Question

Which of the following most accurately explains the cause for the abnormal number of chromosomes during human reproduction that can result in Down's syndrome, Turner's syndrome, or Klinefelter's syndrome?

• A. The occurrence of non-disjunction of homologous chromosomes during meiosis
• B. Duplicative production of extra chromosomes during DNA replication
• C. The abnormal pairing of non-homologous chromosomes during meiosis I
• D. The selective loss of particular chromosomes from sex cells after formation of the mature gamete

Answer 1

The correct option is A The occurrence of non-disjunction of homologous chromosomes during meiosis

Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. There are three forms of non-disjunctions - failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. Non disjunction results in daughter cells with abnormal chromosome numbers (aneuploidy). All the given diseases are examples of aneuploids.