GENETIC FACTORS IN THE ORIGIN OF DIVERSITY 391 



In dogs there is a dominant gene, B, for black hair color; its recessive, 

 b, results in brown color, in homozygotes. Another dominant gene, /, 

 inhibits the action of the genes just mentioned causing the coat to remain 

 unpigmented, white, despite the presence of genes B or b. Dogs having the 

 genotype // will be colored if the proper genes for color are present. Thus a 

 brown dog has the genotype iibb. Some white dogs have the genotype 

 IIBB. What would be expected from matings of dogs of these genotypes? 

 As a result of meiosis the brown dog produces germ cells having the 

 constitution ib, the white dog produces germ cells of IB constitution. The 

 resulting fertilized ova have the genotype liBb, and give rise to white 

 dogs, because of the presence of /. 



When these Fi white dogs are bred together the offspring shown in 

 Fig. 17.7 are expected. We note that twelve of the 16 combinations contain 

 at least one / and hence result in white dogs. Of the four combinations 

 which are homozygous //, three contain at least one B, and hence are 

 black, while one is homozygous bb and hence brown. Thus the 9:3:3:1 

 ratio (p. 388) has been modified to a 12:3:1 ratio. 



Epistasis is not the only type of gene interaction. The expression of 

 many genes is modified by the action of other genes. An example of such 

 modifier genes is afforded by the genes affecting the size of the pigmented 

 areas of hooded rats. Hooded rats are white with black heads and 

 shoulders and a black stripe down the middle of the back and tail. They 

 are homozygous for a recessive gene, //. But in addition to this gene 

 there are other genes which determine the size of the pigmented areas — 

 whether, for example, the black stripes down the back shall be narrow or 

 broad. Such interaction of genes is very common, in fact it is probably the 

 rule. When we speak of a gene "for" a certain characteristic we mean that 

 without the gene the characteristic can not develop but we do not imply 

 that the gene in question works alone in producing the characteristic. 



A type of gene interaction which is very common is the addition of the 

 effect of one gene to that of another. Genes which have cumulative effects 

 of this kind are called multiple genes or polygenes. Many quantitative 

 characteristics have polygenes as their genetic basis. 



Suppose, for example, that a certain species of plant has a tall variety 

 and a dwarf variety, the tall variety averaging 34 inches in height, the 

 dwarf variety 10 inches. Thus the difference between them is 24 inches. 

 We shall also suppose that the dwarf variety has the genotype aabb, the 

 tall variety the genotype AABB. In this case each "capital letter" gene 

 contributes a certain increment in height. If the effect of each "capital 

 letter" gene is the same, each one contributes 6 inches increase in height 



