Determining Gene Order

To determine gene order, you must first identify the parental classes and the double crossover classes. Don't fall into the trap of assuming that the parentals must always beABC and abc. Parental classes are always the largest ones, since the connection between the genes will tend to preserve parental linkages above the expected level of independence. In our example, the AbC and aBc classes are the parental classes. Notice that these are reciprocals of each other--they have "opposite" phenotypes. This should make sense.
The double crossover classes will always be the smallest classes, since they require a crossover between A and B and a crossover between B and C. So our double crossover classes are ABc and abC. Again, note that they are reciprocals.
Now we compare the parentals with the double crossovers. The only gene which has recombined in a double crossover will be the middle one, because the chromatids will have crossed over between the first two genes, then crossed back between the second and third genes. Compare each double crossover with the parental which is most like it. For double crossover ABc, the most similar parental is aBc. (It's the same in two of the genes; the other parental is the same in only one gene.) Comparing these, we see that the only gene that is different is the A/a gene. If we compare the other double crossover to the other parental, we get the same result--only the A/a gene is difrerent. So A must be the gene in the middle. Our actual gene order is B-A-C.
To keep from getting confused, it's an excellent idea to rewrite the offspring chart above with the genes in the correct order, like this:
BAC: 95 BaC: 50
BAc: 5 Bac: 700
bAC: 700 baC: 5
bAc: 50 bac: 95