Definition of Chromosomal crossover

Chromosomal crossover (or crossing over) is the process by which two chromosomes, paired up during prophase 1 of meiosis, exchange some portion of their DNA. Crossing over is specifically initiated in pachytene, before the synaptonemal complex develops, and is not completed until near the end of prophase 1. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome. The result of this process is an exchange of genes, called genetic recombination. Crossing over was first described, in theory, by Thomas Hunt Morgan. The physical basis of crossing over was first demonstrated by Harriet Creighton and Barbara McClintock in 1931.