Chapter *6 



PHENOTYPIC EFFECTS 

 OF GENE ACTION 



Ti 



|he variations in phenotypic 

 ratios that result from the in- 

 teraction of alleles and non- 

 alleles has been discussed (pp. 49-52). 

 Several other phenotypic consequences of 

 gene action are discussed in the present 

 chapter. Let us begin with a discussion of 

 viability effects, which also modify pheno- 

 typic ratios. 



Viability Effects 



In the snapdragon (Antirrhinum) one finds 

 two kinds of full grown plants, green and a 

 paler green called auria. Green crossed by 

 green produces only green, but auria by 

 auria produces seedlings of which 25% are 

 green (AA), 50% auria (Aa), and 25% 

 white (aa). The last type of seedling dies 

 after exhausting the food stored in the seed, 

 because it lacks chlorophyll. Among full 

 grown plants, the phenotypic ratio observed 

 is y 3 green: % auria. In this case, the ab- 

 sence of dominance gives the 1:2:1 ratio 

 characteristic of a cross between monohy- 

 brids in the seedling stage which, following 

 the death of the albino, becomes a 2 : 1 ratio 

 among the survivors. 



In mice, matings between yellow-haired 

 individuals produce F : in the ratio 2 yel- 

 low : 1 nonyellow. It is found after this mat- 

 ing that % of m e fertilized eggs which 

 should have completed development fail to 

 do so and abort early in embryogenesis. 

 Since crosses between nonyellows produce 

 only nonyellows, the nonyellow phenotype 

 69 



must be due to one type of homozygote, 

 yellow must be a heterozygote, and the 

 aborting individuals must be due to the other 

 type of homozygote. The gene symbols usu- 

 ally employed are not satisfactory here, for 

 we now must describe two effects for each 

 gene — color and viability. Moreover, the 

 allele which is dominant for the first effect 

 is recessive for the second, and vice versa. 

 This problem is solved by using base letters 

 with superscripts for each gene (Figure 

 6-1), where the base letter refers to one 

 trait and the superscript refers to the other 

 trait. Let the superscript / be the recessive 

 lethal effect of the gene dominant for yellow, 

 Y, and the superscript L be the dominant 

 normal viability effect of the allele recessive 

 for nonyellow, y. Accordingly, the Fi from 

 crossing two yellow mice (Y'y L X Y l y L ) are 

 1 Y'Y 1 (dies) :2 Y'y L (yellow) : 1 y L y L (non- 

 yellow). 



In both the snapdragon and mouse cases 

 described, death results from the presence 

 of a gene in homozygous condition. Genes 

 that kill the individual before maturity are 

 called lethal genes or lethals — those doing 

 so only when homozygous are recessive 

 lethals, and those acting in this way when 

 heterozygous are dominant lethals. Lethals 

 may act very early or very late in develop- 



yellow 

 Y l y L 



'/2Y 1 , Vay 1 



'AY'Y 

 dies 



ViY'y 1 - 

 yellow 



yellow 



Y 1 y 



VjY 1 , Viy L 



v 4 y L y L 



nonyellow 



figure 6-1. Results of matings between yel- 

 low mice. 



