Final answer:
Crossing over in eukaryotes most frequently occurs between two homologous chromosomes of a diploid germ cell (option 1) during prophase I of meiosis, leading to genetic recombination and increased diversity.
Step-by-step explanation:
Crossing over in eukaryotes most frequently occurs between two homologous chromosomes of a diploid germ cell. During the first phase of meiosis, known as prophase I, homologous chromosomes pair up gene-for-gene along their entire length. This formation, a tetrad, consists of four chromatids. During this phase, crossing-over, also known as recombination, occurs, where genetic material is exchanged between non-sister chromatids of homologous chromosomes. This event significantly contributes to genetic diversity, providing the basis for the production of over 64 trillion genetically unique individuals from each human couple.
During crossing-over, segments of DNA break off from one chromatid and reattach to the corresponding position of the homologous pairing, creating unique combinations of genes, known as recombinant chromosomes. These chromosomes carry some DNA from one parent and some DNA from the other, leading to offspring with a blend of parental traits. Crossing-over is a fundamental mechanism by which genetic variation is introduced during the formation of gametes, and it underlies the essential biodiversity observed within species.