Q.(a) Compare the mechanism of sex determination in humans with that of honey bees, with respect to chromosome number.

Concept: Sex determination mechanisms – chromosomal (XY system) vs haplodiploidy

(a) Mechanism comparison

Humans (XY system):

Diploid organisms with 2n = 46 chromosomes. Females are homogametic (XX, 44 + XX) and males are heterogametic (XY, 44 + XY). Sex is determined at fertilization by the sperm carrying either an X or Y chromosome.

Honey bees (Haplodiploidy):

Females (queen and workers) are diploid (2n = 32) arising from fertilized eggs. Males (drones) are haploid (n = 16) developing from unfertilized eggs through parthenogenesis. Sex is determined by ploidy level, not sex chromosomes.

(b) Gamete formation comparison

Humans:

Both sexes undergo meiosis. Males produce haploid sperm (n = 23, either 22 + X or 22 + Y). Females produce haploid ova (n = 23, all 22 + X).

Honey bees: …

The concept: two evolutionary solutions to sex determination

Sex determination mechanisms reveal how different organisms solve the same biological problem—producing two sexes—through entirely different chromosomal strategies. Humans rely on sex chromosomes in a diploid system, where every individual carries two sets of chromosomes. Honey bees, however, use haplodiploidy, where sex is determined by whether an individual is haploid or diploid. This distinction cascades into how each organism produces gametes.

(a) Chromosome number and sex determination mechanisms

Humans (XY system)

1. Chromosome complement: Both males and females are diploid throughout their life cycle. Females carry 2n = 46 chromosomes (44 autosomes + XX), while males carry 2n = 46 chromosomes (44 autosomes + XY).

2. Determination mechanism: Sex is determined at fertilization by the type of sperm that fertilizes the egg. All eggs carry an X chromosome (since females are XX). Sperm are dimorphic—half carry X and half carry Y. An X-bearing sperm produces a female zygote (XX); a Y-bearing sperm produces a male zygote (XY).

3. Key point: The male is the heterogametic sex (producing two types of gametes), and the presence of the Y chromosome—specifically the SRY gene—triggers male development.

Honey bees (Apis mellifera) – Haplodiploidy

1. Chromosome complement: Females (queens and workers) are diploid with 2n = 32 chromosomes. Males (drones) are haploid with n = 16 chromosomes.

2. Determination mechanism: Sex is determined by fertilization status, not sex chromosomes. Fertilized eggs develop into diploid females; unfertilized eggs develop into haploid males through parthenogenesis. The queen controls this by releasing or withholding sperm from her spermatheca during egg-laying.

3. Key point: Males have no father but do have a grandfather. This creates unusual genetic relationships—a male's daughters are his only direct descendants, and sisters share more genes with each other (75%) than with their own offspring (50%), which has profound implications for social behavior.

(b) Gamete formation: meiosis vs. mitosis

The ploidy difference between the two systems fundamentally alters how gametes are produced.

Humans

1. Both sexes undergo meiosis: Since both males and females are diploid (2n = 46), both must reduce their chromosome number by half to produce haploid gametes.

2. Spermatogenesis (in males): Diploid spermatogonial cells undergo meiosis I (reductional division) and meiosis II (equational division) to produce four haploid sperm (n = 23), half carrying X and half carrying Y.

3. Oogenesis (in females): Diploid oogonial cells undergo meiosis to produce one functional haploid egg (n = 23, always carrying X) and three polar bodies that degenerate.

4. Result: All gametes are haploid, and fertilization restores the diploid state.

Honey bees

1. Females undergo meiosis: Diploid queens and workers (2n = 32) produce eggs through meiosis, reducing the chromosome number to n = 16. These haploid eggs can then be fertilized (→ diploid female) or remain unfertilized (→ haploid male). …