Phenylketonuria (PKU) is an inherited disorder caused by a recessive allele, ddd. The dominant allele is DDD.
Sarah and her parents do not have PKU. Sarah's brother has PKU.
Sarah is planning to have a child with a man who has PKU.
Explain the probability that their first child will have PKU.
You may use genetic diagrams in your answer.
43 exam-style questions on Edexcel GCSE Biology 3.3 Inheritance, covering 3.3.1 Gregor Mendel and the basis of genetics, 3.3.2 Alleles and inherited characteristics, 3.3.3 Monohybrid inheritance, 3.3.4 Determination of sex at fertilisation, 3.3.5 Outcomes from monohybrid crosses, 3.3.6 Inheritance of ABO blood groups, 3.3.7 Inheritance of sex-linked disorders, and 3.3.8 Multiple genes and phenotypic features. Each one has a worked solution and a mark scheme showing where the marks go.