

 anticipate the proportion of white and yellow as exactly as in the 

 case of single mating Fj. 



From the formula above described, we may easily infer that the 

 results of Fi whites and Fi yellows paired inter se are the production 

 of normal monohybrid F2 or F3 and so on, the only difference being that 

 in the former the white is dominant and in the latter it is recessive. 



If matings take place between the F^ yeUow and F^ white, we 

 would expect to have the following gametic combinations in Fo, that 

 is to say, i) a mating between homozygous yellow (yy) and hetero- 

 zygous white (WY) which wiU give a mixed F2 brood (iw : lY) and 

 2) a mating between heterozygous white (WY) and yellow (wY) which 

 will produce a mixed brood consisting of i w : i Y. The results are 

 apparently the same in both matings, but their zygotic constitutions 

 are entirely different, since the former is 



W Y .< YY = W Y + YY = I ^\• : i Y. 

 and the latter 



WY ,■; w Y = Ww -|- WY -f- wY -f YY = 2 W : .2 Y = i ^\' : i Y. 



Of these two sorts of i w : i Y broods, the former is a case of 

 familiar monohybrid back-crosses, in which the recessive yellow is 

 homozygous whUe in the latter both white and yellow are a mixture 

 of homozygous and heterozygous whites and yellows. When paired 

 inter se therefore the white will give three kinds of F3 broods 



1. F2 white X F2 white = Ww x Ww = WW -|- 2 Ww -|- w\v = all white. 



2. F2 white X Fa white = Ww X WY = WW -f- Ww -|- WY -\- wY = 3 w : i y. 



3. F, white ;< F, white = WY x WY = W^V -f 2 \VY + YY = 3 w : i y. 



and the yellow also three kinds of broods as shown below : 



1. F2 yellow X F2 yellow = YY X YY = YY = all yellow. 



2. F2 yellow X F, yellow = Y Y x wY = wY -)- Y Y = all yellow. 



3. Fo yellow x F2 yellow = wY x wY = w^v -)- 2 wY = YY = i w : 2 Y. 



Contrary to a i w : i Y brood of monohybrid back-crosses, both 

 white and yellow paired inter se produce three sorts of broods; in 

 white mating i) aU white, and two sorts of 3W : lY broods, and in 

 yellow matings, i) two sorts of all yellow and 2) iw :3Y broods. 

 Thus the order of inheritance of white and yellow characteristics in 

 this scries of double matings is summarized as below: 



