Q.Explain the mechanism of 'sex determination' in birds. How does it differ from that of human beings?

Birds use ZZ (male) / ZW (female) — the female is heterogametic, so the egg decides sex. Humans use XX (female) / XY (male) — the male is heterogametic, so the sperm decides sex.

Concept. Sex determination is governed by sex chromosomes. The mechanism differs among organisms in which sex produces two kinds of gametes (the heterogametic sex).

Mechanism in birds (ZZ-ZW type).

• The sex chromosomes are named Z and W.
• Males have two similar chromosomes: ZZ (homogametic) — they produce only one type of gamete (Z).
• Females have two different chromosomes: ZW (heterogametic) — they produce two types of eggs (Z-bearing and W-bearing).
• Therefore, in birds the female (egg) determines the sex of the offspring.

Mechanism in human beings (XX-XY type).

• The sex chromosomes are X and Y.
• Females have XX (homogametic) — produce only X-bearing eggs.
• Males have XY (heterogametic) — produce two types of sperm (X-bearing and Y-bearing).
• Therefore, in humans the male (sperm) determines the sex of the offspring. …