Cumulative Duplicate Genes 331 



sules are the CD, Cd, or cD types; the round-capsuled plants 

 are ccdd. The segregation of these types in the F2 is: 



/3D -^ 9Ci)— 

 3C<( 



lc< 



= 15 



\ 



Id ^ led — =1 



Duplicate genes are common among plants but less so in ani- 

 mals. They probably indicate that the plant that contains them 

 has more than two sets of chromosomes, a situation discussed in 

 Chapters 26 and 27. They can be considered as duplicate domi- 

 nant epistatic genes since C and c produce the same phenotype 

 in the presence of D, whereas D and d give plants indistinguish- 

 able phenotypically in the presence of C. 



Cumulative Duplicate Genes 



If two duplicate dominant genes interact to produce a result 

 different from that produced by either one plus the recessive 

 allele of the other, the 15 : 1 ratio becomes 9:6:1. A ratio of 

 this sort was reported by Miyake and Imai for grain color in 

 barley. Some plants have purple grains and others have white 

 grains, and the former type is dominant over the latter. When 

 a dark purple-grained and a white-grained plant were crossed, 

 the F2 ratio was 15 purple : 1 white; but the purple was not 

 of the same intensity in all the purple-grained plants. This 

 ratio w^as caused by two pairs of duplicate genes, Pi and Po, 

 which interacted so that the plants with the two dominant genes 

 Pi and P2 were of a deeper purple than those with Pi and p2 

 or with pi and P2 genes. These two nonallelic dominants have 

 a cumulative or additive effect such as was not found for cap- 

 sule shape in Capsella. 



Nilsson-Ehle showed that the situation in wheat is even more 

 complex, for not only are duplicate genes present but there is 

 incomplete dominance as well. Grain color is either red or white, 

 and the intensity of the red color depends upon the number of 

 color-producing genes present. The genes can be designated 

 Ri, ^1, R2, and ro. If a deep red-grained plant, RiRi R2R2 is 



