Phenotypic Effects of Gene Action 



71 



figure 6-4. Silhouettes showing various types of human red blood cells: normal, in 

 normal homozygote {A), sickle cell trait, in mutant heterozygote (B), sickle cell disease, 

 in mutant homozygote (C). 



come translucent and die. Using suitable 

 techniques, one can compare the kinds and 

 amounts of chemical substances in the blood 

 fluid of normal larvae and pupae with those 

 found in the recessive lethal homozygotes 

 (Figure 6-3). Some substances are found - 

 in equal amounts in both genotypes (pep- 

 tide III), others are more abundant in the 

 lethal than in the normal individual (pep- 

 tide I, peptide II, and proline), still others 

 are less abundant (glutamine) or absent 

 (cystine) in the lethal. Thus it is clear that 

 pleiotropism also occurs at the biochemical 

 level. 



In the case of the yellow mouse, the allele 

 producing yellow coat color as a dominant 

 effect also has a recessive lethal effect. On 

 the presumption that homozygotes for this 

 allele would have yellow body color had they 

 survived, and on the basis that there is no 

 obvious relation between coat color and via- 

 bility, it could be concluded that this too is 

 a case of pleiotropism. 



The coat color of Himalayan rabbits 

 (cV') is usually mosaic; or black at the ex- 

 tremities and white elsewhere (Chapter 1). 

 Has this allele different effects on the same 

 color trait in different parts of the coat? 

 Because individuals with this genotype are 

 completely black when grown under cold 



2 Based upon work of E. Hadorn. 



temperatures, we suspect that the gene has 

 only one effect. This hypothesis is sup- 

 ported by the finding that this genotype pro- 

 duces an enzyme, necessary for pigment for- 

 mation, which is temperature sensitive, being 

 inactivated by temperatures above about 

 34° C. Thus, in a cool climate, the body 

 temperature is less than 34° C at the ex- 

 tremities, and pigment is produced there; on 

 the warm parts of the body no pigment is 

 formed because the enzyme is inactivated by 

 heat. The Himalayan pattern is attributed, 

 then, to a single product of gene action 

 which, because it is subject to modification 

 by the environment, can result in two dif- 

 ferent phenotypic alternatives of the same 

 trait. 



A genetic disease in man called sickle-cell 

 anemia is due to homozygosity for a certain 

 allele. This disease involves the following 

 effects, either singly or in any combination: 

 anemia, enlarged spleen, skin lesions, heart, 

 kidney, and brain damage. As a conse- 

 quence, homozygotes for the gene for sick- 

 ling usually die as adolescents or young 

 adults; this allele, therefore, almost always 

 acts as a recessive lethal. 



It is also found that the red blood cells 

 of these homozygotes may become sickle- 

 shaped instead of being disc-shaped (Figure 

 6-4). Sickle-shaped cells can clump to- 

 gether and clog blood vessels in various parts 



