Final answer:
Crossing over is the exchange of genetic material between homologous chromosomes during prophase I of meiosis I, leading to genetic variation in the resulting gametes.
Step-by-step explanation:
The phenomenon of crossing over in meiosis is best described as the exchange of genetic material between homologous chromosomes. This essential event takes place during prophase I of meiosis I when homologous chromosomes pair up forming tetrads. Crossing over leads to genetic variation by allowing the chromatids of homologous chromosomes to swap equivalent segments of DNA. Each chromosome then contains a mixture of maternal and paternal genes resulting in unique genetic combinations.
During this process, known as recombination, chromosomal segments are broken and rejoined, contributing considerably to the genetic diversity of gametes. Such a significant genetic reorganization is a primary source of variation in offspring, vastly increasing the potential for genetic diversity in a species.