Lecture Notes: Sex Determination in Animals

Introduction

• Speaker shares personal context: wife is pregnant, expecting first child.
• Common question asked: "Is it a boy or a girl?"
• Assumption: 50/50 chance of having a boy or a girl due to human biology.
• Explanation of sex determination systems in different species.

Human Sex Determination

• Genetic Determination System (XY System)
  • Humans use the XY chromosome system.
  • Females: XX, Males: XY.
  • Sex determined by the father (X or Y chromosome from sperm).
  • 50/50 chance of being a boy or girl due to random sperm selection.

Other Genetic Sex Determination Systems

• Birds and Some Reptiles

  • Use the ZW system.
  • Males: ZZ, Females: ZW.
  • Sex determined by the mother (Z or W chromosome from egg).

• Ants (Haplodiploid System)

  • Males have no fathers; develop from unfertilized eggs (haploid).
  • Females develop from fertilized eggs (diploid).
  • Queen mates and stores sperm; fertilized eggs become females.
  • High division of labor in ant colonies.

Environmental Sex Determination

• Temperature-Based Determination

  • Seen in alligators and most turtles.
  • Sex determined by the temperature of the nest during a critical period.
  • Example: Painted turtles - warm temperatures produce females, cool produce males.
  • Mnemonic: "Hot chicks and cool dudes."

• Sequential Hermaphroditism in Clownfish

  • All start as males; mature into females.
  • Dominance hierarchy determines reproductive roles.
  • If dominant female dies, largest male becomes female.

• Location-Based Determination in Green Spoonworms

  • Sex determined by larva's settling location.
  • Open sea floor: becomes female.
  • Lands on another female: becomes male.

Unique Systems

• Whiptail Lizards
  • Nearly all-female species.
  • Eggs hatch into female clones.

Conclusion

• Sex determination varies widely across the animal kingdom.
• Different systems: genetic, environmental, or unique reproductive methods.
• Personal note: Expecting a baby boy.