Meiosis and how genes make phenotypes

Last time

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

Half the cards

A human cell has forty-six chromosomes, but a sperm or an egg must have only twenty-three. If gametes had the full number, it would double every generation. Meiosis halves the number, and as a bonus it shuffles the genes, so no two gametes are the same. After that, we ask how a gene turns into a visible feature.

What meiosis does

Meiosis is a nuclear division in which a diploid cell produces four haploid cells. It involves two divisions, meiosis one and meiosis two, with only one replication of D N A. In meiosis one, the homologous chromosomes are separated, which halves the chromosome number. In meiosis two, the sister chromatids are separated, like mitosis. In humans, forty-six chromosomes become four cells with twenty-three each.

Prophase one

In prophase one, the chromosomes condense, and each homologous pair comes together. Each chromosome is made of two sister chromatids. The pair is called a bivalent. Chromatids of the homologous chromosomes overlap at points called chiasmata. They swap sections of D N A. This is crossing over. Crossing over produces new combinations of alleles on each chromatid. This is recombination, and it makes the chromatids genetically different.

A past-paper example

May June 2023, Paper 41, question 8. A locust testis cell is at late prophase one. Explain how the behaviour of chromosomes in prophase one results in the appearance shown. Homologous chromosomes pair up as bivalents. Chromatids cross at the chiasmata, where crossing over occurs, and the chromosomes look joined at these points. Chromosomes are condensed, so they are visible as short thick structures.

Metaphase one to telophase two

In metaphase one, the bivalents line up at the equator. The way each pair faces is random. This is independent assortment. In anaphase one, whole chromosomes are pulled to opposite poles, not chromatids. The two cells formed are haploid but each chromosome still has two chromatids. In meiosis two, the chromosomes line up again, and the chromatids are pulled apart. Four haploid cells result, each with single chromatid chromosomes.