Inheritance of Sex

Human Chromosomes

  • Humans have 23 pairs of chromosomes in body cells.

  • 22 pairs → control other inherited characteristics.

  • 1 pair = sex chromosomes (determine biological sex).

Male vs Female Chromosomes

  • Males = XY (two different chromosomes).

  • Females = XX (two identical chromosomes).

  • ⚠ You must learn this distinction for the exam!

How Sex is Inherited

  • Gametes (sex cells):

    • Female gametes → always X.

    • Male gametes → 50% X, 50% Y.

  • Punnett Square:

X (egg)

X (egg)

X (sperm)

XX (girl)

XX (girl)

Y (sperm)

XY (boy)

XY (boy)

Outcomes

  • 50% XX (female)

  • 50% XY (male)

  • Ratio = 1 : 1 (male : female).

  • ✅ Remember: These are probabilities. In reality, families could have all boys, all girls, or a mix.

Exam Tip 🚨

If the question asks: “Why are males more likely to inherit certain genetic conditions?” → It’s because males only have one X chromosome. If there’s a faulty allele on the X, they don’t have another copy to mask it (unlike females with XX).