556 HEREDITY 



receive only the genes of one of the parents. On the other 

 hand, the daughter nuclei may receive chromosomes of both 

 parents and in different proportions in different divisions. Again 

 cytological studies of the reduction division show that there 

 is a pairing of chromosomes previous to their separation, and 

 there is evidence that each pair consists of a father and a 

 mother chromosome. Now, if we assume that chromosomes 

 pairing carry genes for contrasting characters, then the separa- 

 tion and distribution of the members of each pair to different 

 daughter nuclei should result in the segregation of genes for 

 contrasting characters and in the production of pure gametes. 

 The trouble with this assumption is that a plant or animal has 

 so many more pairs of contrasting characters than chromo- 

 somes, that it is difficult to explain the numerous combinations 

 that occur when many pairs of contrasting characters are taken 

 into account. Despite the fact that there are some things 

 about segregation we are unable to explain by the mechanism 

 of reduction division, it is generally believed that the two phe- 

 nomena are vitally related. 



The Mendelian Ratio and the Combinations of Gametes. - 

 It is possible to account for the Mendelian ratio 1 : 2 : 1 by 

 taking into account the probable combinations that may occur 

 among gametes during fertilization. A hybrid forms two kinds 

 of gametes equal in number in respect to a pair of contrasting 

 characters. One kind of sperms and eggs may be represented 

 by A and the other by B. Now the probable combinations 

 between the two kinds of sperms and two kinds of eggs in the 



self-fertilization of a hybrid are represented by T /\ J There 



are two chances for A and B to unite to one chance for A to 

 unite with A or B to unite with B. The probable combinations 

 and their ratios are, therefore, A A :2AB: BB or 1:2:1. 

 If the factors represented by either A or B are dominant, then 

 3 : 1 is the ratio of the dominant to the recessive offspring. 



