328 ELEMENTS OF BIOLOGY 



ber of times that heads are tossed will be 50 per cent of the 

 number of tosses. But the first three or even more tosses may turn 

 heads. If there are only six individuals in an F2 generation, all six 

 might by chance be phenotypically Green Smooth. Thus, while the 

 table enables one to predict accurately the ratios that will appear, it 

 does not enable a prediction of the characters of a single offspring. I 

 The letters in the blocks show that genotypically the ratios of ' 

 homozygous individuals for one or the other of the genes is 1:1:1:1. 

 If one employs any of these as a parent in a mating with another 

 homozygous for the same character, it is found that all progeny in 

 turn are homozygous. It is therefore possible to extract a pure reces- 

 sive out of a cross with a dominant, or vice versa. But suppose an 

 individual that is heterozygous for both characters is crossed with 

 one that is homozygous for both the recessive genes. The germ 

 cells possible and the ratios of occurrence of phenotypes are as 

 follows: 



Parents — YySs X yyss 



Germ cells — YS, Ys, yS, ys X ys 



Offspring — YySs, YYss, yySs, yyss 



Phenotypes — i YS: i Ys: i yS: i ys 

 Similarly, crosses may be made between individuals homozygous 

 for both dominants and heterozygous for both characters, with cor- 

 responding ratios. We may then conclude that, given a dominant 

 character, or a number of dominant and recessive characters, it is 

 possible to predict the frequency of occurrence among the offspring 

 of any given type we may desire. 



Polyhybrids. To follow the detail of inheritance in a polyhy- 

 brid cross is unnecessary here. It is sufficient to state that in a tri- 

 hybrid cross the 3:1 ratio of phenotypes is again advanced as is 

 shown in the accompanying diagram (Fig. 206) representing three 

 pairs of allelomorphic genes in the germ cell chromosomes, to 

 27:9:9:9:3:3:3:1, 64 possibilities in all. A moment's consideration will 

 now indicate the complexity of human inheritance, where there 



