In a two-point linkage analysis, genes a and b have been found to be 26 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 33 m. u. apart rather than 26 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