Pleiotropism, Penetrance and Expressivity 



71 



5.5 

 6.6 



oh5^6 



o 



fl^k 6 



FIGURE 10-3. A pedigree of Polydactyly in man. 



6.6 



5.5 6.6 6.6 



6.6 5.5 5.5 



5.6 

 6.7 



i5c55i 



6.6 

 6.6 



Penetrance and Expressivity 



One of the reasons for the ease with which 

 the principles of transmission genetics were 

 estabhshed is the fact that each of the geno- 

 types used expressed itself repeatedly in 

 approximately the same way, despite the nor- 

 mal fluctuations of the environment. We 

 shall refer to the ability of a genotype or of 

 its parts to be expressed phenotypically in 

 one way or another as penetrance. Most genes 

 studied up to now are fully penetrant. 



Consider a pedigree for Polydactyly (Figure 

 10-3), a rare condition in human beings in 

 which individuals may have more than five 

 digits on a limb. The topmost female was 

 aff"ected, having five fingers on each hand, 

 but six toes on each foot. Her husband was 

 normal with respect to this trait. This couple 

 had five children, of whom three were affected. 

 This result suggests that Polydactyly is due 

 to a single dominant gene, P, so that the par- 

 ents would be, then, mother Pp, father pp. 

 Consistent with this hypothesis is the result of 

 the marriage of one of their aff'ected daughters 

 to a normal man which produced two sons, 

 one of whom was aff"ected, and this affected 

 son, in turn, had five children including some 

 aff'ected and some unaffected. 



But examine now the left side of this pedi- 

 gree. Shown here is the first-born son who 

 was unaffected, yet had an affected daughter. 

 How may this be explained? It might be 

 supposed that this son was genotypically pp 

 and that his daughter was a mutant individual, 

 Pp, derived from an egg or a sperm in which 

 the p gene underwent mutation to the P 

 allele. The following reasoning argues 

 against this interpretation, however. It was 

 noted already that Polydactyly is rare, so 

 that mutations from p to P must be still more 

 rare. Therefore, the chance that such a mu- 

 tation will occur in a sex cell of one of two 

 normal parents in this pedigree is very small. 

 Examination of other pedigrees for Polydac- 

 tyly also reveals other cases in which two 

 normal individuals have an affected child. 

 It is extremely improbable, then, that such a 

 rare mutation, if it occurs at random among 

 normal individuals, would occur so often 

 among the normal ones in Polydactyly pedi- 

 grees. 



A different explanation is that the first- 

 born son was in fact Pp but that the P was 

 not penetrant in him, though it was in his 

 daughter. This interpretation is supported 

 by the kind of expression the P gene produced 



