During prophase I each replicated chromosome (comprising two chromatids) pairs with its homologous partner, ie. the diploid number of chromosomes produces the haploid number of homologous pairs. Crossing over occurs when all four chromatids are at synapsis (exactly aligned) non-sister chromatids may cross over, break and reassemble so that parental gene combinations are replaced by recombinants, This is a major source of genetic variation.