Organize offspring into four groups based on their combinations of recessive and dominant traits. For instance, if you mate a fruit fly with red eyes and short wings--dominant traits, for the purpose of example--and one with green eyes and long wings--recessive traits--you may find that out of 1,000 offspring, 425 have red eyes and short wings, 400 have green eyes and long wings, 75 have green eyes and short wings and 100 have red eyes and long wings.
Add together the number of offspring that mix--again, "recombine"--recessive and dominant traits. For the example, 75 fruit fly offspring have green eyes--recessive--and short wings--dominant--and 100 have red eyes--dominant--and long wings--recessive. In genetic terms, these are known as "mutants" and there are 175 of them in total.
Express the number of recombinants or mutants as a rate, or percentage. If 175 of 1,000 offspring express recombinant mutations, the recombination rate is: 175/1000 = .175 = 17.5% recombinant rate.