Why gametes must be haploid
Gametogenesis is the formation of gametes (sperm and ova). Body cells are diploid (2n = 46 chromosomes in humans), carrying two matching sets of chromosomes. If gametes were also diploid, fertilisation would double the chromosome number in every generation, which the body cannot allow. To prevent this, gametes are made by meiosis, a special reduction division that halves the number, producing haploid cells (n = 23). Meiosis also shuffles the genes, so each gamete is genetically unique. When a sperm and an ovum fuse at fertilisation, the diploid number of 46 is restored in the new individual.
Spermatogenesis
Spermatogenesis is the production of sperm in the seminiferous tubules of the testes. It begins at puberty and continues throughout life. A diploid germ cell grows and then divides by meiosis to give four small, motile sperm. Each sperm has a head containing the nucleus and an acrosome (with enzymes to penetrate the ovum), a middle piece packed with mitochondria for energy, and a long tail (flagellum) for swimming.
Oogenesis
Oogenesis is the production of ova in the ovaries. It also uses meiosis, but the cytoplasm divides unequally: one diploid cell forms one large ovum plus small polar bodies that break down. This concentrates yolk and cytoplasm into the single ovum, giving the future embryo a food store. Oogenesis begins before birth, pauses, and completes one ovum per menstrual cycle from puberty.
Example
One diploid cell in the testis produces four functional sperm, but one diploid cell in the ovary produces only one functional ovum (the polar bodies degenerate). Both, however, are haploid with 23 chromosomes.
Key idea
Meiosis: 1 diploid cell (2n = 46) → 4 haploid cells (n = 23). Fertilisation: n + n → 2n, restoring 46 chromosomes.