Which event contributes to genetic variation during meiosis?

• A. Crossing over between homologous chromosomes.
• B. The replication of DNA prior to meiosis.
• C. The separation of sister chromatids during mitosis.
• D. The fusion of two gametes during fertilization.

Answer: (A)

Crossing over between homologous chromosomes during prophase I of meiosis creates new combinations of alleles on each chromosome. When non-sister chromatids exchange segments, the resulting chromatids are recombinant, so the gametes produced carry allele combinations that never existed in either parent. This reshuffling is a primary source of genetic diversity in offspring.

DNA replication before meiosis simply duplicates the genetic material; it does not by itself generate new allele combinations. Separation of sister chromatids during mitosis is part of the mitotic process (and in meiosis II it separates sister chromatids as well) but it doesn’t introduce new genetic variation. Fertilization follows meiosis and combines paternal and maternal gametes, increasing variation in the offspring, but it is not a meiosis event that creates variation.