FACTS OF INHERITANCE 153 



formed by two gametes, must contain two factors. 

 It is a double structure, and, when it comes to 

 form gametes, these single structures are produced 

 by the separation of the two factors present in 

 any zygotic cell. The factors representing the 

 characters are said to segregate from one another in 

 the process. In a zygote produced by the union 

 of similar gametes, the segregation is between like 

 factors, and all the gametes produced are alike. 

 But a zygote which has been formed by two 

 dissimilar gametes, each bearing one of the factors 

 corresponding to a pair of characters, must, on 

 forming gametes, give rise to gametes of two sorts, 

 and must give rise to them in equal numbers. 

 On this simple hypothesis is afforded a ready 

 explanation of the various experimental facts 

 given above. 



" A blue hen is producing equal numbers of 

 ' black ' and * white ' eggs let us say 2n of 

 each. To fertilise these eggs are brought large 

 numbers of spermatozoa of the two sorts, black and 

 white, in equal numbers. Every black egg, then, 

 has an equal chance of being fertilised by a black 

 or a white spermatozoon. In the former case it 

 will form a black, and in the latter a blue, bird. 

 From our 2n black eggs we shall obtain n black, 

 and n blue birds; that is to say, the mating of 

 blue with blue must, on the assumption of the 

 purity of the gametes, give black, blue, and white 

 in the ratio 1:2: 1." 



WALTZING MICE. Let us take another illustra- 

 tion relating to the quaint Japanese waltzing 

 mice, which waltz round and round in circles and 

 have only one semicircular canal of the ear well 

 developed. When waltzing mice are crossed with 



