HYBRIDIZING PLANTS 129 



If we compute the areas of the sections within the popula- 

 tion square, we find: 



3/4x3/4 9/16 Round yellow 



3/4 X 1/4 3/16 Wrinkled yellow 



1/4x3/4 3/16 Round green 



1/4 X 1/4 1/16 Wrinkled green 



Again, Mendel proceeded to test his theory carefully by 

 carrying the experiment to the F3 generation. He was able 

 to show that all the wrinkled green F2 plants bred true; all 

 the wrinkled yellow plants bred true for wrinkled, but 2/3 of 

 them split up into a 3 : 1 ratio for yellow. All the round green 

 plants bred true for green, and 2/3 of them had a 3 : 1 ratio 

 with respect to round. And finally the round yellow plants 

 behaved variously; only one out of every nine bred true for 

 round yellow; some gave 9:3:3:1 ratios like the Fi; and 

 others bred true for yellow, but not for round; and some bred 

 true for round but not for yellow. He had evidence enough 

 to propose a second law which has now become known as the 

 law of independent assortment or the second law of Mendel. 

 It can be stated as follows: the members of different 



PAIRS OF FACTORS SEGREGATE INDEPENDENTLY IN GAMETE 

 FORMATION. 



This did not complete Mendel's work, but his later work 

 is not of such fundamental importance. His work has since 

 been repeated by many workers and found to be sound. For 

 instance, the experiment of the yellow-seeded variety crossed 

 with the green-seeded variety was repeated by a group of 

 people, and the final results for the F2 were 146,802 yellow 

 and 48,675 green or a ratio of 3.016 to 1. We might note at 

 this point that even with nearly 200,000 F2 plants, the ratio 

 is not exactly 3:1. The larger the number of F2 plants, the 

 closer the ratio approaches 3:1, but seldom does the ratio 

 actually become exactly 3:1. If you make a cross between 

 varieties in a garden, which involves a single character differ- 

 ence, the chances of obtaining a perfect 3 : 1 ratio in the F2 



