Imagine that five hypothetical genes are arranged on a chromosome in the following order: G--H--I--J--K. Which two genes are most likely to have a crossover occur between them?

G and K

Explanation:

Crossing-over is a genetic phenomenon that occurs in meiosis, specifically, Prophase I. It is when chromosomal segment (genes) are exchanged between non-sister chromatids of homologous chromosomes. Crossing-over occurs only to genes that are UNLINKED i.e. genes located on different chromosomes or far apart on the same chromosome.

This unlinked genes are said to have the highest RECOMBINATION FREQUENCY. Crossing-over allows genes on the same chromosome but far apart from each other, assort independently, which allows alleles to be recombined on the same chromosome. This phenomenon does not occur to genes that are close on the same chromosome as they will be inherited together as a unit.

In this case, G and K are the farthest apart, hence, they will have the highest RECOMBINATION FREQUENCY i.e. the likelihood for homologous crossing-over to take place during meiosis.