Q4. Phenylketonuria is called an inborn error of metabolism. Which word in that phrase points to a gene, and which to an enzyme pathway?
“Inborn” points to the inherited cause, a mutation in the gene for phenyl alanine hydroxylase (NCERT page 69); “error of metabolism” points to the pathway that fails: the affected individual lacks the enzyme that converts phenylalanine into tyrosine (NCERT page 75). Beyond the NCERT text: both PAH copies carry variants, and enzyme activity may be very low rather than absent (MedlinePlus Genetics).
Q5. A carrier (Pp) and a person with phenylketonuria (pp) have a child. What is the chance that the child is affected?
The pp parent always passes on p; the carrier passes on p with chance ½. So the chance that the child is pp, affected, is ½.
Q6. List the effects of the single phenylketonuria mutation that NCERT names on pages 69 and 75.
NCERT names mental retardation (now called intellectual disability) and a reduction in hair and skin pigmentation (page 69), and the build-up of phenylalanine, its conversion into phenylpyruvic acid and other derivatives, and their excretion in urine (page 75). Beyond the NCERT text: these describe untreated disease, and “PKU can often be managed by following a diet that is low in phenylalanine” (MedlinePlus Genetics).
Q7. Assertion: phenylketonuria shows pleiotropy. Reason: phenylketonuria is inherited as an autosomal recessive trait. Are both true, and does the reason explain the assertion?
Both are true, but the reason does not explain the assertion. Pleiotropy comes from one mutated gene having several phenotypic effects through a metabolic pathway (NCERT page 69); the mode of inheritance (NCERT page 75) is a separate fact.