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 cach 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 oceur 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 
A B 
self-fertilization of a hybrid are represented by i} e< i . There 
A B 
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, AA :2AB:BB or 1:2:1. 
If the factors represented by either A or B are dominant, then 
3:1 1s the ratio of the dominant to the recessive offspring. 
