recombination frequency

Meaning: In meiosis I, crossing over between homologous chromosomes can separate alleles that sat together on one chromosome. New allele pairings are recombinants; original pairings are parental types. A crossover is more likely between genes that are far apart, so the frequency measures distance. Genes on different chromosomes assort independently and give 50% recombinants; linked genes give less than 50%.

Equation: recombination frequency (%) = (recombinant offspring ÷ total offspring) × 100. The result is a percentage, with no SI unit. 1% = 1 map unit = 1 centimorgan.

On the MCAT: A passage gives offspring counts from a test cross such as AaBb × aabb. The two most common phenotype classes are parental, the two least common recombinant. Calculate the frequency, then use pairwise distances (A–B 10, B–C 5, A–C 15) to order genes: B lies between A and C.

Common confusion: A value of 50% does not mean 50% linked; it means the genes behave as unlinked, on different chromosomes or far apart on one. Values cannot exceed 50% because each crossover involves two of the four chromatids. Double crossovers restore parental pairings and go undetected, so map distance underestimates true distance for far-apart genes.