How often two genes cross-over can tell us how far apart the genes are from each other. This is called recombinant frequency (rf). These calculations allow us to create a gene map like this one. Circle in red the two genes that are closest together. Underline in green the pair of genes that are furthest apart. Genes that are closer together on the chromosome are more likely to be inherited together. Put a blue X through the gene that is most likely to be inherited with vestigial wings.