Meiosis

Last time

Before we start: last time, in Codominance, sex linkage and pedigree diagrams, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Why you are not a copy

Brothers and sisters have the same parents, yet only identical twins look exactly alike. The reason is meiosis, the cell division that makes gametes. It halves the chromosome number and shuffles the chromosomes, so every gamete is different.

Meiosis

Meiosis is reduction division, in which the chromosome number is halved from diploid to haploid. It produces genetically different cells. It is used to make gametes: sperm and eggs in animals, pollen nuclei and egg cells in plants.

The numbers

A human body cell is diploid, with forty-six chromosomes in twenty-three pairs. Meiosis makes gametes that are haploid, with twenty-three, one from each pair. At fertilisation, twenty-three plus twenty-three restores forty-six.

Why halve it?

If gametes had forty-six chromosomes, the zygote would have ninety-two. And the number would double every generation. Halving it in meiosis keeps the chromosome number constant from generation to generation.

Why gametes differ

In meiosis, each pair of chromosomes is separated, and which one goes to which gamete is random. With twenty-three pairs, there are over eight million possible combinations of chromosomes. Random fertilisation adds even more variation: any sperm can fuse with any egg.

Mitosis or meiosis?

Here is the comparison every exam asks for. Number of cells, chromosome number, and identical or different. The quickest test: if it makes gametes, it is meiosis.

Where it happens

In humans, meiosis happens only in the testes and the ovaries. In flowering plants, it happens in the anthers and the ovules. Everywhere else in the body, cells divide by mitosis.