Why is crossing over likely to occur when genes are located further apart? I do not see the connection between one gene and another gene located on the same chromosome and what that has to do with crossing over. i thought crossing over was mainly between alleles of the same gene across two homologs. This seems irrelevant to the presence of another gene.
Because there is some crossing over of DNA when the chromosomes segregate, alleles on the same chromosome can be separated and go to different daughter cells. There is a greater probability of this happening if the alleles are far apart on the chromosome, as it is more likely that a cross-over will occur between them.
The relative distance between two genes can be calculated using the offspring of an organism showing two linked genetic traits, and finding the percentage of the offspring where the two traits do not run together. The higher the percentage of descendants that does not show both traits, the further apart on the chromosome they are.
Wikipedia http://en.wikipedia.org/wiki/Genetic_linkage
