Sex Determination — How Chromosomes Decide 🌟

Why do some babies grow up to be boys and others girls? The secret lies in tiny packages of DNA called sex chromosomes. Let’s explore! 🔬🧬 :contentReference[oaicite:0]{index=0}

1. Early Clues

  • Henking’s discovery (1891): He noticed an “X body” during insect sperm formation; only half the sperm had it. Later scientists confirmed it was a chromosome, now called the X-chromosome. :contentReference[oaicite:1]{index=1}

2. Two Main Systems of Male Heterogamety

🦗 XO System (e.g., Grasshopper)

  • Every egg carries one \(X\).
  • Some sperm carry an \(X\); others carry none (\(O\)).
  • \(X\text{-sperm} + X\text{-egg} \rightarrow XX\) (female)
  • \(O\text{-sperm} + X\text{-egg} \rightarrow XO\) (male)
  • Males have one \(X\) plus autosomes; females have two \(X\) chromosomes. :contentReference[oaicite:2]{index=2}

🧑‍🔬 XY System (Humans, Many Insects & Mammals)

  • Males: \(XY\) (the \(Y\) is smaller). Females: \(XX\).
  • Males make two kinds of sperm — one with \(X\), one with \(Y\) — so they’re heterogametic. :contentReference[oaicite:3]{index=3}

3. Female Heterogamety in Birds 🐦

  • Females are \(ZW\) and lay two egg types: one with \(Z\), one with \(W\).
  • Males are \(ZZ\) and make identical sperm.
  • This flip (female makes different gametes) is called female heterogamety. :contentReference[oaicite:4]{index=4}

4. Human Sex Determination 👶

Humans have 23 chromosome pairs. Twenty-two pairs are autosomes. The last pair decides sex.

  • Females: \(XX\), Males: \(XY\).
  • \(50\%\) of sperm carry \(X\); \(50\%\) carry \(Y\).
  • All eggs carry \(X\).
  • \(X\text{-sperm} + X\text{-egg} \rightarrow XX\) → girl 👧
  • \(Y\text{-sperm} + X\text{-egg} \rightarrow XY\) → boy 👦
  • Chance is \(\frac{1}{2}\) for either sex each time.
  • The father’s sperm decides the baby’s sex, not the mother’s — so blaming moms is scientifically wrong. 💖 :contentReference[oaicite:5]{index=5}

5. Haplodiploid System in Honey Bees 🐝

  • Fertilised egg (diploid, 32 chromosomes) → female (queen or worker).
  • Unfertilised egg (haploid, 16 chromosomes) → male drone (parthenogenesis).
  • Drones make sperm by mitosis.
  • A drone has no father and cannot have sons, but he does have a grandfather and can have grandsons. 😮 :contentReference[oaicite:6]{index=6}

Important Concepts for NEET 📚

  1. Chromosomal systems \(XO\) & \(XY\) and how male heterogamety works. :contentReference[oaicite:7]{index=7}
  2. Female heterogamety in birds: \(ZW\) vs. \(ZZ\). :contentReference[oaicite:8]{index=8}
  3. Human sex is decided by the father’s \(X\) or \(Y\) sperm; probability is \(50\%\). :contentReference[oaicite:9]{index=9}
  4. Haplodiploid sex determination in honey bees (32 ♀ vs 16 ♂ chromosomes). :contentReference[oaicite:10]{index=10}
  5. Historical milestone: Henking’s identification of the X-chromosome. :contentReference[oaicite:11]{index=11}