Using Stranger Matches in Visual Phasing

With the standard three-sibling visual phasing, it’s usually easy to tell who owns each crossover. However, if you’ve adapted the technique for two siblings plus others, such as a nephew (like I did) or a half sibling, sometimes you can’t. In those cases, stranger matches can be very helpful. In this post, I’m going to demonstrate the use of strangers in visual phasing, using an example from my own results.

In a previous post, I showed how I did visual phasing with two siblings and a nephew (the son of a 3rd sibling). On each chromosome, I start the same way, with my mother (my nephew’s grandmother) in the extra view. I the set the crossover points and label the ones I can.

On Chromosome 4 (figure 1), I could label all of them except for 2 – one at 27.3 Mb and one at 174.5 Mb (outlined in red). I know these crossovers belong to either L or J, but I can’t tell which.

Chr 4 - Fig 1
Figure 1

Following the same technique I used previously, and with the help of some known cousin matches, I was able to figure out most of the chromosome, with the exception of those two unlabelled crossovers.

Figure 2Time to bring in some strangers to help, starting at the crossover on the left, at 27.3 Mb.

Because I know this crossover belongs to L or J, and because I know it’s a paternal crossover, I need to find some paternal matches to L and J over that section of Chromosome 4.

Since our mother has been tested and her kit uploaded to GEDMatch, I’ve already used the Phasing utility on GEDMatch to create phased kits for both L and J. I run the GEDMatch Tier 1 Matching Segment Search utility on the two phased paternal kits and look for matches around the 27.3 mark on Chr 4.

Matching kits at 27I found 6 people who match L from about 22 Mb to 34 Mb. Those same people match J from 22 Mb to 27.3 Mb. So, even though I have no idea who these matches are (hence the term “stranger matches”), it doesn’t matter. This is enough to tell me that the crossover at 27.3 must belong to J. That enables me to complete the first section.

crossover 27

Let’s look at the other end, around 174.5.

crossover 174This one is less clear-cut. Neither of us have matches that cross 174.5. We have the same two matches (Karen and Mary B) from 155 Mb to 163 Mb, as expected. And after 174.5, we have different matches, as expected. But who has the Cooper matches and who has the Sharpe matches? To figure this out, I needed to dig into these matches a bit further.

First, I ran the Multiple Kit Analysis on GEDMatch for each kit against L, J & R, to see if any of them matched on other segments besides the one on Chr 4. The only one that did was John M. (the last line of the J table), who matches J and R on Chr 17, in addition to the match with J on Chr 4.

comparison 17Looking at the visually phased Chr 17, John M would appear to be a Sharpe match (paternal grandmother)

Chr 17.pngAs well, I looked up John M in J’s FTDNA matches (his last name appeared in GEDMatch – I removed it for privacy) and found that he has a tree attached to his DNA results. He has ancestors from the same small town in New Brunswick as my grandmother’s ancestors, including one with a surname in that line. Since our paternal grandfather and grandmother came from different countries, it would be highly unlikely that we match on Chr 4 on one line and Chr 17 on a different. And since both segments are of a decent size (12cM and 14.9cM), it’s unlikely that one is a false match.

So while I haven’t (yet) figured out exactly who our common ancestor is, I’m quite confident that John M. is a Sharpe match.

Therefore, the crossover at 174.5 also belongs to J.

complete 174.pngAnd Chromosome 4 is complete

Chr 4 complete.pngWith the help of some stranger matches.

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