Recombination mapping

Deducing a gene map from recombination data

    The position of locus A on a chromosome is known, the positions of loci B and C are unknown. Dihybrid crosses establish that the pairwise recombination distance between loci A & B and A & C are 5 cM and 3 cM, respectively. Two possibilities then exist for the position of of locus C: either it is between A and B, or it is outside A on the opposite side from B.

   A third dihybrid cross is done with loci B and C. The observed recombination distance of 2 cM establishes that locus C is between A and B. Were it outside the A-B region, the expected recombination distance between B and C would be 8 cM, which is inconsistent with the data.

    [ Given the initial data, can C be on the outside of B from A ? Explain. If it were, what would the expected recombination distance between A & C ? ].

Figure after 2002 by Griffiths et al.; all text material 2013 by Steven M. Carr