Crossing Over

Crossing over is the process during meiosis in which two homologous chromosomes exchange genetic material. It occurs in prophase I when non-sister chromatids of a homologous pair swap segments of DNA. Crossing over creates new allele combinations on each chromosome, contributing to genetic diversity in the gametes.

• Crossing over happens between homologous chromosomes (for example, between the chromosome 1 from mom and the chromosome 1 from dad). It does not occur between sister chromatids of the same chromosome (sisters are identical, so exchanging between them would change nothing).
• The physical evidence of crossing over is the chiasma (pl. chiasmata), which is the X-shaped connection where homologous chromatids have exchanged pieces.
• Crossing over is one reason why siblings (except identical twins) are genetically unique. It shuffles genes - even genes on the same chromosome can be reshuffled if crossing over happens between them.

• If a question asks about sources of genetic variation in meiosis, crossing over is a key answer (along with independent assortment). For example, "Name one event in meiosis that increases genetic variability" -> crossing over in prophase I.
• You might see a diagram of paired chromosomes with segments swapped - identifying that process as crossing over (or "homologous recombination") is essential. Also, problems on genetic linkage and mapping rely on understanding that crossing over frequency between two genes indicates their distance apart.