Sex Determination and Genetic Diagrams

This lesson covers sex determination by sex chromosomes, sex-linked inheritance (X-linked conditions), and family pedigree diagrams. This is essential content for Edexcel GCSE Biology (1BI0) Topic 3: Genetics.

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Sex Chromosomes

In humans, biological sex is determined by the sex chromosomes.

• Humans have 23 pairs of chromosomes (46 in total).
• 22 pairs are autosomes (non-sex chromosomes) — these are the same in males and females.
• 1 pair is the sex chromosomes — these differ between males and females.

Sex	Sex Chromosomes
Female	XX (two X chromosomes)
Male	XY (one X chromosome and one Y chromosome)

How Sex Is Determined

• Egg cells (from the mother) always carry an X chromosome.
• Sperm cells (from the father) carry either an X or a Y chromosome.
• If a sperm carrying an X chromosome fertilises the egg → the offspring is XX (female).
• If a sperm carrying a Y chromosome fertilises the egg → the offspring is XY (male).

Therefore, the sex of the offspring is determined by the father's sperm, not by the mother's egg.