CONTINUITY OF THE RACE 



597 



Single 



Fig. 24-10. The inheritance of combs in fowls is due to two dominant interacting or supplementary genes. In a cross 

 between pea- and rose-combed animals a new type of comb, walnut, is produced. Crossing walnut-combed fowls 

 gives the typical 9:3:3:1 two gene ratio, but the interaction of the genes show a distribution of comb types as 

 shown here. The double recessive comes out to be a fourth kind, single comb. 



tirely new phenotype may appear. When 

 two dominant genes interact so as to pro- 

 duce a new phenotype, they are called sup- 

 plementary genes. One of the simplest cases 

 of this interaction occurs among combs in 

 chickens. Three kinds of combs are well 

 known: single, pea, and rose (Fig. 24-10). 

 Both pea and rose are dominant to single. 

 What happens when pea and rose, two 

 dominants, are crossed? The result is a new 

 kind of comb, called walnut! This demon- 

 strates the combined action of two domi- 

 nant genes, namely, that each supplements 

 the other and the result is a comb different 

 from that each would produce alone. The 

 proof of this conclusion can be shown 

 merely by crossing two walnut-combed ani- 

 mals. The offspring appear in the ratio of 

 9 walnut : 3 rose : 3 pea : 1 single, which 

 means that wherever the genes for rose and 

 pea come together their supplementary ac- 

 tion produces a walnut comb. Supplemen- 

 tary gene action is different from blending 

 because the genes are located on different 

 chromosomes and are therefore not alleles. 

 Moreover, it is possible to produce a pure 



breed of walnut-combed chickens simply by 

 making test crosses with the double reces- 

 sive single comb to establish homozygous 

 genes. Once this is done, the mating of two 

 homozygous birds will result in all walnut- 

 combed offspring and the pure breed is 

 established. You will remember that this 

 procedure is impossible with those showing 

 incomplete dominance. 



Another slight variation in supplemen- 

 tary gene action can be demonstrated in 

 guinea pigs. Black coat (BB) is dominant 

 to brown coat (bh). In addition to these 

 genes there is a third gene (C) which is 

 essential in order that the color appear at 

 all. A double recessive (cc) is an albino. If 

 a brown guinea pig ( CCbb ) is mated with 

 an albino (ccBB) the offspring will all be 

 black ( CcBb ) which might seem surprising 

 until the genes are examined. Whenever tlie 

 gene for color (C) is in combination with 

 the supplementary gene (B), the pheno- 

 type will be black, but if it occurs only with 

 the recessive (c) no color appears. If two of 

 these Fi pigs are mated, the F2 generation 

 will have the ratio of 9 black, 3 brown, and 



