centiMorgan (cM)

A centiMorgan (or cM for short) is a measure of a matching segment length in genetic genealogy. It is a more accurate measure of the real, respective length of a segment than the raw base-pair count. This because the segment length is being determined from the sparsely measured SNPs which are not evenly scattered among the DNA strands. Thus the cM is an adjusted length based on the frequency of occurring SNPs that happen to be measured.

Each testing company reports a different number of cM’s. To determine the half-identical, or haploid amount, look at the total matching length between a mother and daughter. (Or less accurately any parent and child. We restrict it here as some companies include X matching in their reported length. Reported total cM’s for the haploid are often higher than the actual base pair count but very roughly correspond to millions of base pairs. That is, 1 cM is roughly equivalent to 1,000,000 base-pairs.