The three-point mapper

You will write three small functions that map three linked genes from the eight classes of a test cross. The classes are written as three letters, such as "ABC" and "aBc". The heterozygote was ABC over abc.

The idea. The two commonest classes are parental. The two rarest are double crossovers. A recombinant for two genes is an offspring in which the two genes are not in a parental arrangement.

Your tasks.

1. Finish find_classes. It gives two lists: the two parental classes and the two double crossovers.
2. Finish distance. It gives the distance in map units between the genes at places i and j (0, 1 or 2), from the counts and the first parental class.
3. Finish coincidence. It gives the coefficient of coincidence from the counts, the two distances in map units and the list of the double crossovers.