Half Siblings and DNA Matching

Question

My sister and I are half-siblings. We have always know this and my mother reportedly had an affair during WW2. This was confirmed with ancestry DNA as we only share 1603cM.

I'm trying to filter out matches that could be on my paternal side. I have a match with someone who I share 475cM with. They are not dNA matched to my sister.

Would it be possible to still be related to this person via my half-sister/Mum as we can inherit different DNA even if we were full siblings or would sharing as much as 475cM across 23 segments only possible via a different paternal side?

Answer 1

You can make a reasonable assessment of whether your match could be on your maternal side by consulting the Shared cM Project tool (link here). If you plug in 475 for the number of cM you match, you'll see that the possible relationships falls into one of three different level groups. The relationships within a group have equivalent amounts of relatedness. Close examination will show that the least-related group is the one with 8.1% likelihood, which is the one including 1C2R, first cousin twice removed. (The other groups have comparable relationships of 1C1R and 1C, which are closer relationships.) You'll want to consider the relationships in this least-related group because these will have the lowest possible cM values for your relationship, which will be the same relationship that your sister shares and thus shows whether an undetectable DNA match for her is possible.

To see the possible cM value ranges for each of the possible relationships in the 8.1% group, you'll probably want to open a second window using the same link but this time avoid filling in a cM value in the input box. This will avoid graying out many of the relationship boxes in the chart lower down on the page.

For each relationship in the group, find the corresponding entry in the chart and examine the third line of entry, which gives the cM range for that relationship. Note the lowest number of that range. After doing this for all the relationships in the group, you'll know the lowest cM value experienced for someone with the same relationship you have with your match (someone like your sister if the match is on your maternal side). My quick look at the table finds low cMs of 12, 43, 46, and 57. So the lowest cM seen by the project is 12 cM, and not zero. I believe you can take this to mean your match is almost certainly paternal, since based on this your sister would be expected to show a match of some size if you were related maternally to the match.

Answer 2

As half siblings on your mother's side, you and your half sibling will share approximately half of your mother's chromosomes and none of your father's (assuming your sibling is not related to him in some other way).

What that means is that each match that you have on your mother's side has a 1/2 chance of also being a match of your half sibling. For 23 matches to not match, the chances are about 1 / (2**23) or less than one in a million. So you would be safe to say that your 23 segment match is almost certainly on your father's side.

Basically, 1 / (2**7) is about 1 in 100 chance. So you could be reasonably certain that most of the people you match with who have 7 or more segments, that your half-sibling does not match with, are most likely on your father's side.

Answer 3

My half sibling and I have been trying to figure out his biological father for some time. The best investigative tool I've found to get started with is gedmatch.com, where you can upload your kits and then query matches to both kits (which will yield your mother's side matches) or just one kit, your half brother's, which will effectively be filtered to have only his unknown father's matches.