My Big Fat Chromosome 20

I never would have guessed 10 years ago that I would be blogging about my Chromosome 20. 10 Years ago I was definitely interested in genealogy, but knew virtually nothing about DNA. Even if I did know anything about DNA I would not have guessed that it would have anything to do with genealogy.

My Chromosome 20

My Chromosome 20 actually isn’t that big and fat. Actually it is one of my smallest chromosomes. However, I have more matches there than on any other chromosome. In fact, over 1,000 – more than a quarter of my matches – are on Chromosome 20. This is pretty amazing considering I have 23 chromosomes counting my X Chromosome. If my matches were spread out evenly over these 23 chromosomes, I would expect each chromosome to have about 4% of my matches. This representation shows the ridiculous number of matches I have on Chromosome 20. They are on the bottom of the image in light blue.Joel Hartley Circle Chart

This particular representation is for just my FTDNA Family Finder matches. I believe the threshold was set relatively high and this was done a while ago. However, at the time and threshold, it appears that more than half of all my matches were at Chromosome 20.

How To Explain All the Matches? Colonial Massachusetts?

I had a difficult time explaining all the matches I had on Chromosome 20. Most were on my paternal side as that is where most of my matches are. I had guessed that these may have been due to a colonial effect as that had been suggested in various places. My great grandmother’s mother was a Bradford and was descended from the Mayflower Bradfords. A lot of those early Pilgrims married other related Pilgrims. In fact, some of my Chromosome 20 matches were descended from a Brewster who was one of the Pilgrims that I am also descended from. Then there were a few who seemed to be related on my Irish Frazer side. Finally I had a match with Bonnie from the Frazer DNA Project I am working on. She matched on Chromosome 20 but was outside my large triangulation groups.

Chromosome 20 Triangulation Groups

I also have Triangulation Groups (TGs) for Chromosome 20 – very large ones. In fact, gedmatch would overload when I tried to run an analysis I had so many. I have 2 paternal TGs and one maternal TG. There also may be sub-TGs within those.  I have roughly 650 matches in these combined TGs. So now, based on testing my mother, I knew if my matches were maternal or paternal and if they were in TGs, but I still didn’t know much about where the common ancestors could be other than a vague guess about colonial Massachusetts. What I did was ignore Chromosome 20. I gave up even adding matches to my spreadsheet because I had so many. These matches tended to be around 13 cM with some higher and some lower.

Sticky Segments Or Pileup Areas?

While looking for a Chromosome 20 explanation, I read about sticky segments and pileup areas. Sticky segments are those that came down intact for many generations. They don’t want to go away. However, a few sticky segments wouldn’t explain over 1,000 matches. It seemed like I had a pileup, so I looked into those. Pileup areas are areas are described by Jim Bartlett in his comment on one of his blogs:

I do find that each person tends to have two kinds of pileup areas: 1) are fairly narrow, are widespread, and are outlined in this ISOGG article:; and 2) are also fairly compact (7-9cM) and are unique to each person. I believe these are caused by a unique set of markers in our personal DNA that makes it easy to form matches with others in that region. These are characterized by many segments in a narrow range, which do not generally Triangulate, and the Matches don’t see this as a pile-up area, only you do.

However, my case didn’t seem to match some of the explanations of sticky segments or pileup areas. My matches were larger and did triangulate. Furthermore, they were not in areas of the chromosomes described in the ISOGG article above.

Enter Kathy Johnston and Her Crossover/Segment Analysis

At the beginning of 2015, Kathy posted her instructions on an FTDNA Forum for analyzing DNA based on the 3 siblings. She showed how to determine the 4 grandparents’ contributing DNA for each of these siblings.  I discovered her post at the end of 2015. Could this help me figure out my Chromosome 20? I tried Kathy’s method and got some surprising results.

Finding Chromosome 20 Crossover Points

Finding crossover points in Chromosome 20 was not as easy as it has been in other chromosomes. According to Kathy, usually there will be one owner of a crossover point. This owner will appear in 2 out of the 3 comparisons at a crossover point. In this one, I found only one clear owner. That was my sister Heidi at position 47. For the other ambiguous crossover points, I gave a double initial separated by a slash.

Chr 20 Crossovers

Below, the gedmatch comparison is transformed into a maternal/paternal Chromosome 20 map. The green area means that Heidi matches Joel on the 3rd segment. This match is a Fully Identical Region (FIR). This means they match the same maternal grandparent and the same paternal grandparent. For Joel, I move those grandparent to the right as I have no crossovers until the last crossover point.

Chr 1 Segment 1

Sharon has no match with her 2 siblings in the same area, so that will mean she shares the complementary grandparent on her maternal and paternal DNA. This will be represented by 2 different colors. I again extend that double segment to Sharon’s crossover points.

Chr 1 Segment 2

Looking at the earlier gedmatch comparison, in the 2 segments to the right of Heidi’s existing mapped segment, there is a Half Identical Region (HIR). That means a grandparent matches on one chromosome and doesn’t match on the other. This will be shown as 2 different colors in this area when comparing Heidi to Joel. This first HIR choice is chosen randomly as no names or side (maternal/paternal) have yet been assigned to the grandparents.

Chr 1 Segment 3

Next, we have an illogical situation.

Chr 20 Crossovers

In the next to last segment, the smaller one, Sharon is no match with Heidi or Joel and Heidi and Joel have a half match. That is illogical because if Sharon doesn’t match with Joel, that is the same orange/purple scheme continued in the small segment for Sharon. Then if Sharon and Heidi are opposites, it goes back to green/blue for Heidi in that small segment. Those are the same colors that Joel already has, so that means that Heidi and Joel can’t be HIR which means they should have one matching color and one non-matching color. However, look at that small segment again in the first two rows. The red is strong in the first row. In the second row, I hardly see any red – with red indicating no match at gedmatch. Therefor, I’m going with the first comparison of Heidi and Sharon. Plus this goes with the matches that I will mention soon that Sharon has. I make Sharon and Heidi opposite in Sharon’s little segment and extend that segment to the end.

