Question

The given pedigree depicts the inheritance of a rare hereditary disease affecting muscles. What is the most likely mode of inheritance of this disease?

• A. Autosomal dominant
• B. Autosomal recessive
• C. X-linked dominant
• D. X-linked recessive
• E. Y-linked

Answer 1

The correct option is D X-linked recessive

The pedigree shows that 50% of sons of normal parents are affected but all daughters are normal which means that the allele for the disease should be present in the recessive condition. Father expresses all recessive traits since he has one copy of both X and Y chromosome; the disease is present in the mother on her X-chromosome who serves as carrier mother and passes the trait to 50% of sons. If it was an autosomal trait; the progeny would have shown it in equal frequency irrespective of their sex. Further, the progeny of the affected father and normal mother is normal which means that it is an X-linked recessive disorder. the affected father has passed the affected X-chromosome to daughters who receive the second copy of X-chromosome from the normal mother and hence the disease is not expressed in a heterozygous daughter. But these daughters will serve as carriers of the disease. Duchenne muscular dystrophy is a disease that causes progressive weakening of the muscles. It is a sex-linked disorder. The allele for the disorder can express itself only in homozygous genotype, not in the heterozygous genotypes. As we know that the allele which can express itself in homozygous genotypes but not in heterozygotes are the recessive alleles. Hence, the correct answer is option D.