Recombination Frequency vs Map Distance: Why It Maxes Out at 50 Percent

Recombination frequency is the fraction of offspring that are recombinant. Map distance is measured in map units (centimorgans), where 1 map unit is defined as 1 percent recombination. For genes that are close together the two numbers match. For genes that are far apart they do not.

When they are equal

If two genes are close, a single crossover between them is the main event, and percent recombinants equals map units. A 12 percent recombinant count means 12 map units.

Why frequency caps at 50 percent

Genes on different chromosomes, or very far apart on the same chromosome, show 50 percent recombinants, which is the same as independent assortment. Frequency can never exceed 50 percent, because an even number of crossovers (such as two) returns the parental combination.

Double crossovers hide distance

Two crossovers between a pair of genes can restore the parental arrangement. That offspring is counted as non-recombinant, so the measured frequency underestimates the real distance. Map distances between far genes are better found by adding up shorter adjacent intervals in a three-point cross.

Quick rules

Common traps

• Equating the two for all distances.
• Reporting more than 50 percent recombination.
• Counting parental double crossovers as recombinants.

One-minute recap

• 1 percent recombinants = 1 map unit for nearby genes.
• Recombination frequency maxes at 50 percent.
• Far apart genes are underestimated, so add intervals.