Sexual Reproduction — Study Notes

Look in a mirror and you'll spot a bit of your mum's nose, maybe your dad's ears — but you're not a stamped copy of either parent. That blend traces back to one biological trick: exactly how many chromosomes get passed on, and how they're combined.

Why Body Cells and Sex Cells Don't Carry the Same Chromosome Count

Every ordinary body cell in an organism holds a fixed, full set of chromosomes — call this the diploid number. Sex cells, called gametes, are built differently: each one carries only half that amount, known as the haploid number. This isn't a random quirk of biology — it's a requirement. At fertilisation, one gamete from each parent combines to form a single new cell. If each gamete arrived carrying a full diploid set already, that new cell would end up with twice the usual amount, and the count would keep climbing generation after generation. Halving first, then adding two halves back together, is what keeps a species' chromosome total fixed over time. The special cell division responsible for producing this halved set happens before any combining of gametes can take place at all.

Worked Example — Chromosome Counts in a Rice Plant

1. An ordinary rice plant cell is diploid, holding 24 chromosomes in total.
2. A gamete produced by that plant is haploid, carrying half of 24, which works out to 12 chromosomes.
3. When two gametes combine: 12 + 12 = 24, so the resulting cell is diploid once more — matching the parent generation exactly, not doubling it.

Try the same three steps on any organism you're given — halve for the gamete, add the two halves back for the new individual — and the chromosome maths always checks out.

This rice example is just the starting point — the exam-precise definition, the paired-chromosome vocabulary examiners look for, and further worked questions with mark-scheme-style answers are all covered step by step in the full lesson below, alongside an audio walkthrough and the worksheet.