Jeff’s father has Machado-Joseph Disease (MJD), a neurodegenerative condition caused by a rare autosomal dominant allele (M).

Jeff’s father has Machado-Joseph Disease (MJD), a neurodegenerative condition caused by a rare autosomal dominant allele (M). The M allele is 100% penetrant by age 80, but 40% of people with the dominant M allele show no symptoms yet by age 50. Jeff is 50 and has no symptoms. What is that chance that Jeff’s biological child will have symptoms of MJD by age 80?
a. 17.
b. 112.
c. 115.
d. 1/5.