BLUF: MITx Course: Genetics: Population Genetics and Human Traits – Free Starts NOW!

Several years ago, I took their course: DNA – The Secret of Life; taught by Prof. Eric Lander – short bite-sized videos (some of which I watched several times); interspersed with a quiz; including labs and aside material – very close to a real life classroom experience – better than a Zoom (you can pause and rerun). Self-paced but covers a semester class. These MITx classes are available FREE! I highly recommend this class for anyone wanting to take a deep-dive into DNA. In one segment you’ll learn that the DNA base pairs are read in the 5 prime to 3 prime direction…

Link: https://www.edx.org/learn/biology-life-sciences/massachusetts-institute-of-technology-genetics-population-genetics-and-human-traits?utm_medium

[22BY] Segment-ology: Free DNA Classes from MITx TIDBIT by Jim Bartlett 20230816

A Segment-ology TIDBIT

This is an important point: The DNA Matching algorithms are based on finding a Shared DNA Segment. My DNA Segment matches your DNA Segment. We say: I match you; but, technically what we mean is my DNA matches your DNA – our DNA segments overlap and match – they match enough to satisfy the algorithm.

Above 15cM, the matching algorithms are designed to determine an Identical By Descent (IBD) segment. My DNA is identical to your DNA (all the SNPs are the same) over enough DNA that it can only be that way because we both got that segment of DNA from the same Ancestor. This is the foundational concept of autosomal DNA testing.

If a person only shares one IBD segment with us, we call that person a Match. We match the Match. We are basically equating the Match to the Shared DNA Segment. And most of our “Matches” share only one IBD segment with us. But not all…

Some of our “Matches” share two or more DNA segments with us. In these cases, we need to be careful how we speak. Each of these Shared DNA Segments is an independent event. Most of the time, multiple Shared DNA Segments will be from the same Common Ancestor, but that’s not a requirement of the biology. From my spreadsheet of over 20,000 shared segments, I can attest that there are many instances of multiple segments from one Match coming from different Ancestors (as well as many instances of Matches who are genealogically related to me in multiple ways).

I raise this point because I found myself assigning my 3C Match with 6 Shared DNA Segments to our Common 2xG grandparents. That is, in my spreadsheet, I assigned all 6 segments to the same 2xG grandparents. Analyzing each of these resulting Triangulated Groups, I found one that was “off”, “strange”, “out of kilter”… The other Matches in that TG were related to me on a different line. A little investigation into my 3C’s Ancestry revealed we were also 7C on a different line.  

I also raise this point to illustrate the importance of segments in genetic genealogy.

The point here is that we don’t really “match” a Match, we only match a part of their DNA.

Notwithstanding… like most of us, I will continue to say that “I have a Match” and that “I match person A”. It’s just important to remember that part of our DNA matches part of our Match’s DNA.

[22BF] [ Segment-ology: We Match Segments, Not Matches TIDBIT by Jim Bartlett 20230813

Bottom Line Up Front [BLUF]: Yes, but caution.

Here is the original statement that prompted this blog post:

The chance that three fourth cousins will all share the same matching segment is practically zero.

A bold statement – repeated several times – that has implications for Triangulated Groups. It appears this was part of the education material provided by AncestryDNA for their DNA Circles feature [Hat Tip to Debbie Kennett – the material is no longer online].

This means you and two 4C Matches sharing the same matching segment [all three of you descending from 3 different children of the Common Ancestor].

Mitigating factors:

1. Shared DNA segments in a Triangulated Group (TG) are rarely “the *same* matching segment”. We are almost always talking about overlapping segments of different sizes. So that gives some wiggle room. Maybe the odds are just small (not practically zero) with a group of different sized segments in a TG.

2. As Debbie pointed out to me, these were simulations by Ancestry, using “perfect” data. In genetic genealogy our data is usually somewhat messier than simulated data, so there is even more wiggle room. Maybe the odds are on the low end…

3. Another factor is that the data has grown substantially since the simulations were done for the Circles feature. The information has been removed.

The bottom line for me becomes: If you find 4C Matches in one TG from more than two other children of the Common Ancestor, take a closer look at it. It is possible, but there may be other factors at play.

