How do  find the recombination frequency of two genes: take for instance a cross between two flies, one with mutant short antennae (s) and mutant big feet (b) and one with wild type long antennae (s+) and wild type small feet ( b+). You get the following progeny:

9 short antennae, small feet11 big feet, long antennae37 short antennae, big feet43 long antennae, small feet