How does sexual reproduction contribute to genetic variation?

Sexual reproduction contributes to genetic variation through the processes of meiosis and fertilisation.

Sexual reproduction involves the fusion of male and female gametes in a process known as fertilisation. Each gamete carries half the number of chromosomes of a normal cell, which are derived from the parent organism. These chromosomes are not identical copies of the parent's chromosomes, but rather a mix, due to a process called meiosis.

Meiosis is a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell. During meiosis, homologous chromosomes pair up and exchange segments of DNA in a process called crossing over. This leads to a shuffling of genetic material, creating new combinations of genes. This is the first way that sexual reproduction contributes to genetic variation.

The second way is through the random assortment of chromosomes. When the homologous pairs line up in the middle of the cell during meiosis, it is completely random which chromosome from each pair goes into each daughter cell. This means that each gamete produced has a different combination of chromosomes from the parent organism.

Finally, during fertilisation, one gamete from each parent combines to form a zygote. The combination of these two sets of chromosomes results in a unique individual, different from both parents. This is because the zygote receives a random mix of traits from each parent, leading to even more genetic variation.

In summary, sexual reproduction contributes to genetic variation through the processes of meiosis and fertilisation. The shuffling and recombination of genetic material during meiosis, followed by the random combination of these genes during fertilisation, results in offspring that are genetically unique. This genetic variation is crucial for the survival and evolution of species.