Segment-ology CONCEPT – For Matches forming a TG (overlapping segments in a range), the odds decrease with each generation going back and with each additional child of the Common Ancestor. Take a critical look within TGs beyond 3C Matches spread over more than 2 other children. The odds are very small with Matches from 3 other children (total of 4 children).  This is not a “rule”.

Important Note: This does not mean that we cannot have DNA Matches from 4 or more children. We can! Instead of a double negative let me say: We can have 4C Matches from more than 3 other children of the Common Ancestor – we can have 7C Matches from 5 other children of the CA. It just means that there is more than one segment (TG) involved.  Over the different children, we should expect to see several TGs. We can have over a hundred Matches in a TG going back to 7XG grandparents, for example. We just need to carefully screen for the number of children per TG.

Takeaway: It’s hard to have a hard “rule” on this subject. However, it makes sense to pay attention to our data. The further back we go (in generations), the more constrained our options become.

I’m inviting discussion on this Segment-ology CONCEPT, and on your experience with TGs and numbers of Common Ancestor children.

[08E] Segment-ology: Can Three Fourth Cousins Share the Same Segment? By Jim Bartlett 20230812

A Segment-ology CONCEPT and Thought Stimulator

Per Wikipedia: A haplotype is a group of alleles in an organism that are inherited together from a single parent, and a haplogroup is a group of similar haplotypes. Your atDNA segment from an Ancestor is likewise a group (or string) of alleles (SNPs) that is inherited from a single parent – an atDNA haplotype. The Match segments in a Triangulated Group (TG) have this same string of SNPs – they have matching shared DNA segments – and this group would then be a Haplogroup (Hg).

A Triangulated Group of segments would be a Haplogroup.

Wikipedia also notes that in human genetics, the haplogroups most commonly studied are Y-Chromosome (Y-DNA) haplogroups and mitochondrial DNA (mtDNA) haplogroups, each of which can be used to define genetic populations.

In exactly the same vein, a Triangulated Group (TG) defines a genetic population. It’s the population of descendants who carry the same segment of DNA passed down by an Ancestor. DNA test takers in this population have shared DNA segments with the same string of SNPs – they match each other!

An mtDNA Hg is often many thousands of years old (because the mtDNA rarely changes). A Y-DNA Hg is usually somewhat closer, and with a Big-Y test, is often found within a genealogical timeframe. My estimate is that an atDNA Hg (a TG) is usually 5-9 generations old – generally within a genealogical timeframe. We could argue that a TG Hg is a better tool than Y or mt. For me, it is a very good tool. In any case, each DNA Hg tool has strengths in genetic genealogy.

Note that the process of Triangulation culls out most, if not all, false shared segments. A few false Match segments (under 15cM) may slip in; but your own DNA, as the base in a TG, is true. If such an under-15cM Match is critical to you, you need to check for Triangulation with that Match segment as the base.

MUSING….

Dr. Tim Janzen – one of the earliest pioneers in atDNA (and my early mentor), has often advocated for a database of unique atDNA segments from our Ancestors. I used to think of this as a giant TG database and wonder how we would describe each TG. Now I think it would be an atDNA Haplogroup database, but still wonder how we would describe each Hg. Each segment would be unique to a specific Ancestor and would be on a specific chromosome (with start and end points). Note the chromosome could be maternal or paternal, depending on each Match’s ancestry. This segment would manifest itself in a TG, with shared segments from other descendant Matches. Each Match would likely have his or her own unique TG. These TGs taken together would represent an atDNA Hg from that Ancestor.

NB: if we can phase our data, we could actually record the SNP alleles (ACGTs) in each TG (or atDNA Hg)! Alternatively, by comparing raw DNA data among the Matches in a TG, we could probably determine the individual the SNPs. Remember your TG segment is the equivalent of phased DNA.

This post is about an atDNA Haplogroup. It’s a concept to think about. Your thoughts are welcome here.

[14B] Segment-ology: A Triangulated Group is an atDNA Haplogroup by Jim Bartlett 20230802