Biology 9700 · AS & A Level · Passage of information from parents to offspring
Passage of information from parents to offspring — practice question
Phenylketonuria (PKU) is a genetic disorder that causes the amino acid phenylalanine to build up at a higher concentration in the blood. If a newborn baby is not treated, this may result in brain damage. Because of this, babies may be screened for PKU shortly after birth. Normally, any excess 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 the meaning of a recessive mutation.
(b)[3]
Using suitable symbols, finish the diagram to show how two parents who do not have PKU can produce children with PKU or without PKU.
(c)[3]
PKU can occur if a short length of the RNA made during transcription of the gene coding for PAH is lost. Suggest what effect this would have on the protein later produced.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “recessive: shown only in homozygote” …