284 THE CONTINUITY OF THE RACE 



was transferred to the stigmas of an individual of the other variety and 

 so produced a cross-fertilization. 



The dihybrid Fu The F\ seeds were all yellow and round, as we should 

 expect from the results already given for monohybrid crosses between 

 round and wrinkled and between yellow and green. 



The dihybrid F 2 . On planting the Fi seeds and allowing the flowers 

 produced by them to self-fertilize, Mendel obtained four kinds of seeds — 

 yellow and round, yellow and wrinkled, green and round, and green and 

 wrinkled. Here were not only the original parent types — yellow and 

 round and green and wrinkled — but also two new combinations — yellow 

 and wrinkled and green and round. When the number of each of the 

 four types was counted, it was found that 315 were yellow and round; 

 101 were yellow and wrinkled; 108 were green and round; and 32 were 

 wrinkled and green. The proportion of yellow to green is (315 + 101): 

 (108 + 32), approximately Y±.y±, and the proportion of round to wrinkled 

 is (315+ 108): (101 +32), again approximately Y^'-Y^'i but how are 

 we to account for 101 individuals that are yellow and wrinkled and 108 

 that are green and round? 



MENDEL'S SECOND LAW: INDEPENDENT ASSORTMENT 



Mendel saw that the two new combinations — yellow and wrinkled, 

 and green and round — could be accounted for if he assumed that segrega- 

 tion between the genes for yellow and green was independent of the 

 segregation that takes place between the genes for round and wrinkled. 

 That is, although the hybrid F\ received its genes for yellow and round 

 from one gamete and its genes for green and wrinkled from the other, 

 these genes would not tend to stay together when the F\ generation 

 formed its germ cells. Such independence in segregation would result 

 in the Fi individuals forming four kinds of male and female gametes in 

 equal numbers: 



1. Containing a gene for yellow and a gene for round. 



2. Containing a gene for yellow and a gene for wrinkled. 



3. Containing a gene for green and a gene for round. 



4. Containing a gene for green and a gene for wrinkled. 



If one of each of the four types of male gametes then fertilized one 

 of each of the four types of female gametes, four classes of phenotypes 

 should be produced, in the proportion of %q that are yellow and round, 

 %6 that are yellow and wrinkled, Ye that are green and round, and Y.6 

 that are green and wrinkled. 



This can be more clearly expressed if we use a diagram to describe 

 the various individuals and germ cells involved. We have already used 



