Chapter 10 



PLEIOTROPISM, 



PENETRANCE AND EXPRESSIVITY 



Pleiotropism 



h: 



"ow MANY phenotypic effects 

 does a gene have? In com- 

 . paring the phenotypes of two 

 genetically different lines of rabbits, one chin- 

 chilla (c''''c'''') and one white {cc) there is only 

 one apparent phenotypic difference — the 

 presence vs. the absence of coat pigment. 

 Saying that the c"'' gene has many effects — 

 to produce pigment on the ears, on the trunk, 

 on the limbs, on the tail, and to produce no 

 pigment in the intestine, none in the pancreas, 

 etc., complicates the description without add- 

 ing any more meaning. In the case of 

 Himalayan rabbits (cV), the coat itself is 

 usually variegated, being black at the extremi- 

 ties and white elsewhere (Chapters 1 and 9). 

 Does that mean this allele has a different kind 

 of action in different parts of the coat? No. 

 For the pigment differences are attributed 

 rather to the effect of temperature upon the 

 action (at less than 34° C) or inaction (at 

 more than 34° C) of an enzyme, produced by 

 this genotype, which transforms nonpig- 

 mented into pigmented material. Thus, where 

 the body temperature is less than 34° C, as it 

 is at the extremities, pigment is produced, 

 while on the warm parts of the body no pig- 

 ment is formed because of heat inactivation 

 of this enzyme. The Himalayan pattern is 

 attributed, then, to an allele whose single effect 

 is modified by differences in the environment. 

 In discussing the MN blood groups (Chap- 

 ters), it was stated that M produces M antigen 

 while M' produces N antigen. In this case 



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it might be thought first that the gene has 

 two effects, since one allele produces M but 

 not N antigen and the other produces N but 

 not M antigen. But, again, the lack of an 

 effect cannot be counted as an effect, and 

 it is simpler to think of this gene as having 

 the ability to produce a single antigen whose 

 specific nature depends upon the particular 

 allele that is present. This, then, is a case in 

 which one trait may be affected in different 

 qualitative ways. In Chapter 9 we discussed 

 a multiple allelic series for eye color in 

 Drosophila. There also we were dealing with 

 one trait, in that case eye color pigment, differ- 

 ent alleles affecting it in an apparently quanti- 

 tative manner. In the last Chapter we also 

 considered a case of inheritance in the snap- 

 dragon. But there again we were dealing 

 not with two different effects of a gene, one 

 on pigmentation and the other on viability, 

 but rather on the single activity, chlorophyll 

 production, which had lethality as the con- 

 sequence of its failure. 



The question posed refers then to whether 

 or not a gene has effects upon two or more 

 traits which are apparently independent of, 

 or unrelated to, each other in their origin. 

 Such effects of a gene we can call multiple, 

 manifold, or pleiotropic effects. 



None of the examples just mentioned dealt 

 with such multiple effects. However, we have 

 already discussed a case which seems to fulfill 

 these requirements. This is the case of the 

 yellow mouse. The allele which produces 

 yellow coat color as a dominant effect also 

 has a recessive lethal effect. On the presump- 

 tion that homozygotes for this allele would 

 have had yellow body color had they survived, 

 and on the basis that there is no obvious 

 relation between coat color and viability, it 

 could be concluded that this is a case where 

 the gene shows pleiotropism. 



Studies have been made to test the idea 

 that, in general, genes are pleiotropic. The 

 procedure in one of these studies ^ was as 

 ^ Based upon Th. Dobzhansky's work. 



