HEREDITY 99 



upon a ' factor ' residing in one of the chromosomes. 

 To give a mental picture we may suppose that it is one 

 of the little chromatin beads as shown in Fig. 46, H. 

 Each gamete, as we have already seen, has one set of 

 chromosomes (in the fowl the number is seventeen instead 

 of the three in the species of Cyclops figured), but the 

 zygote has two sets. Hence while each gamete has but 

 one chromatin bead or factor for feather colour, the 

 organism (zygote) has two. Hence its hybrid nature if 

 the two are dissimilar. The gametes which issue from 

 this hybrid individual have, however, again only one set 

 of chromosomes, and therefore only the one colour factor, 

 and therefore conform to Mendel's la\v. 



The reason why Mendel's law is difficult or impossible 

 to detect in many cases is certainly that the characteristics 

 under consideration are complex ones, depending upon 

 the interaction of many such factors, which exist in pairs 

 in the zygote and in single in the gamete. The variety 

 of ways in which the double set of factors of the zygote can 

 be sorted out into the single set of the gamete, and the 

 still greater variety of combinations that can result in 

 syngamy of the various kinds of gametes will then result 

 in such an enormous variety of zygotes as to render 

 impossible the discovery of segregation, which is only 

 detectable when the different types can be divided into 

 a few well-marked categories. 



