Mapping genes with recombination

Crossovers are more likely between genes that are far apart. Turn that around and you get a tool: count how often two linked genes get separated, and you know roughly how far apart they are.

Distance from crossover frequency

Working out where genes lie in a species’ genome is genetic mapping. The result is a genetic map, a chart of the linear order of genes along a chromosome and the distances between them. It places each gene at its locus, the gene’s fixed position on the chromosome.

The logic follows from the cookbook picture: a swap that starts at a random page rarely falls between neighboring recipes. A crossover can happen almost anywhere along a chromosome. Two genes close together are rarely separated by one, so a testcross gives few recombinants. Two genes far apart are often separated, so it gives many.

Calculating map distance

The percentage of offspring in which two genes were separated by crossing over is their recombination frequency. It gives the map distance between them:

Map distance = (number of recombinants ÷ total offspring) × 100

Map distances are measured in map units (mu). One map unit equals a 1% recombination frequency.

Check yourselfA testcross of two linked genes gives 460 + 440 nonrecombinant offspring and 52 + 48 recombinants. How far apart are the genes?

Recombinants are 52 + 48 = 100 out of 1,000 offspring, a recombination frequency of 10%. The genes are 10 map units apart.

Putting genes in order

Map distances between several pairs of genes also reveal their order. Suppose genes A and B are 5 map units apart, B and C are 12 apart, and A and C are 17 apart. Only one order fits all three distances: A, then B, then C, with B in the middle, because 5 + 12 = 17.

To measure the distance between any two genes, the setup follows the same pattern as the fly cross:

Check yourselfTwo genes give 2% recombinant offspring in a testcross, and another two give 30%. Which pair is closer together on its chromosome?

The pair with 2% recombinants, at 2 map units apart. Fewer recombinants means crossovers between those genes are rare, which happens when the genes are close together.

Where this method works

Mapping by crosses needs organisms that are easy to breed and produce many offspring quickly, such as fruit flies and many crop plants. It doesn’t work for most species, including humans, whose generations are long and who can’t be bred in planned crosses. Faster molecular methods that don’t depend on crosses now map genes in those species.