How does crossing over during meiosis create variation?

Crossing over during meiosis creates variation by exchanging genetic material between homologous chromosomes.

During meiosis, the process of cell division that produces gametes (sex cells), one of the key stages is called Prophase I. In this stage, homologous chromosomes - pairs of chromosomes, one from each parent, that carry the same genes but not necessarily the same versions of those genes - come together in a process known as synapsis. This forms a structure called a tetrad, which consists of four chromatids.

At this point, an important event called crossing over occurs. This is where non-sister chromatids - chromatids from different members of the homologous pair - exchange segments of their DNA. The points at which this exchange occurs are called chiasmata. The result of this process is that each chromatid now carries a unique combination of genetic material from both parents.

When these cells then go on to form gametes, the gametes will each carry one of these unique combinations of genetic material. This means that each gamete - and therefore each offspring produced when these gametes combine - will be genetically unique.

This process is a major source of genetic variation in sexually reproducing organisms. It means that even siblings, who share the same parents, can be genetically different from each other (except in the case of identical twins, who come from the same fertilised egg). This genetic variation is crucial for the survival and evolution of species, as it provides a wide range of traits on which natural selection can act.

In summary, crossing over during meiosis creates variation by shuffling the genetic material from both parents and creating unique combinations of this material in the offspring. This process is a key driver of genetic diversity and evolution.