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.

Verified step by step guidance

1. Identify the inheritance pattern: Pseudohypertrophic muscular dystrophy primarily affects boys and is usually transmitted by unaffected mothers. This pattern suggests a sex-linked inheritance, specifically linked to the X chromosome.
2. Determine the type of allele: Since the disorder manifests in individuals who have only one copy of the affected gene (mostly males who have only one X chromosome), and females (with two X chromosomes) are typically carriers without showing symptoms, the allele is likely recessive.
3. Examine why it affects males more: Males have one X and one Y chromosome. If the X chromosome carries the recessive allele for this disorder, there is no corresponding allele on the Y chromosome to counteract it, leading to the expression of the disorder.
4. Understand carrier status in females: Females have two X chromosomes. Even if one X chromosome carries the recessive allele, the presence of a normal allele on the other X chromosome generally prevents the development of the disorder, making females carriers.
5. Explain rarity in females: For a female to exhibit symptoms of pseudohypertrophic muscular dystrophy, she must inherit the recessive allele from both parents (one from an affected or carrier father and another from a carrier mother). This scenario is much less likely, hence the disorder's rarity in girls.

Key Concepts

X-Linked Recessive Inheritance

Pseudohypertrophic muscular dystrophy, also known as Duchenne muscular dystrophy, is primarily caused by mutations in the dystrophin gene located on the X chromosome. This type of inheritance means that males, who have only one X chromosome, are more likely to express the disorder if they inherit the mutated allele. In contrast, females have two X chromosomes, so a mutation would need to be present on both for the disorder to manifest, making it rare in girls.

Alleles and Dominance

Alleles are different forms of a gene that can exist at a specific locus on a chromosome. In the case of pseudohypertrophic muscular dystrophy, the disorder is caused by a recessive allele. This means that an individual must inherit two copies of the recessive allele (one from each parent) to express the disorder, which is why it is predominantly seen in males who inherit the single affected X chromosome.

Genetic Carrier Status

Carriers are individuals who possess one copy of a recessive allele but do not exhibit the associated phenotype. In the context of pseudohypertrophic muscular dystrophy, females can be carriers if they have one normal and one mutated X chromosome. They typically do not show symptoms because the normal allele can compensate for the defective one, allowing them to pass the mutated allele to their offspring without being affected themselves.