In humans, nearsightedness and phenylketonuria are both inherited as single-locus autosomal recessive traits that assort independently. Assume that a family has been identified in which both parents were phenotypically normal, but their first child was phenylketonuric and their second child developed nearsightedness.

a. What is the probability that their next child will show both defects? Show your work.

b. What is the probability that their next child will exhibit neither defect? Show your work.