INHIBITORY GENES 387 



phenotypes in the second hybrid generation. In this 

 particular cross the red corn has the genotype AA BB cc 

 {A and B being the genes which produces red) and the 

 white one may have the genotype A A bb CC (in which 

 C is the gene which changes red to purple). The Fi 

 hybrid is AA Bb Cc and produces gametes ABC, Abe, 

 AbC, Abe which combine to produce F2 of 9 ABC; 3 ABe; 

 3 AbC; 1 Abe. The 9 ABC are purple; the 3 ABe are red; 

 the 3 AbC and the 1 Abe are white because they lack one 

 of the genes for red. 



Inhibitory Genes are genes which prevent the usual 

 action of other genes. If a certain kind of white corn 

 is crossed with red corn the hybrid is white, notwith- 

 standing that it receives both red genes from the red 

 parent. There are several ways in which this may hap- 

 pen of which one of the simpler will be explained. The 

 red parent has the genotype AABBii and the white 

 parent AAbbll which gives the hybrid the composition 

 of A A Bb li. It produces gametes ABI, ABi, Abl Abi, 

 and they recombine to produce a second hybrid genera- 

 tion of 9 ABI; 3 ABi; 3 Abl; 1 Abi. All those with I 

 are white because this gene prevents the formation of red, 

 and Abi is white because it lacks B. There are there- 

 fore 13 white to 3 red in the second hybrid generation 

 of this cross. Other crosses can be made with different 

 kinds of whites but all containing / which will, of course, 

 give still different phenotype ratios in F2. 



Cumulative Genes are those which produce different 

 intensities or amounts of the same character accord- 

 ing to how many of them are present. A very simple 

 case of this was described in the four o'clock where the 

 red parent has two " red " genes RR and the white has 

 two corresponding genes rr. The hybrid has only one 

 red gene (Rr) and is only about half as red (i.e., pink) 

 as the red parent. A certain red-grained wheat crossed 

 with a white gives a half -red hybrid. When the 

 hybrid is self -pollinated it produces a second generation 



