In а twо-pоint linkаge аnalysis, genes a and b have been fоund to be 16 m.u. apart on the same chromosome. A third gene, c, has just been discovered and found to be located between a and b. A three-point linkage analysis with a, b, and c indicates that a and b are actually 21 m.u. apart rather than 16 m.u. Why does the three-point analysis give a different map distance for a and b than does the two-point linkage analysis, and which is more accurate?