Fun with map units...

The rate of crossover depends on a lot of things like species, gender, age, and so on, but most importantly (for our purposes), it depends on distance between the genes.

This concept is so important that geneticists talk about distance in "map units", where 1 map unit = 1% probability of crossing over.

sudoku with smiley faces By the way, what is the maximum rate of crossover? You might be tempted to say "100%" (I know I am). However, talking about a 100% crossover rate doesn't really make sense. In fact, at a crossover rate of 50% or more, you can't tell the difference between linked genes and unlinked genes. So the highest crossover rate that makes sense is 50%, or 50 map units.

If you like Sudoku, you'll love map units. Give them a try:

Genes A and B crossover at a rate of 10%,
Genes A and C crossover at 7%
Genes B and C at 17%.

Solution:
Start with B and C, since they are the farthest apart
A has to be BETWEEN B and C, so this is the only place it can go, 7 map units from C.
Just to verify, A is also 10 map units from B.