Final answer:
Crossing over between two genes on homologous chromosomes during meiosis I produces gametes with both parental and recombinant genotypes, increasing genetic variation.
Step-by-step explanation:
If crossing over occurs only between two genes on homologous chromosomes, the combinations produced in the gametes will include both the parental (original) and recombinant (mixed) genotypes. This genetic recombination happens during meiosis I, more specifically in prophase I, when homologous chromosomes exchange segments of their genetic material.
For example, if we have two genes, A and B, with alleles A and a, B and b, respectively, the resulting combinations in the gametes after crossing over could be AB, Ab, aB, and ab. Here, AB and ab represent the parental genotypes, while Ab and aB are the recombinant genotypes, created due to the exchange of alleles between the chromosomes.
The occurrence of crossing over thus increases genetic variation and contributes to the genetic uniqueness of individuals within a species.