Chr 1 Segment 4

I filled in some of the no matches and FIRs on the right. On the left, I was left with 2 illogical no matches again, so I chose the redder of the 2. This left me with having to guess a HIR on the left. I am only allowed one guess, so I left this blank for now.

Chr 1 Segment 5

Adding Real Grandparents

It would be nice to add actual grandparents here and not just speak of my orange grandparent, for example. I can do this using two of Sharon’s matches.

Sharon's Chr 20 Matches

These 2 important matches Sharon has are both on the paternal side. James is related to my grandfather and Bonnie is in the Frazer DNA Project on my Frazer grandmother’s side. Coincidentally, the orange match above goes with the orange on my chromosome map. That would make my paternal grandfather Hartley orange and paternal grandmother Frazer green.

Joel’s Matches

Here’s my Frazer match with Bonnie. 47 to 54 is in my green Frazer region on my map. So that is a relief.

Joel's Frazer 20 Match

Below is my only maternal match. It is with a cousin on my maternal grandmother’s line. She matches only with me because she tested at 23andme and hasn’t uploaded to gedmatch yet.

Joel match Judith 20

However, Judy gets me unstuck on my maternal side. Her match is telling me that from zero to 8, I can identify my grandparent. I already have blue from 6 to 8 (from using my brighter red logic). So I just need to extend the blue all the way to the left on my maternal segment line. That gives me a solid blue on Chromosome 20 on my maternal side.

Chr 20 Final Segment

This is as far as I can figure out now without further guessing. Perhaps when cousin Judy gets her DNA uploaded to Gedmatch, I will know more. So what does this tell me about my 1,000 plus Chromosome 20 matches and 600 plus matches that appear to be in Triangulation Groups?

Mystery Solved?

I think it is. These matches correspond to the area on the map above between 16 and 49. By the above mapping these massive amount of matches are solidly in Frazer territory for me. Instead of my huge block of matches being in colonial Massachusetts, I see that they are on my Frazer line. That came as quite a surprise. These ancestors were in Ireland mostly. I assume that many of these ancestors got out of Ireland. Perhaps they moved to the United States and married people who were descended from colonial Americans. That would explain some of the other colonial matches.

Summary, Application and Conclusions

  • When you are looking for DNA matches, it helps to know where you are looking
  • While I was looking at my largest group of matches, I was looking in the wrong place even though I had some reasonable assumptions
  • Kathy Johnston’s method cuts through bad assumptions and replaces them with sound logic
  • Phasing by parents cuts the looking in half but didn’t help me with identifying a huge block of Chromosome 20 matches. However, Kathy Johnston’s method is twice as good as phasing as it separates all matches to areas of 4 grandparents.
  • This method needs 3 siblings and some known tested relatives.
  • If I have this mapped correctly, any maternal match after 6 million for Sharon will be on the Rathfelder line and any maternal match for me will be on the Lentz line.
  • Interestingly, I have only about 42 matches for my sister Sharon on this Chromosome. Given that the makeup of her Chromosome 20 is mostly opposite of her 2 sibling, this makes a lot of sense.
  • I forgot to mention that my sister Heidi has almost as many matches as I do on Chromosome 20. Her shorter Frazer segment compared to mine would explain the slightly fewer matches.
  • The fact that all these matches are on my Frazer line doesn’t necessarily mean that they are Frazer matches. They could be McMaster, Clarke, Spratt or any other known or unknown ancestor of my Frazer grandmother.


10 Replies to “My Big Fat Chromosome 20”

  1. Hi Joel, nice blog and post. Kathy J also showed me how to do this and I have mapped my mother, her brother and sister’s chromosomes to all 4 of their grandparents. The “2 out of 3” rule works well when it does – which is most of the time. Problems occur when you move from fully to non-matching segments. Sometimes 2 siblings have crossovers at the same point which can be hard to detect. That’s a good technique to pay attention to how much red is in the gedmatch comparison. I’d not thought of that for resolving these types of quandaries, obvious though it is. Keep up the good work!

    1. Thanks Barton, Kathy had mentioned your name. She also helped by getting me to lower the gedmatch thresholds. That brought up more Half Identical Region matches where I thought they might be.

  2. Can this same idea by used to help identify chromosomes for grandparents of 3 fist cousins and a nephew of one of them?

    1. Can we expand on this? If you have 3 siblings’ information and you add a cousin’s information can you use what you got from your 3 siblings to help the cousin/s if they do not have siblings? Let’s just say one single cousin wants to glean to find matching info from your cross-over findings.
      I have just started looking into the mass amount of information related to DNA recombination/crossover and barely dipped my fingers into autosomal DNA info in the first place. In other words I’m still under layman’s terms/use. Thank you for any help you may be able to provide in understanding.

      1. It depends on the level of cousin and whether you know how you match the cousin by genealogy. If the level of cousin is 2nd or further out and you know their genealogy, you would already know what grandparent they match on. If the cousin is a first cousin, then yes, it would help to narrow down which of the two choices of grandparent line the 1st cousin matches on. If the cousin is 2nd or further out and you don’t know how they are related, then the information would be helpful if you know whether they are a maternal or paternal match. In my case, I have my mother tested and know whether my matches are maternal or paternal by phasing. If you don’t have a parent tested, then the mapping would not help as much as you wouldn’t know which side of the chromosome the match is on without guessing. The guessing may be in the form of geography. So if the two choices of matches were from Slovakia or England, it may be more obvious depending on the genealogy of the cousin match.


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