Consider two homologous chromosomes, one with dominant alleles at all gene loci and the other with recessive alleles at those loci. In prophase I of meiosis, homologous chromosomes line up side by side in a process called synapsis. The alignment is very precise and places the genes on one homologue opposite the equivalent genes on the other homologue. While together, the chromatids of different homologues overlap in one or more places. At the point of overlap an exchange of chromosomal pieces takes place. This is called crossing-over. This exchange is reciprocal; equivalent pieces of chromosomes are exchanged. Although the exchanged pieces have the same genes, they may not have the same alleles. After crossing-over, 2 of the 4 chromatids have dominant alleles at some loci and recessive alleles at others. This recombination of alleles is a major source of genetic variability in populations.