96 SEX AND HEREDITY 



producing in equal numbers eggs carrying black factors 

 and eggs carrying white factors, and similarly the cocks 

 are producing the same two kinds of spermatozoa. We 

 have thus the following possible combinations in syngamy : 



Eggs. Spermatozoa. Zygotes. 



Black x Black Black. 



White x Black Gray. 



Black x White : Gray. 



White x White = White. 



As the combination of the different classes of gametes 

 is supposed to take place by chance, we will get on an 

 average of a large number about equal numbers of each 

 combination i.e. the zygotes will be in the proportions 

 of one black, two gray, one white. 



It must be understood that these numbers must not 

 be expected to be realised exactly. The position can be 

 paralleled by putting an equal number of white and black 

 counters into a box, shaking them up, and then drawing 

 them out in pairs without looking at them. Theory will 

 then lead us to expect that 25 per cent, of the pairs will 

 consist of two whites, 25 per cent, of two blacks, and the 

 remaining 50 per cent, of one white and one black. It 

 will of course be only by chance that these percentages 

 are exactly realised, just as if we tossed up a penny a 

 hundred times we should probably get an approximately 

 equal number of heads and tails, but only by chance 

 exactly fifty of each. Thus in one experiment with the 

 Andalusian fowls the offspring of a number of them 

 totalled up to 



Black, 41 ; Gray, 78 ; White, 39. 



The further breeding of these offspring of the gray 

 hybrids is easily understood. Since blacks lack the white 

 factor altogether, when bred together they can give 



