282 



EVOLUTION AND GENETICS 



they are handed down to the next generation from each hybrid 

 parent, and the number of possible combinations received from 

 the two parents is therefore sixteen. Out of the sixteen combina- 

 tions some are duphcated. The phenotypic ratio is 9:3:3:1. 

 Nine are dominant for both characters, three for one, three for 

 the other and one for neither (Fig. 161). In this diagram it is 

 evident that the number of genotypes in the dihybrid is greater 

 than the number of phenotypes, as was the case in the monohy- 

 brid. 



In the F2 generation many individuals are homozygous for one 

 character and heterozygous for the other. Such individuals, when 



inbred, are capable 



SY 



Male gametes 



Sy 



s Y 



sy 



m 



(D 

 +-> 



£ 



bid 



s 



of producing off- 

 spring in which the 

 homozygous char- 

 acter always ap- 

 pears while the 

 heterozygous char- 

 acter behaves as in 

 monohybrids. An 

 SsYY pea vine, for 

 example, can pro- 

 duce only yellow 

 seeds but they will 

 appear in the ratio 

 of three smooth to 

 one wrinkled. 



In experiments 

 with animals simi- 

 lar results have 

 been obtained. It 

 has been found that a guinea-pig with short black hair crossed 

 with a long-haired albino produces a short-haired black Fi genera- 

 tion. When these Fi guinea-pigs are inbred they produce an F2 

 generation with four t3^pes of animals, viz., short-haired black, 

 short-haired white, long-haired black and long-haired white. Of 

 these the short-haired black animals may belong to four different 

 genotypes, the long-haired all)inos to only one and the others to 

 two each. Figure 162 illustrates the kinds of guinea-pigs based 

 on the reassortment of three different characters. 



SY 



Sy 



sY 



sy 



Fig. 161. — Diagram to illustrate the F2 generation of 

 Mendel's hybridization of smooth yellow and green 

 wrinkled peas. Ratio of phenotypes 9:3:3:1; ratio 

 of genotypes 1:2:2:4:1:2:1:2:1. The phenotypes 

 are numbered in the upper right hand corners of 

 the squares, the genotypes in the lower corners. 



