How can crossing over increase genetic variation?

Think of a homologous chromosome pair as two versions of the same chromosome: one inherited from each parent. They carry corresponding genes in the same order, though the versions of those genes may differ. During prophase I of meiosis, the homologous chromosomes pair, and corresponding genes align. A crossover can exchange matching segments between chromatids from opposite homologs; these are called non-sister chromatids. The resulting chromatids can carry a new combination of maternal and paternal DNA. [S1]

Here is a simplified diagram description. Assume two gene locations, A/a and B/b, lie in that order on a chromosome pair:

Before crossover

• Maternal-origin chromatid: A — B
• Paternal-origin chromatid: a — b
• The other two chromatids in the paired chromosomes are not shown.

Crossover point: between the A/a location and the B/b location, on the two shown non-sister chromatids.

After crossover

• Recombinant chromatid 1: A — b
• Recombinant chromatid 2: a — B

The labels are an illustrative assumption, not a claim about particular genes. In this simplified example, the exchange leaves each chromatid with a different pairing of the versions at the two locations. That is what “increased variation” means here: offspring can inherit combinations not present together on either of these original chromatids.

The key result described here is a reshuffling of DNA already present on the paired chromosomes: the example changes which versions occur together, not the versions themselves. So it is useful to distinguish a new combination of alleles from a newly created allele. The source supports the combination-changing role of crossing over; it does not establish that every chromosome or every meiosis has a crossover at a particular location or frequency. [S1]

Also, the diagram shows just one possible exchange between two chromatids. It is a teaching model, not a map of every event in a cell. The position of an exchange affects which sections stay together and which are recombined.

Crossing over changes combinations within a chromosome by exchanging corresponding sections between paired homologs. Independent assortment is a separate process: homologous chromosome pairs are arranged and separated in ways that can produce different combinations of whole chromosomes. OpenStax identifies both crossing over and independent assortment as sources of genetic variation. [S1]

A clear short explanation is: “Crossing over can make recombinant chromatids by swapping corresponding DNA sections between homologous chromosomes. This creates new combinations of maternal and paternal DNA; independent assortment also contributes to variation.” This keeps the two processes distinct and avoids presenting crossing over as the sole source.

Use this template, and label the example as simplified:

1. Before: Draw two paired homologous chromosomes. Label one maternal-origin: A — B and the other paternal-origin: a — b.
2. Mark the exchange: Show a crossover between the two gene locations, involving one non-sister chromatid from each homolog.
3. After: Label the exchanged chromatids A — b and a — B; note that the two unshown chromatids are omitted for simplicity.
4. Explain the result: “The exchange produced recombinant combinations of the versions at these locations.”
5. Add the qualification: “Independent assortment is another source of genetic variation.”

The source explains the exchange and its contribution to variation, but this diagram does not predict the outcome of a particular meiosis or establish how often a crossover occurs at a given location. [S1]