Sexual Reproduction
Sexual reproduction introduces genetic variation into a population, which is essential for adaptation and survival. It involves the fusion of two haploid gametes (n) — one from each parent — to form a new diploid (2n) individual.
During gamete formation, meiosis ensures that the chromosome number is halved.
- Meiosis I separates homologous chromosomes.
- Meiosis II separates sister chromatids.
Both gametogenesis processes contribute to variation, but they produce highly specialized cells with different structures and roles.
Spermatogenesis vs. Oogenesis (Overview)
- Spermatogenesis produces male haploid gametes — sperm.
- Oogenesis produces female haploid gametes — ova (eggs).
These cell types are structurally and functionally different, which reflects the differences in how each process proceeds.
Fertilisation has two key functions:
- 1. Bringing together the haploid gametes to form a new diploid organism.
- 2. Triggering activation of developmental pathways in the egg.
Chromosomal Sex Determination
Male (XY)
Y Chromosome
- ~59 million base pairs; highly variable in length.
- Mostly composed of non-functional repetitive DNA.
- Known protein-coding genes ≈ 48.
- Includes SRY, which encodes a 204-amino-acid Testis-Determining Factor (TDF).
- TDF belongs to the HMG box family of DNA-binding proteins → functions as a transcription factor regulating downstream genes.
Female (XX)
X Chromosome
- ~155 million base pairs.
- Represents ~5% of the haploid genome.
- Contains both housekeeping and specialised genes.
- Includes genes such as:
- Wnt-4 → essential for initiating the female pathway.
- DAX-1 → critical for early ovary development.
X-Inactivation
- One X chromosome is randomly inactivated in all female embryonic cells.
- Results in a mosaic pattern (maternal X active in some cells, paternal X in others).