Basics: Sharing and Half-Identical Regions

If two people share a sufficiently long segment of DNA, it means that they are related. In fact, one can try to estimate the degree of relatedness, very approximately, by looking at the lengths of the segments they have in common and the number of common segments of various lengths. Those who have compared their DNA with others at 23andMe and similar websites are familiar with this.

A common segment is identified by specifying a chromosome and a range of locations—for example, Chromosome 1, bases 1,000,000 to 2,000,000.

Now, chromosomes come in pairs. One chromosome in each pair comes from one's mother's DNA, and the other chromosome in each pair comes from one's father's DNA.

There are 22 homologous pairs of autosomes—chromosomes which are present as a pair in both sexes—numbered 1 to 22. In addition, a woman has two homologous X chromosomes. A man, on the other hand, has a single X chromosome and a single smaller Y chromosome.

Sharing is reported by 23andMe for the autosomes and for the X chromosome. (Sharing isn't reported as such for the Y chromosome, nor is it reported for the mitochondrial DNA, which is yet other genetic material. These are reported instead as paternal and maternal haplogroups, but that's another matter.)

When a segment is said to be shared between two people, what's really meant is that there is a match between a region on one of the two homologous chromosomes in the first person and one of the two homologous chromosomes in the second person. This is sometimes described as a half-identical region. (The X chromosome in males is different. That chromosome is unpaired, so there's no issue of half-identity; a match on a region there is simply a match.)