THE INTERACTION OF GENES 



F2 families. Where two genes which segregate independently do not 

 interact, they give Mendel's familiar 9:3:3:1 ratio. When they 

 interact this ratio is simplified; classes are combined in most of the 

 ways possible with complete dominance (Fig. 38). 



Interaction in the simplest case consists of a mere anticipation: the 

 effect of one gene may be cut out by that of another which is said 

 to be epistatic to it. 



Epistasy comes in at all stages of development, early and obvious 

 or late and inferential. A gene removing life, a lethal gene, must 

 obviously be epistatic to any other genes affecting the life it cuts 

 short. A gene removing an organ is likewise obviously epistatic to 

 one modifying that organ. A gene suppressing, or failing to produce, 

 a substance, is obviously epistatic to one modifying that substance, 

 or requiring it for its own activity. But the conclusion whether the 

 substance is modified on the one hand, or is required on the other, 

 is usually to be deduced only from the epistasy itself. 



Epistasy expresses itself in two types of F2 ratio according to 

 whether the epistatic allelomorph is dominant or recessive. 

 Albinism in rodents is due to a gene which is recessive, and also of 

 course epistatic to the various colour and pattern genes. The 

 9:3:4 ratio is therefore characteristic of Fg's segregating for 

 albinism. White leghorn fowls, on the other hand, owe their 

 absence of colour to a gene, which is, again, epistatic to other colour 

 and pattern genes just like the albino gene in rodents. But it is 

 dominant. Thus they will give a 12 : 3 : i ratio in Fg. 



Fig. j8. — Continued from page 156 



action, give two phenotypic classes understandable on the assumption that the appear- 

 ance of the character in question requires a particular allelomorph of each gene to 

 be present. Failure in either or both genes results in absence of the character. These 

 three cases differ only in the dominance relations of the effective allelomorphs, both 

 being dominant in complementary action, both recessive in duplicate action, and 

 one of each kind in recessive suppression. 



Tw^o other cases, dominant and recessive epistatic action, give three classes, one 

 gene, said to be hypostatic, causing a difference in phenotype only in the presence 

 of a particular allelomorph of the other gene, said to be epistatic. The cases differ 

 only in the dominant or recessiveness of the effective allelomorph of the epistatic 

 gene. 



In the sixth case, of additive action, the two genes have individually indistin- 

 guishable, but cumulative, actions. 



The classes which are confounded are separated only by dotted lines in the repre- 

 sentation of their various interactions, and the relative frequencies of the pooled 

 classes as they would appear in F.j segregations are shown in the small circles. 



157 



