Question

A man suffering from a disease marries a normal woman and they have 5 daughters diseased and 3 normal sons. All the daughters inherit the disease. The disease is

• A. Sex linked recessive
• B. Sex linked dominant
• C. Autosomal dominant
• D. Sex limited

Answer 1

Answer: (B)

Sex linked dominant

According to the question, father is diseased and mother is normal. All the daughters of first filial are diseased while sons are normal which means that daughters have inherited the diease from the diseased father; the trait is sex linked. If it would have been an autosomal trait, the father would have transmitted the disease to the sons also. Further, since the daughters are diseased, not the sons; the trait is X-linked, not Y-linked. This is due to the fact that fathers transmit their X-chromosome to daughters. As shown, the mother is normal, hence the daughter must have inherited only one copy of affected allele from father. This means that the trait is expressed in heterozygous condition in the daughters of first filial generation, thus the trait is dominant. Correct answer is B.