386 CHARACTER EXPRESSION OF GENES 



gene. It is to be emphasized that the chromosome 

 mechanism distributes these genes in the usual way and 

 gives rise to exactly the same number and kinds of geno- 

 types as in other characters, but that the number and 

 kind of phenotypes vary with the way in which the 

 particular genes express themselves. The following para- 

 graphs illustrate common types of interaction of genes: 



Complementary Genes are such that the presence of 

 both is necessary to produce the character. There are 

 two sorts of white corn each of which has one of the com- 

 plementary genes necessary to produce red corn. When 

 they are crossed the hybrid is red and the F2 consists of 

 9 red to 7 white. This result can be readily understood 

 by considering the combinations of genes in the second 

 generation. If we suppose that the one white parent 

 carries the genes A and b and the other carries the genes 

 a and B, both dominants being necessary to produce red 

 color, we can account for all the facts. The hybrid then 

 has the genotype AaBh (since one parent is supposed to 

 be A Abb and the other oaBB). The hybrid produces 

 four kinds of gametes AB, Ab, aB, ab in equal numbers. 

 These recombine according to Law 2 to give 9 AB ; 3 aB ; 

 3 Ab; 1 ab (see Punnet t square, Chap. XXIX). The 

 9 AB are of course red ; the 3 Ab are white like one grand- 

 parent; the 3 aB are white like the other grandparent; 

 and the 1 ab is also white. The seven are white because 

 they lack either one or both of the genes necessary to 

 make red. 



Supplementary Genes. — This name is given to genes 

 which change or modify the characters produced by other 

 genes but do not produce any visible effect of themselves. 

 For example, if pure red corn is pollinated by a certain 

 kind of white corn a purple hybrid is produced. The 

 white carries a modifying or supplementary gene which 

 changes red to purple, but produces no color in the 

 absence of either of the genes for red. If this purple 

 hybrid is self-pollinated it gives purple, white, and red 



