GCSE Biology (AQA)

Topic Summaries

DNA synthesis and inheritance

Determining sex

  • Ordinary human body cells contain 23 pairs of chromosomes. 22 of these pairs are somatic cells that control characteristics. The other 1 pair are germ-line cells that carry the genes that determine sex. In females, the sex chromosomes are the same (XX), whereas in males, the sex chromosomes are different (XY).
    • Genotype ratio XX:XY = 1:1
    • Probability of offspring: XX = 50%, XY = 50%
  Mother
X X
Father X XX XX
Y XY XY

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