Model Assignment:
Scenario / Puzzle
Biology B1XX
Biology and Human Affairs
Microtheme #3
Instructions
As described in class, phenylketonuria (PKU) is a rare inherited metabolic disorder caused by a recessive allele. PKU patients lack an enzyme necessary for the breakdown of the amino acid phenylalanine, a normal dietary component. Excessive amounts of phenylalanine affect the development of the brain in young children, leading to mental retardation. At birth, however, a PKU baby has a normal brain. If diagnosed at birth, PKU can successfully be treated by keeping the child on a special diet very low in phenylalanine. When the child is six years old, his brain is assumed to be fully developed and he can be returned to a normal diet.
A young woman with PKU had successfully been treated by dietary restriction in childhood and was now on a normal diet. Her first baby, however, was born severely retarded, with a abnormally small brain and skull (microencephaly). What do you think had happened? Your answer should include a description of the baby's probable genotype(s) and an explanation of why the woman herself was not born mentally retarded.
Length
One page double-spaced.
