AncestryDNA recently introduced a new feature that maps your ethnicities onto your individual chromosomes. This beta chromosome painter can be found in the “DNA Story” tab.
The chromosome painter builds on technology that Ancestry’s scientists introduced with SideView™. SideView™ is a technological advance that can apportion your ethnicity estimates to the contributions from each parent. It does this by comparing your DNA to that of your matches.
Think of it this way: if both of your parents have tested, all the system has to do is compare your DNA to theirs to determine which copy of chromosome 1—we have two copies of each chromosome—came from Mom and which came from Dad. The same with each of the other chromosomes. Even if only one parent has tested, the chromosomes can be sorted by process of elimination. This sorting process is called phasing.
But if neither parent has tested, things get trickier. Consider a match who shares a single DNA segment of 25 cM with you. The algorithm will be able to phase that small portion of your DNA into Parent 1 and Parent 2, but it won’t be able to tell which parent is which, nor will it have any insight into the rest of that chromosome or any of the others, for that matter.
Now consider a first cousin match, like the one shown here. These two cousins share about 1,100 cM across 34 segments, with at least one segment on every chromosome except numbers 14 & 21 and the X chromosome. The algorithm still doesn’t know whether this is a maternal cousin or a paternal cousin, but it can at least conclude that all 34 of those segments came from the same parent. We’ll call this Parent 1. By process of elimination, the complementary segments on each chromosome must have come from Parent 2.
Another first cousin on this side is likely to add an additional 600 cM or so to our Parent 1 phasing. The algorithm knows the segments came from the same side because the two cousins will also match one another. (This phasing algorithm can be misled by endogamy.)
More distant cousins can help, too, especially if they share segments on two or more chromosomes. The algorithm will phase your DNA using cousins on both sides, sometimes assigning DNA regions based on a positive segment match and sometimes by elimination.
With a large enough database, the method should be able to phase all or almost all of your genome. AncestryDNA is positioned to make the most of this technology, because they have the largest database of the genealogy companies, more than 22 million people as of June 2022. Compare that with the next largest database, 23andMe, which has roughly 13 million genotyped customers.
Once the DNA is assigned to Parent 1 and Parent 2, the ethnicity estimates can be divided up accordingly.
Which brings us back to the new chromosome painting feature. All of the major genealogy DNA companies estimate ethnicity piece-by-piece along the chromosomes, and both 23andMe and FamilyTreeDNA offer chromosome paintings. These are visual representations of where along your chromosomes each ethnicity is found.
AncestryDNA is now showing us the same information, but with the added benefit of sorting the chromosomes by Parent 1 and Parent 2. Even if neither parent has tested there.
To see your chromosome painting, click the blue “View Breakdown” button below the SideView™ circle (see first image above), scroll down, and click “Explore now” in the Chromosome painter panel. Yes, it’s a bit hidden; I hope they’ll make this feature easier to find once it’s out of the introductory beta phase.
It’s pretty neat! You can see that one of my parents is mostly Germanic and Irish (teal and medium blue segments) while the other is mostly French (green). This aligns well with the family histories of my German–Irish father and Cajun mother.
A Deeper Analysis
For this comparison, I imported segments from both companies into separate DNA Painter chromosome maps and tweaked the colors to be similar. In both maps, my paternal chromosome is above my maternal one.
At first glance, the most obvious difference is that AncestryDNA favors solid chromosomes of a single ethnicity while 23andMe shows more variation across most chromosomes. This was a bit surprising given that AncestryDNA has a more fine-grained reference panel (77 regions) than 23andMe (44 regions), so I’d expect AncestryDNA to be able to give me more precision.
Of course, which company is more correct is an open question. On this front, I’m inclined to believe 23andMe. Why? Consider my paternal chromosomes. Only two of them (chromosomes 3 & 4) show any ethnic mixing. My father’s mother was 100% German while my grandfather was about 60% Irish and 40% German. It’s unlikely that the chromosomes I inherited from my father would be all one or the other.
What’s more, AncestryDNA shows chromosomes 9, 11, and 20 as all Scottish and chromosome 15 as all Sweden & Danish. I have no known ancestry from either region, and any unknown heritage should be in segments much smaller than an entire chromosome.
Then there’s the Spanish segment on my maternal chromosome 8 (purple).
Roughly 2,000 Canary Islanders settled in southern Louisiana between 1778 and 1783, including some of my ancestors. Although an imperfect proxy, my tree suggests I am about 5.5% Spanish. Yet, Ancestry finds only one segment making up 1% of my genome. By comparison, 23andMe estimates me at 9.7% Spanish & Portuguese.
What’s more, this is my mom’s chromosome 8 at AncestryDNA:
And here’s the Spanish & Portuguese estimate for me from 23andMe on chr 8:
The Spanish fragments just barely overlap!
I suspect the two issues are related, that Ancestry’s ethnicity algorithm penalizes a switch from one ethnicity to another, causing chromosomes to be assigned a uniform ethnicity when they shouldn’t be and impacting where the real breaks should occur.
The Good News
Fortunately, the chromosome painting is still a beta feature, meaning that AncestryDNA‘s scientists and programmers are still working on it. And it’s a great addition! AncestryDNA recently announced that their annual ethnicity update will happen in the coming weeks. I am looking forward to seeing how the chromosome painting feature improves!