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Sex Chromosomes: X and Y

44 + XY
X
Y
44 + XX
X
X
Humans have 23 pairs of chromosomes. The 23rd pair are the sex chromosomes.

Meiosis: Cell Division for Gamete Formation

44 + XY
44 + XX
Meiosis reduces chromosome number by half
22 + X
22 + Y
Sperm Cells
22 + X
22 + X
Egg Cells

Gametes: Sperm and Egg Cells

Produces two types of sperm:
22 + X
(50% of sperm)
22 + Y
(50% of sperm)
Produces only one type of egg:
22 + X
(100% of eggs)
22 + X
(100% of eggs)
The father's sperm determines the child's gender because only males carry the Y chromosome.

Fertilization: Union of Sperm and Egg

22 + X
Sperm
+
22 + X
Egg
44 + XX
OR
22 + Y
Sperm
+
22 + X
Egg
44 + XY

Gender Determination Outcomes

44 + XX
X
X
44 + XX
X
X
44 + XY
X
Y
44 + XY
X
Y
50% probability for daughter (XX) and 50% for son (XY)

Sex Chromosomes

23 pairs total: 22 autosomes + 1 sex chromosome pair.

  • Female (XX): two X chromosomes
  • Male (XY): one X, one Y

Y chromosome carries SRY gene → triggers male development.

📊 Did you know? X chromosome has ~900 genes, Y has ~70.

Meiosis = special cell division in testes/ovaries.

  • Halves chromosome number: 46 → 23
  • Each gamete gets one sex chromosome
  • Male: sperm with X or Y
  • Female: eggs always X
🎯 Key: After meiosis, gametes are haploid.

Sperm (father):

  • 50% X (22+X)
  • 50% Y (22+Y)

Eggs (mother):

  • 100% X (22+X)

→ Father determines gender because only he contributes Y.

Fertilization combines sperm (23) + egg (23) → zygote (46).

  • X sperm + X egg = XX (♀)
  • Y sperm + X egg = XY (♂)
🧬 SRY gene on Y chromosome starts male development at ~7 weeks.

Offspring gender probabilities:

  • Daughter (XX): 50%
  • Son (XY): 50%

Reason: Equal number of X and Y sperm.

🌍 Real-world: Slight bias (~105 males : 100 females) at birth.