Final answer:
Crossing-over, an important source of genetic variation, occurs between non-sister chromatids of homologous chromosomes during prophase I of meiosis, resulting in the potential for a high number of genetically unique offspring.
Step-by-step explanation:
Crossing-Over in Eukaryotes
Crossing-over most frequently occurs between non-sister chromatids of homologous chromosomes. This process happens during prophase I of meiosis, where homologous chromosomes pair up gene-for-gene, forming a tetrad. The close proximity of the chromatids allows exchange of genetic material, or recombination, which can occur multiple times between the same pair of chromosomes. This event significantly contributes to genetic diversity by creating unique combinations of genes.
The significance of this process lies in its role as a source of genetic variation. During meiosis, the exchange of genetic material leads to the formation of recombinant chromosomes. As a result, when these chromosomes segregate into gametes, each gamete carries combinations of genes that are different from those of the parent organisms, allowing for the potential production of a large number of genetically unique offspring. For instance, each human couple has the potential to produce more than 64 trillion genetically unique children.