Meiosis — Study Notes

Look in the mirror: half of your chromosomes trace back to your mother, half to your father, yet you are not a fifty-fifty blend of either parent — you are a brand-new combination. The cell division that builds this combination, one sex cell at a time, is called meiosis.

How Meiosis Halves the Chromosome Number

Every ordinary body cell carries its chromosomes in matching pairs — one member of each pair inherited from each parent. Biologists call this full paired set the diploid number, written 2n. A sex cell, or gamete, cannot carry this full set: if it did, combining two gametes at fertilisation would double the chromosome total in every new generation. So a gamete carries just one chromosome from each pair — the haploid number, written n. Meiosis is the division that produces this outcome: a parent cell splits twice in succession, and the matching chromosome pairs — known as homologous chromosomes — are pulled apart so only one partner from each pair lands in a given gamete. Fertilisation then reunites two half-sets into one complete set again, which is why the numbers balance across generations.

Worked Example — Finding a gamete's chromosome number

1. A grasshopper's body cell has a diploid number of 2n = 24, meaning it holds 12 pairs of matching chromosomes.
2. During meiosis, the two partners in every pair separate, and only one enters each newly forming cell.
3. Since all 12 pairs each surrender a single partner, every gamete finishes with n = 12 chromosomes — exactly half the starting total.

Notice the shortcut buried in there: whatever diploid figure a question hands you, dividing it by two gives the haploid answer immediately.

That's the headline halving — it doesn't yet explain why brothers and sisters never turn out identical, or the precise phrasing examiners look for when a question asks why the number halves. Those pieces, alongside a full audio walkthrough and worksheet, are waiting in the full lesson below.