Meiosis and chromosome mutationsEdexcel A-Level Biology B: Mind map

Exam questions on Meiosis and chromosome mutations

1. A fruit fly, Drosophila, has 8 chromosomes in each of its body cells. Sperm are made by meiosis in the testes of the male fly.
   Explain why meiosis I, rather than meiosis II, halves the chromosome number.2 marks
2. A baby girl is born with Down's syndrome. A karyotype of her cells shows 47 chromosomes, including three copies of chromosome 21. Both of her parents have a normal karyotype of 46 chromosomes.
   Explain how non-disjunction during meiosis could have caused this baby's condition.2 marks
3. Many patients with chronic myeloid leukaemia have a Philadelphia chromosome in their white blood cell precursors. It forms when a piece of chromosome 22 breaks off and joins chromosome 9, while a piece of chromosome 9 joins chromosome 22. The change is found in the leukaemia cells but not in the patient's other body cells, such as skin cells.
   Explain why the Philadelphia chromosome is unlikely to be passed on to the patient's children.3 marks