Genes that are located on the same chromosomes are linked genes. Alleles for these genes tend to segregate together during meiosis unless they are separated by crossing-over. The closer together two genes are on a chromosome, the less likely their alleles will be separated by crossing-over. A pair of genes with a larger recombination frequency are likely farther apart, while a pair with a smaller recombination frequency are likely closer together.
The figure shows two alleles that are closest together - tail-feather length aand feather color which is only 4 units in distance.