How to Calculate Map Units From Recombination Frequency

A map unit (1 centimorgan, cM) is 1 percent recombination between two genes. To convert, count recombinant offspring, divide by total offspring, and multiply by 100; the result in percent is the distance in map units. That is all the arithmetic. The work is in picking out which offspring are recombinants.

The method

1. Find the parental classes. They are the two most common phenotypes from a test cross.
2. Everything else is recombinant. Add up the counts in the other classes.
3. Divide by the total number of offspring, including parentals.
4. Multiply by 100. The percent equals map units.
5. Add distances for adjacent intervals. Distances between neighboring genes add up along a chromosome.

Worked problem 1: two genes

A test cross gives 420 AB, 410 ab, 90 Ab and 80 aB, out of 1,000 offspring.

• Parentals: AB and ab (830).
• Recombinants: Ab and aB (170).
• 170 / 1,000 = 0.17, so 17 percent, or 17 map units.

Worked problem 2: from a percent

Genes X and Y show 8 percent recombination. In 2,500 offspring, how many are recombinants?

• 8 percent of 2,500 = 200 recombinants, split roughly equally between the two recombinant classes.

Worked problem 3: three genes in a line

Three genes are tested pairwise: A to B is 10 map units, B to C is 6 map units, A to C is 16 map units.

• The largest distance is between the outer genes, so B is in the middle: A to B to C.
• Check: 10 + 6 = 16. The distances add.

Why it tops out at 50 percent

Even genes very far apart, or on different chromosomes, give at most 50 percent recombinants in a test cross. Multiple crossovers can undo each other, so large raw distances underestimate the true distance. Long maps are built by adding short intervals.

Common traps

• Counting parentals as recombinants. The largest two classes are parental.
• Using only one recombinant class. Add both.
• Dividing by the parental count. Divide by the total.
• Expecting more than 50 percent. Observed values stay at or below 50 percent.

One-minute recap

• Recombinants / total x 100 = percent = map units.
• Largest two classes are parental.
• Largest pairwise distance spans the outer genes.