Molecular Genetics Problem 2

Molecular Genetics: Problem 2

Pseudohypertropic muscular dystrophy is a disorder that causes gradual deterioration of the muscles. It is seen only in boys born to apparently normal parents and usually results in death in the early teens. (a) Is pseudohypertrophic muscular dystrophy caused by a dominant or recessive allele? (b) Is its inheritance sex-linked or autosomal? (c) How do you know? Explain why this disorder is always seen in boys and never girls.

(a) Pseudohypertropic muscular dystrophy is a recessive allele because if it were dominant then the heterozygous female would die before she was sexually mature, and thus could not pass on the trait

(b) Pseudohypertropic muscular dystrophy is a sex-linked trait because it only occurs in males.

(c) It is never seen in girls because for a girl to be homozygous recessive (and express the trait) XmXm, her father would have to be hemizygous recessive XmY) and her mother would have to be heterozygous or homozygous recessive (XmXM or XmXm). It is impossible for a homozygous recessive male or a homozygous recessive female to have children because they will die before being old enough to have children.

The Punnett square above shows that in a cross between two apparently normal parents, but the mother is a carrier, three children will be normal but one, always a boy will inherit the disease.