Final answer:
Crossing-over is an exchange of genetic material between non-sister chromatids of homologous chromosomes that occurs during prophase I of meiosis I, leading to increased genetic variation in gametes.
Step-by-step explanation:
Crossing-over is a critical process that occurs during prophase I of meiosis I. During this phase, homologous chromosomes pair up closely and form a structure called a tetrad, where non-sister chromatids of these chromosomes can exchange segments of genetic material. This genetic exchange is an intricate dance of DNA, where corresponding segments break and rejoin, effectively shuffling alleles to produce recombinant chromosomes. Such recombination contributes to the genetic diversity of gametes, ensuring that offspring have a mixed inheritance rather than a direct copy of one parent's chromosomes.
It's important to note that crossing-over happens specifically during prophase I of meiosis I, a phase that is unique to the formation of sex cells or gametes. This process of genetic material exchange increases variation within a species, which is a critical aspect of evolution and natural selection.