Final answer:
Crossing-over during meiosis switches genes between homologous chromosomes during prophase I, resulting in new genetic combinations which contribute to genetic diversity in offspring.
Step-by-step explanation:
Crossing-over during meiosis switches genes between the two homologous chromosomes. This process takes place during prophase I of meiosis I, where homologous chromosomes form pairs and line up down their entire length in a configuration known as a tetrad. At this stage, non-sister chromatids align very closely and exchange genetic material in a process also called recombination.
The crossover can lead to multiple exchanges within the same pair of chromosomes, thereby creating unique combinations of genes that contribute to genetic variation. This is significant as it contributes to the diversity of genetic material in gametes, which ultimately leads to the potential for genetic diversity in offspring.
For example, if allele A is located on one chromatid and allele a is on the corresponding site of its homologue, after crossover, the chromosomes could carry the combinations Ab, aB, ab, or AB. This increases the genetic variation because the gametes produced will carry different combinations of these alleles.