Crossing over is a process that happens during the early
prophase of meiosis.
Just before a sperm or egg-producing cell divides, the chromosomes pair up, and may exchange bits of genetic material. This genetic recombination is part of what ensures that no two people will be genetically alike, except for identical twins, or clones. It means that a child can have different alleles -- groups of genes that express a certain trait-- than its parents.
Genes that stay together are likely to be physically closer together on the chromosome. So, by studying the crossing over process, geneticists can get information about gene location.