Biology 9700 · AS & A Level · Passage of information from parents to offspring
Passage of information from parents to offspring — practice question
Phenylketonuria (PKU) is an inherited disorder that causes the concentration of the amino acid phenylalanine in the blood to increase. If a newborn baby is not treated, it may result in brain damage. Because of this, babies may be screened for PKU shortly after birth. In normal conditions, any surplus phenylalanine is changed into the amino acid tyrosine by the enzyme phenylalanine hydroxylase (PAH). PKU may arise from a recessive mutation in the gene that codes for PAH.
(a)[2]
Explain what is meant by a recessive mutation.
(b)[3]
With suitable symbols, fill in the diagram below to show how two parents without PKU can have children with PKU or without PKU.
(c)[3]
PKU may occur if a short section of the RNA made during transcription of the gene coding for PAH is lost. Suggest what effect this would have on the protein that is then produced.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “recessive trait shown only when two copies are present” …