Question

Pseudohypertrophic muscular dystrophy is an inherited disorder that causes gradual deterioration of the muscles. It is seen almost exclusively in boys born to apparently unaffected parents and usually results in death in the early teens. Is this disorder caused by a dominant or a recessive allele? Is its inheritance sex-linked or autosomal? How do you know? Explain why this disorder is almost never seen in girls.

Short answer

A recessive allele causes the disease because the parents would have been affected if a dominant allele had caused it.

The disease is a sex-linked trait because it affects only males.

The disease is not seen in girls because it is a rare case that girls would receive recessive alleles from both parents.

Step-by-step solution

Pseudohypertrophic muscular dystrophy caused by a recessive allele

Alleles are the alternative forms of a gene. All genes have two alleles: one dominant and the other recessive. The recessive allele is expressed only in homozygous conditions.

However, the dominant allele is expressed in both heterozygous and homozygous conditions. In a heterozygous condition, the dominant allele masks the expression of a recessive allele.

A recessive allele causes the disease and can be transmitted from an unaffected parent to a diseased son. If a dominant allele causes the disease, then either of the parents would have been affected by the disease.

Explanation for pseudo hypertrophic muscular dystrophy

Disorders caused by sex-chromosomes (X or Y) are called sex-linked traits, and those caused by autosomes and not sex chromosomes are called autosomal traits.Sex chromosomes cause pseudo hypertrophic muscular dystrophy because this disease occurs exclusively in males and not females.

Thus, it exhibits a sex-linked inheritance. If it were an autosomal trait, both males and females would have an equal chance of inheritance.

Disease not observed in females

Pseudohypertrophic muscular dystrophy is exclusively observed in males.As a recessive allele causes this disease, it is never seen in females. They would have to inherit two recessive alleles from each parent to acquire the disease.

Males who inherit recessive alleles on their X chromosome do not survive and die before they attain puberty. Thus, the chance that girls would receive both recessive alleles is very case. As a result, the disease is not seen in girls.