Final answer:
A double crossover between two genes during meiosis can involve two, three, or four chromatids, leading to genetic variation.
Step-by-step explanation:
A double crossover between two genes could involve two, three, or four chromatids. During prophase I of meiosis, homologous chromosomes line up and form a tetrad configuration, which contains four chromatids. Material from non-sister chromatids may break off and reattach at the corresponding position on the homologous chromosome. This exchange of genetic material, or recombination, can lead to multiple crossovers within the same pair of homologous chromosomes, creating unique combinations of genes and thus, genetic variation.