Importance of meiosis 

Meiosis is a type of cell division that produces four genetically different haploid cells. 

This type of cell division serves two important roles: 

1. Production of haploid gametes - This allows sexual reproduction to take place. 
2. Creates genetic variation - This increases diversity, allowing natural selection to take place. 

The random fusion of gametes during fertilisation also leads to genetic variation. 

Sexual reproduction

Meiosis is needed to produce haploid gamete cells containing only half the number of chromosomes as a body cell.

In sexual reproduction two gametes (sex cells) fuse together to form a zygote.

As each gamete contains half the normal number of chromosomes (n), the resulting zygote cell contains the diploid number of chromosomes (2n). 

Half of these chromosomes are from the father's sperm cell and half are from the mother's egg cell 

Genetic variation

Meiosis is needed to produce cells which are genetically different from one another. 

Two events within meiosis lead to genetic variation (these will be covered in more detail later in the lesson): 

• Crossing over (or recombination)
• Independent segregation (or random assortment)

Crossing over

Crossing over (or recombination) occurs during prophase I of meiosis.  

Steps of crossing over:

1. During prophase I, the homologous chromosomes condense and pair up.
2. The chromatids of each chromosome then twist around one another, forming a chiasmata.
3. When the chromosomes are separated during anaphase I, the chromatids break at the chiasmata and then reconnect to the chromatid from the homologous chromosome.

This swaps alleles between the homologous chromosomes to produce different combinations on each chromosome. 

In the diagram below, you can see that the process of crossing over produces four genetically different daughter cells. 

Each cell has a different chromatid (and a different set of alleles), increasing the genetic variation of the offspring. 

Independent segregation (random assortment)

During metaphase I, pairs of homologous chromosomes line up along the cell's equator. However, whether the paternal or maternal chromosomes appears on the left or right is completely random. As a result, which chromosomes end up in each daughter cell is also random. This principle is known as independent segregation or random assortment. 

To understand the principle of independent segregation, it is best to look at two chromosome arrangements. The diagram below shows the possible arrangements of two pairs of homologous chromosomes.

Looking at the diagram above, we can see that arrangement 1 and arrangement 2 produce daughter cells with four different combinations of chromosomes. 

Human cells contain 23 pairs of homologous chromosomes and so independent segregation produces a huge number of different chromosome combinations. Gametes with different chromosome combinations increase genetic variation of offspring. 

Calculating genetic variation due to independent segregation

It's important to understand how to calculate the number of genetically distinct gametes produced as a result of independent segregation. 

To calculate this, we use the formula 2ⁿ where 'n' represents the number of chromosome pairs. 

For example, humans have 23 chromosome pairs so there are 2²³, or 8,388,608, potential chromosome combinations that could result from independent segregation alone. 

You may also need to calculate the number of genetically different zygotes that are possible from two parents.

The formula for this is (2ⁿ)².

This is essentially the same formula as before, but squared to account for the combination of two gametes. 

For humans, the equation would be (2²³)², meaning there are a possible 7 x 10¹³ genetically different zygotes.