GENES, MOLECULES AND PROCESSES 



environment have often been shown to affect the expressivity of 

 genes. In fact genotype and environment work in parallel. 



Sometimes variation in the expression of a gene is such that it is 

 not shown by all the individuals, even the homozygotes, carrying 

 it; it does not, as it were, penetrate the whole population of 

 individuals carrying it. The gene "antennalcss," for example, studied 

 in Drosophila by Gordon and Sang, removes either both or only one 

 of the fly's antemiae according to its degree of expressivity. A 

 proportion of the antennaless flies even show no effect of the gene 

 at all. Its penetrance is incomplete. But the degree of penetrance 

 (and expressivity) varies with both genotype and environment, and 

 both effects are shown in relation to sex. In the early emerging flies, 

 penetrance is lower in males than in females; the reverse is true in 

 the later part of the hatch. In both sexes the penetrance is at a 

 minimum in a culture at about the fourth day of emergence and at a 

 maximum at eight days. 



These variations in expression are due to changes in the food of 

 the larvae which arise from staling of the culture medium. This, in 

 turn, is partly explained by the discovery that increase in the supply 

 of vitamin Bg reduces penetrance. Indeed this vitamin enables all 

 the flies to get over some of the difficulties, and some of the flies to 

 get over all of the difficulties, presented by the shortcomings of the 

 antennaless gene. 



These experiments have had to do with genes in the homozygous 

 conditions. When we examine heterozygotes we are concerned 

 with dominance and we can regard dominance as a measure of the 

 expressivity and penetrance of the heterozygous gene pair. We 

 might, therefore, expect that dominance would also be subject to 

 modification by genotype and environment. The effect of the 

 genotype has indeed often been established — for example, in cotton, 

 butterflies, and poultry. In the mouse, as we saw, agouti is normally 

 dominant to non-agouti. The gene umbrous, however, shifts the 

 expression of the agouti heterozygote towards non-agouti. The 

 Fg ratio of 3 agouti to i non-agouti becomes 1:2:1. Umbrous 

 thus modifies the dominance of agouti and, indeed, this is the only 

 known effect of the gene. 



The effect on dominance of the environment is, on the other 

 hand, less widely known. The recessive gene cubitus interruptus in 



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