Phenotypic Effects of Gene Action 



73 



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 6.6 



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figure 6-5. A pedigree of Polydactyly in man. 



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 6.6 



that the P gene produces in different affected 

 individuals in this pedigree. These may 

 have the normal number of fingers but have 

 extra toes, or they may have the reverse; 

 they may have different numbers of toes on 

 the two feet, or they may have extra fingers 

 on one hand and the normal number on the 

 other. The expression of Polydactyly, so far 

 as the number of extra digits is concerned, is 

 clearly quite variable. Accordingly, since it 

 is possible to have no expression on one 

 limb of an individual known to be Pp it must 

 also occur that, on occasion, expression fails 

 on all four limbs of an individual with this 

 genotype. 



The ability of a given gene or gene com- 

 bination to be expressed phenotypically in 

 one way or another is called penetrance. 

 The P gene in heterozygous condition, there- 

 fore, has a penetrance of less than 100%, 

 sometimes failing to produce any detectable 

 phenotypic effect when present. Although 

 a polydactylous person is certain to carry P, 

 a normal phenotype can represent either the 

 Pp or pp genotype. Since Polydactyly is 

 rare it is usually quite safe to score as pp 

 the genotype of a normal individual who 

 marries into a line of descent containing P. 



The expression of P when heterozygous is 

 not only quite variable with respect to the 

 number and position of extra digits, but 

 further variability of expression is demon- 

 strated by the different degrees of develop- 

 ment which the extra digits show. The term 

 expressivity is used to refer to the kind or 

 degree of effect produced by a penetrant 

 genotype. In individuals where P is non- 

 penetrant when heterozygous there is no ex- 

 pressivity, and when P is penetrant its ex- 

 pressivity is variable. 



What factors are involved in the produc- 

 tion of variable penetrance, or, in cases of 

 penetrance, of variable expressivity? A 

 study of a genetically uniform line of guinea 

 pigs shows that Polydactyly occurs more fre- 

 quently in the litters from younger than from 

 older mothers. In this case the physiological 

 changes accompanying age modify pene- 

 trance. In another case, a genetically uni- 

 form line of Drosophila flies shows a greater 

 percentage of penetrance of an abnormal 

 abdomen phenotype when moisture content 

 during development is high than when it is 

 low. Both of these examples illustrate that 

 variations in penetrance can be produced by 

 variations in the environment of different 



