Final answer:
Crossing over and genetic recombination take place during prophase I of meiosis I, resulting in new gene combinations on chromosomes and contributing to genetic diversity in sexually reproducing organisms.
Step-by-step explanation:
Crossing Over and Genetic Recombination Take Place
Crossing over and genetic recombination occur during prophase I of meiosis I. During this phase, homologous chromosomes pair up and exchange segments of DNA, resulting in recombinant chromosomes which lead to genetic diversity. Such exchange of genetic material can happen multiple times with the same pair of homologous chromosomes.
This process is crucial for producing genetically unique offspring in sexually reproducing organisms. In humans, this contributes to the potential to produce a significant number of genetically unique children—each human couple has the potential to produce more than 64 trillion genetically unique offspring due to these mechanisms.
The genetic recombination ensuing from crossing over results in new combinations of alleles, which means that the chromatids, formerly identical, become unique. By the time chromosomes segregate during meiosis II, this diversity is further shuffled and passed on to the offspring, contributing to the variation observed within a species.