Question

Study the two cases carefully. What would be the correct interpretation of the two cases ?

Case	Mother	Father	Children
Case I	With disease	Normal	Sons always with diseases
Case II	With disease	Normal	Sons and daughters could show disease

• A. Case I : $X$-linked recessive disease
  Case II : Autosomal recessive disease
• B. Case I : $Y$-linked recessive disease Case II : $X$-linked recessive disease
• C. Case I : and II : $X$-linked recessive disease
• D. Case I : $X$-linked dominant disease
  Case II : Autosomal dominant disease

Answer 1

Answer: (A)

Case I : It is an example of sex-linked recessive disease. The sex-linked inheritance follows criss-cross inheritance where father does not pass the sex-linked allele of a trait to his son. The same is passed to the daughters from the where it reaches the grandson. In the given case, the mother is affected, father is normal and sons are always diseased (just as mother). Therefore, $X$- linked recessive allele is being passed from the mother to the sons. The recessive allele expresses itself in males because $Y$ chromosome does not carry any corresponding allele. So, mother who is homozygous for the recessive trait, transfers the trait to all the sons.
Case II : Here, both sons and daughters have the possibility of getting disease thus the gene is autosomal. If we consider this gene dominant then there are two possibilities.

Possibility $1$ can match with the given case. If we consider the gene for the disease to be recessive then there are two possibilities :

Possibility $2$ matches with the given case. But if we look at options, option (a) is best suited regarding both the cases.