390 INTRODUCTION TO EVOLUTION 



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dies 2 yellow: 1 normally colored 



In this case the 1:2:1 ratio is changed to a 2:1 ratio. Here it is the 

 homozygous state of the dominant gene which is lethal. 



Sometimes the homozygous condition of the recessive gene is lethal. 

 In com, for example, there is a recessive gene (vv) which results, in 

 homozygotes, in failure to produce chlorophyll; the seedling is white and 

 dies as soon as the food stored in the seed is exhausted. Heterozygous corn 

 {Ww^ is green just like homozygous corn {WW). When heterozygous corn 

 is self-pollinated {Ww x Ww) the following results are obtained: ^/^ WW 

 (green): % Ww (green): 14 ^^ (white). The white seedlings wither, 

 leaving only the green ones. If the ww individuals did not sprout at all 

 we should know of the presence of the gene w only through its effect in 

 reducing the proportion of viable seeds produced. Doubtless many reces- 

 sive lethals are known only through their effects in reducing fertility. 

 Later on we shall have occasion to mention genes which affect viability 

 without being completely lethal (pp. 450-454). Genes which in homo- 

 zygotes are lethal, or lower viability, are widespread in natural popula- 

 tions. The most abundant mutations produced by irradiation (X rays, etc.) 

 are changes in genes causing them to become lethal or to reduce via- 

 bility. 



Interaction of Genes 



In the example of the 9:3:3:1 ratio in guinea pigs the pairs of genes were 

 independent both in inheritance and in effect. The color of an individual, 

 for example, had no effect upon whether it was short-haired or long-haired. 

 At times, however, pairs of genes may be independent in inheritance but 

 not be independent in their effect upon the phenotype. An example of such 

 an interaction of genes is afforded by epistasis, a situation in which one 

 pair of genes prevents or hides the expression of another pair of genes. This 

 interaction of genes assumes various forms. We shall mention but one 

 example. 



