(a) : This chart shows inheritance ofan autosomal recessive trait like phenylketonuria. An autosomal recessive trait may skip a generation. It appears in case of marriage between two heterozygous individuals $(Aa × Aa = 3\ Aa + 1\ aa)$, a recessive individual with hybrid $(Aa × aa = 2\ Aa + 2\ aa)$ and two recessive $(aa× aa = all\ aa)$. Phenylketonuria is an inborn, autosomal, recessive metabolic disorder in which homozygous recessive individual lacks the enzyme phenylalanine hydroxylase. The heterozygous individuals are normal but carriers.