Question

Study the pedigree chart given below. What does it show?

• A. This pedigree chart is wrong as this is not possible.
• B. Inheritance of a recessive sex-linked disease like haemophilia.
• C. Inheritance of a sex-linked inborn error of metabolism like phenylketonuria.
• D. Inheritance of a condition like phenylketonuria as an autosomal recessive trait.

Answer 1

Answer: (D)

Inheritance of a condition like phenylketonuria as an autosomal recessive trait.

According to pedigree, offspring of normal parents show the trait which means that the trait is recessively present in the normal parents; the trait is recessive. In a first filial generation, both son and daughter show the trait. As we know that a sex-linked recessive trait can be expressed in either sex not in both; the X-linked trait is expressed in male progeny while the female progeny serves as a carrier and the Y-linked trait is expressed in sons only. Presence of trait in both son and daughter in F1 generation confirms that it is an autosomal trait. The second parent generation must have a carrier parent and a recessive homozygous parent to pass it to 1/3rd of progeny.

The correct answer is D.