Epistasis of a Recessive Gene 323 



Epistasis of a Recessive Gene 



In summer squash, one dominant gene is epistatie, producing 

 the same result when interacting with either the dominant or 

 the recessive of the other pair. Similarly, a recessive gene may 

 be epistatie and, if so, the F2 dihybrid ratio becomes 9:3:4. 

 An example of the epistasis of a recessive gene is found in the 

 common bean, Phaseolus vulgaris. Gene P is dominant to p and 

 gene B to b. Genes P and B interact to produce a purple color 

 on the seed coat, and P and b interact to produce a yellow or 

 brown color. The recessive gene p, however, is epistatie to the 

 other genes and produces a white seed coat with either of them. 

 This relationship could be illustrated by a "checkerboard," as 

 in Fig. 90. 



Many examples of the epistasis of a recessive gene, resulting 

 in a 9 : 3 : 4 ratio, have been found. Baur showed that in the 

 snapdragon, a white-flowered and an ivory-flowered plant, when 

 crossed, produce a magenta-flowered Fi, while the F2 segregates 

 into a ratio of 9 magenta : 3 ivory : 4 white. In flax, Linum 

 usitatissimum, Miss Tammes showed that a lilac-flowered plant 

 crossed with a certain type of white-flowered plant produces a 

 blue-flowered Fi, whereas the F2 ratio is 9 blue : 3 lilac : 4 white. 

 The relationship is not confined to plants. In such rodents as 

 the mouse and the guinea pig, the normal wild type has a peculiar 

 gray color, called agouti. This color is the result of two inter- 

 acting genes, C and A. If two rodents which are heterozygous 

 for both genes are crossed, the offspring fall into the ratio of 9 

 agouti {CA) : 3 black [Ca) : 4 white [cA + lea). 



Most epistatie recessive genes are genes affecting color. A 

 possible chemical explanation for them was suggested a number 

 of years ago and throws some light on the general question of 

 gene interaction. It can be assumed that (in the general situ- 

 ation) gene A produces a colored substance and that a produces 

 a colorless substance. These substances would be chemical com- 

 pounds. It can be further assumed that B produces a chemical 

 that can react with the colored substance produced by A to 

 change its color. In the specific example of ShuU's beans, P pro- 

 duces a yellow pigment. This substance reacts chemically with 

 the enzyme secreted by B so that it is converted from a yellow 



