336 Interaction of Genes 



ratio of 27 triple dominant : 27 double dominant : 9 single domi- 

 nant : 1 recessive. If such duplicate, cumulative genes show 

 incomplete dominance, the situation is the one that Nilsson- 

 Ehle found in wheat. In some strains red color in the grain 

 results from the action of two pairs of genes, but in other strains, 

 three such pairs are found, RiRi, R2R2, and RsR^. In these 

 strains the shade of red depends upon the number of large-let- 

 tered genes. When a very deep red plant {RiRi R2R2 RsRn) is 

 crossed with a white (r 1^1 ^2^2 ^3^3), the Fi is intermediate (RiVi 

 722^2-^3^3) and the F2 ratio is: 



1— 

 6 



deepest red — (6 genes for red) 



V. -very deep red — (5 genes for red) 



15 — deep red — (4 genes for red) 



20 — intermediate — (3 genes for red) 



15 — pale red — (2 genes for red) 



6 — very pale red — (1 gene for red) 



1 — white — (0 genes for red) 



Duplicate, cumulative, incompletely dominant genes and 

 their relation to quantitative characters are discussed further in 

 the next chapter. 



Modifying Genes 



In the above examples of gene interaction, the genes acted 

 apparently with relatively equal weight. In a number of in- 

 stances, however, a certain character is determined by one gene, 

 but is modified slightly by others. All the interacting genes in 

 this case do not appear to have equal strength, but there seems 

 to be one "main" gene whose effect can be varied slightly by 

 other, often numerous, "minor" genes. Such minor genes are 

 known as modifying genes because they produce no effect other 

 than a slight modification of a character which is determined 

 fundamentally by another gene. A good example of modifying 

 genes is the genes which modify gene W in mice, a gene that 

 results in a coat with white spots. The extent of the spotting 

 is determined by these other genes. If only a few are present, 

 the area covered by the white spots is small; but in animals 

 with more such genes, the white area is increased so that some 

 animals are almost entirely white. That these are modifying 

 genes is shown by the fact that they have no effect unless the 

 "major" gene, W, is present. In ww mice, there are no white 



