Sunday, December 17, 2017

X-DNA case study: Molly

I generally find DNA segment analysis impractical but recently had a personal use for it to determine how I may be related to a reasonably distant match (MM) I will call Molly. Molly matches my full sister and me at 23andMe as follows:

Molly does not match a maternal first cousin or a mutual second cousin at 23andMe, but this is not enough to convince me that she is a paternal match. However, I observed that she matches my sister and me at an X-DNA location where we only share paternal X-DNA.

some half and completely identical segments shared with my sister

Our sex-determinining chromosomes have unique inheritance rules. Females generally have two X chromosomes, one from each parent, while males generally have one X chromosome inherited from their mothers and one Y chromosome inherited from their fathers. As paternal sisters share the same X chromosome inherited unchanged from their father (his only X chromosome), I know that the lighter shaded half-identical segment on the right of my 23rd chromosome pair is paternal X-DNA. We share very little maternal X-DNA, indicated by the relatively small darker shaded completely-identical segment on the left.

My working hypothesis is therefore that Molly is a paternal match to us. As males only inherit X-DNA from their mothers I will also assume that Molly is related to us via our paternal grandmother's line (Liverpool Irish), excluding three-quarters of our ancestral lines.