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CHAPTER 10 



in the affected individuals in this pedigree. 

 These individuals may have the normal num- 

 ber 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 P gene, therefore, has a penetrance of 

 less than 100%, sometimes failing to produce 

 any detectable phenotypic effect when pres- 

 ent. So, while 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 indi- 

 vidual who marries into a line of descent con- 

 taining P. 



It has already been mentioned that the way 

 that P is expressed in an individual is quite 

 variable with respect to the number and posi- 

 tion of extra digits. Further variabiUty of 

 expression is demonstrated by the different 

 degrees of development which the extra digits 

 show. The term expressivity is used to refer 

 to the kind of effect produced by a genotype 

 when it is penetrant. So, in individuals 

 where P is nonpenetrant there is no expres- 

 sivity, and when P is penetrant its expressivity 

 is variable. 



What factors are involved in the production 

 of variable penetrance, or, in cases of pene- 

 trance, of variable expressivity? A study of a 

 genetically uniform line of guinea pigs showed 

 that Polydactyly occurred more frequently 

 in the litters from younger than from older 

 mothers. In this case the physiological 

 changes accompanying age modified pene- 

 trance. In another case, a genetically uni- 

 form line of Drosophila flies showed a greater 



percent of penetrance of an abnormal abdo- 

 men phenotype when moisture content dur- 

 ing development was high than when it was 

 low. These are both examples of how varia- 

 tions in penetrance can be produced by varia- 

 tions in the environment of different indi- 

 viduals of essentially identical genotype. 



You are already familar with the effect of 

 variations in genotype upon penetrance, 

 under essentially constant environmental 

 conditions. Remember that the penetrance 

 of an allele may depend upon the nature of 

 its partner allele in cases of complete or par- 

 tial dominance, and that the penetrance of 

 one or a pair of alleles may be modified by 

 its epistatic-hypostatic relations to nonalleHc 

 genes (Chapter 7). Similarly, it can be shown 

 that variable expressivity may be the conse- 

 quence of differences in either or both the 

 environment and the genotype. 



Several additional points should be made. 

 The terms penetrance and expressivity were 

 used to compare the phenotypic events which 

 occur in different individuals. That is, once 

 any phenotypic expression occurred within 

 an individual, the genotype was said to be 

 penetrant, and all other phenotypic compari- 

 sons between penetrant individuals were 

 considered matters of expressivity. In fact, 

 however, one can also correctly speak about 

 penetrance within an individual in those cases 

 where the particular genotype has two or 

 more occasions to express itself. Thus, for 

 example, the gene for Polydactyly has two 

 apparently equal chances to be penetrant in 

 the case of the hands, and two apparently 

 equal chances to be penetrant in the case of 

 the feet. So the genotype may be penetrant 

 in one hand (six fingers) and not in the other 

 (five fingers), it may be penetrant in the feet 

 (6.6) and not in the hands (5.5). When differ- 

 ences in penetrance (or expressivity) are 

 shown by essentially duplicate parts of the 

 same individual (one hand having seven and 

 the other six digits, or one hand having one 

 large extra digit and the other, one small ex- 



