Question

Study the pedigree chart. What does it show?

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

Answer 1

The correct option is A Inheritance of a condition like phenylketonuria as an autosomal recessive trait

As is visible trait skips generations; the unaffected parents have diseased progeny which means that the trait is present in the recessive condition in parents. If it was a dominant trait; the parents would have expressed it. An almost equal number of affected males and females occur which means that trait is not dependent on the sex of the individual; it is an autosomal trait which makes options B and C wrong. Thus, it is an autosomal recessive trait. Since 25% sons and 50% of daughters are affected; both parents are the recessive carriers and have passed the trait to progeny. Again in the second-parent generation; both carrier parents have transferred the disease to sons which means that the pedigree is possible. This makes option D wrong. Phenylketonuria (PKU) is an inborn metabolic error with impaired metabolism of phenylalanine causing its increased blood level. It is caused by the defective gene for phenylalanine hydroxylase which is inherited in an autosomal recessive manner.

So, the correct answer is 'Inheritance of a condition like phenylketonuria as an autosomal recessive trait'.