Crossing over

Definition

The reciprocal exchange of DNA segments between homologous chromosomes during prophase I of meiosis, producing recombinant chromosomes and increasing genetic variation in gametes. The physical manifestation of this process is visible cytologically as a chiasma.

Etymology

Example

In Drosophila melanogaster, crossing over occurs only in females; males exhibit achiasmate meiosis with no recombination, making the species a classic model for mapping gene distances via recombination frequency.

Synonyms

• chromosomal crossover
• genetic recombination (broad)

Related Terms

• chiasma
• recombination frequency
• Genetic linkage
• Meiosis
• synaptonemal complex
• tetrad
• interference
• map distance

Usage Notes

Often distinguished from 'genetic recombination,' which encompasses all mechanisms generating new allele combinations, whereas crossing over specifically denotes the physical exchange between homologs. In some arthropods (e.g., certain Lepidoptera and Diptera males), crossing over is suppressed or absent, affecting inheritance patterns and linkage mapping strategies.