12.6 Sex Determination

Sex determination refers to the biological mechanisms that establish whether an organism develops as male or female. In most animals, sex is determined by specific chromosomes called sex chromosomes (or allosomes), while the remaining chromosomes are called autosomes.

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Types of Sex Chromosomes

Organisms carry two types of chromosomes:

• Autosomes — non-sex chromosomes that carry genes for general body traits.
• Sex chromosomes (Allosomes) — chromosomes that determine the sex of an individual.

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1. The XX-XY System (Humans and Drosophila)

This is the most commonly studied system.

• Homogametic organisms produce only one type of gamete with respect to sex chromosomes (females produce only X-bearing eggs).
• Heterogametic organisms produce two types of gametes (males produce X-bearing sperm and Y-bearing sperm).

The father determines the sex of the offspring:

• If a Y-bearing sperm fertilizes the egg → Male (XY)
• If an X-bearing sperm fertilizes the egg → Female (XX)

$\text{XX (mother)}\times \text{XY (father)}\to \frac{1}{2}\text{ XX (female)}+\frac{1}{2}\text{ XY (male)}$

This system is found in humans, Drosophila (fruit fly), and many other mammals.

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2. The ZZ-ZW System (Birds and Butterflies)

In this system, the sex chromosome letters are changed to Z and W to distinguish it from the XX-XY system.

Key difference from XX-XY: In the ZZ-ZW system, the mother determines the sex of the offspring because she is heterogametic.

• If a Z-bearing egg is fertilized → Male (ZZ)
• If a W-bearing egg is fertilized → Female (ZW)

This system is found in birds (e.g., chickens), butterflies, and some fish.

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3. The XO-XX System (Grasshoppers — Protenor Type)

In this system, there is no Y chromosome. The 'O' represents the absence of a second sex chromosome.

• Males produce two types of sperm: X-bearing and O-bearing (no sex chromosome).
• If an X-bearing sperm fertilizes the egg → Female (XX)
• If an O-bearing sperm fertilizes the egg → Male (XO)

This system is found in grasshoppers and some other insects.

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4. The SRY Gene — Molecular Basis of Maleness

At the molecular level, the SRY gene (Sex-determining Region Y) located on the Y chromosome is the master switch for male development in humans.

• The SRY gene encodes a transcription factor (a DNA-binding protein).
• It directs the undifferentiated (bipotential) gonads to develop into testes.
• In the absence of SRY, the default developmental pathway leads to the formation of ovaries.
• Testes then produce testosterone, which drives the development of male secondary sexual characteristics.

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Summary Comparison Table