Crossing Over

Crossing over is the reciprocal exchange of segments of DNA between chromatids of homologous chromosomes. Sometimes referred to as recombination, crossing over is a characteristic of prophase in the first meiotic division. Normal crossing over is a major source of genetic variation in human reproduction and is used by researchers to help identify the relative location of genes. Errors in crossing over are a common source of mutation.

In the absence of crossing over, germ cells would contain chromosomes that were identical to those of ancestors. It would, in theory, be possible to directly trace a single chromosome back generations. In the extreme case, there would only be a handful of distinct chromosomes, altered only by acquired mutations over the years. Instead, during meiosis, homologous chromosomes approach each other and exchange sequences of DNA, thus making the chromosomes of germ cells slightly different from those of the surrounding somatic cells. Because this transfer of genes occurs by the physical breaking and transfer of DNA, scientists can draw inferences from the frequency with which certain traits tend to cross over. First, they can conclude that two traits that are often seen together are likely caused by genes that lie in close proximity to each other. Genes that are close together on a chromosome are likely to travel together when a crossing over event occurs. Scientists can also conclude that traits encoded by genes that cross over frequently are encoded by genes located at the ends of chromosomes as opposed to loci near the center of the chromosome.