Question

Marfan syndrome is an autosomal dominant disorder that affects connective tissue in the skeletal system and skin. The figure shows a family affected by Marfan syndrome.

grandparent 1: does not have marfan syndrome. grandparent 2: has marfan syndrome. Grandparent 3: has marfan syndrome. grandparent 4: does not have marfan syndrome.
parent 1: does not have marfan syndrome. parent 2: has marfan syndrome. child 1: question mark.

What is the probability that Child 1 will have Marfan syndrome?
A
25%
B
50%
C
75%
D
100%

Answer 1

Answer: B — 50%.

Reason: Let M = Marfan allele (dominant), m = normal. Each affected grandparent married an unaffected one, so their affected children must be heterozygous (Mm). Parent 1 is unaffected → mm. Parent 2 is affected but had an unaffected parent → must be Mm. Cross mm × Mm gives 50% Mm (affected) and 50% mm (unaffected).