Answer:
Phenylketonuria (PKU) is an autosomal recessive disorder caused by a defect in a gene which codes for phenylalanine digesting enzyme. It leads to the lack or deficiency of this enzyme in the body due to which phenylalanine accumulates in the body.
As it is an autosomal recessive disorder, it must require homozygous recessive condition in order to express the phenotype.
The hetrozygous organisms act as carrier of the defective gene however, they do not show the presence of the disease.
Let the alleles of the PKU be P (dominant) and p (recessive).
Father is hetrozygous or hybrid for PKU i.e have Pp as a genotype. So, the gametes formed would be P and p.
Mother is homozygous recessive i.e. have pp as her genotype. So, the gametes formed would be only p.
The cross is shown in the figure attached.
The probability of a child with PKU or pp as genotype = 2/4 = 0.5
