# Meiosis

**Starts with one human cell containing 46 chromosomes**

**Ends up with **

**4 cells each containing 23 chromosomes**

Meiosis occurs prior to sexual reproduction. The chromosomes are copied and each one attaches itself to the chromosome it was copied from to form an X shape. Each chromosome inherited from one parent matches itself up with the corresponding chromosome from the other parent.

During this phase genes inherited from one parent may be transferred over to the matching chromosome inherited from the other parent in a process known as crossing over (shown by red moving onto blue and vice versa). Crossing over produces genetic variation.

All the matched chromosomes form a line down the middle of the cell, which then splits to form two cells. These then split again to form either sperm cells in the testes or eggs in the ovaries, see below:

Simplified, if we look at just 4 chromosomes, meiosis would be as follows:

This produces 4 cells which form into either sperm if produced by the male or eggs if produced by the female.

These cells are called sex cells or gametes, and in humans will contain 23 chromosomes.

In humans the 4 cells that are created by meiosis each contain 23 chromosomes as follows:

Chromosome 23 will either be an X or Y. Your mothers egg cell can only contain an X chromosome but your fathers sperm cell can provide an X or Y see below:

The probability of inheriting a male or female chromosome can be shown in a punnet square.

The probability of producing a male or female is 8 out of 16, or 1 out of 2.

Therefore there is an equal chance of their being a girl or a boy.

**NOTE**

This is usually shown by using only two sperm and two eggs in the punnet square as follows:

Here the probability of producing a male or female is 2 out of 4, or 1 out of 2.

Therefore there is an equal chance of there being a girl or a boy.