Linkage Map Of Chromosome (problem)

by RayWay
(Iran-Isfahan)

When i was reading Campbell, i got to the "Linkage map". imagine on a chromosome, we have 3 different genes: b, vg, and cn(cn is between b and vg)recombination frequencies between b and vg is 17%. between b and cn is 9% and between vg and cn is 9.5%. But the sum of b-cn and cn-vg isn't 17% (9%+9.5%=18.5%). Why is that and which one(17% or 18.5%) we use?

Study Linkage.