MENDELISM 687 



multiplication. In the example given, the two parents were similar 

 and so the resulting squares were regular, but the same method 

 is used where the parents are different. Mendel, for example, 

 tested his theory by a ' back cross ' of the heterozygote to one 

 of the original homozygous parents. If this is the recessive, its 

 only possible gametes are ry, and those for the heterozygote 

 are the same as before. We have, therefore, a rectangle of 4 x i 

 squares (Fig. 534) , and it is clear that individuals of the genotype 

 RrYy, Rryy, rrYy and rryy should be formed in approximately 

 equal numbers. Mendel got in one experiment 31 round yellows, 

 26 round green, 27 wrinkled yellow, and 26 wrinkled green. It 

 must be stressed that the mathematical ratios must not be 

 exactly expected in practice, since they depend on random 

 pairing of gametes, and on random sampling of the factors, 

 and with small numbers there may be wide variations. This 



Fig. 534. — Backcross ' chequer-board '. 



is especially important in animals where each litter is usually 

 small. In a litter of eight guinea pigs, bred from heterozygous 

 normal/angora parents, for instance, one would not expect always 

 to have six normals and two angoras ; five and three or seven and 

 one would both be very probable, and even eight and nought 

 or four and four not unlikely. But the totals of a large number of 

 litters would approximate to a proportion of 3 : i. 



Although the main principles of Mendelism were confirmed 

 by the early geneticists, they soon found that there were excep- 

 tions and difficulties. In particular, they found that different pairs 

 of characters were not always independently inherited, or, in 

 other words, that certain characters were associated together. 

 There are two possible explanations of this : that the same factor 

 determines more than one character, and that there are different 

 factors but that they do not separate in the formation of the 

 gametes. Each is sometimes true, but the second is the more 

 important. Its discussion is best left until we have dealt with the 

 cytological features of cell division. 



