The grandson 7 depicted in the pedigree of Figure 7.8 is afflicted by a lethal, X-linked recessive disease [25]. Problem 11 of Chapter 1 notes that if the carrier females for such a disease are fully fit, then they have a population frequency 4µ, where µ is the mutation rate to the disease allele. In view of this fact, demonstrate that the mother 4 has a chance of approximately 5/13 of carrying the disease allele.

Consequently, her next son has a chance of 5/26 of being affected.

(Hints: Either the grandmother 1 is a carrier, or the mother 4 is a new mutation and passes the disease allele to the grandson 7, or the grandson 7 is a new mutation. The presence of unaffected uncles 5 and 6 and an unaffected brother 8 modifies the probabilities of these contingencies. Because µ is very small, you may approximate the probability of a carrier female passing either the normal or disease allele as 1/2. You may also approximate the prior probability of a normal female or normal male as 1.)