226 GENETICS 



tirrhinum (the snapdragon), in tomato plants. Long and 

 short styles in Oenothera; long and short wings in Droso- 

 phila ; long and short hair in various rodents ; long and short 

 legs in Dexter-Kerry cattle; long and short fingers in man. 

 Differences in color that give typical Mendelian inheritance 

 are numerous in Drosophila. 



In other cases of difference in dimensions or color or the 

 like, the progeny are intermediate between the parents, and 

 there may be many different gradations. This is the case for 

 stature in man, for ear length in rabbits, for depth of color 

 in negro-white crosses, for many dimensions in organisms. 



The key to the understanding of these cases was given by 

 the discovery that when such crosses occur, the descendants 

 in the F2 generation are much more varied than those in the 

 Fi generation. This is of course necessarily the case if the 

 characters depend on differences in genes. When parents dif- 

 fer in a single pair of genes AA and aa, the offspring all have 

 in Fi the same constitution, Aa. But in F2 there are three 

 different constitutions AA + 2Aa + aa, so that F2 is much 

 more varied than Fi. Again suppose that the parents differ 

 in two pairs of genes, so that one parent is AABB, while the 

 other is aabb. Then the Fi generation all have the consti- 

 tution AaBb, and are therefore all alike. But in the F2 gen- 

 eration there are nine diverse combinations of the genes, as 

 shown in table 2, on page 117; they therefore show much 

 more diversity than occurs in Fi. 



An illustration will show how this works. Suppose that two 

 races differ in size, in consequence of a difference in two pairs 

 of genes. The larger race carries the genes ABAB, while the 

 smaller one has the corresponding genes abab. Assume that 

 each one of the four genes ABAB increases the size by a 

 certain amount above that of the race abab. When the two 

 races are crossed, the Fi generation all have the constitu- 

 tion ABab. Having two of the four genes that increase the 

 size, they are intermediate in size between the two original 



