DNA tests like 23andMe and AncestryDNA provide estimates of your genetic ancestry by comparing your DNA to reference populations from around the world. These tests cannot pinpoint your ancestry down to an exact percentage like “50% Irish”, but they can give you a reasonable estimate in the form of a range, like “45-55% Irish”. There are a few key reasons why DNA ancestry tests are limited to providing a range rather than an exact percentage:
Limitations of Reference Populations
The reference populations that DNA testing companies use to compare your DNA to are just samples of people intended to represent broader population groups. For example, AncestryDNA’s “Ireland” reference population includes DNA samples from 443 people who claim Irish ancestry. This is obviously just a tiny fraction of the millions of people with Irish descent worldwide. Since the reference panels are limited, your results are unlikely to match any one population perfectly.
Inheritance is Random
The DNA you inherit from your ancestors is passed down randomly. You receive 50% of your DNA from each of your parents, but there is randomness in exactly which DNA segments you get from each parent. So even full siblings only share around 50% of their DNA on average. This randomness means that your DNA results will not precisely reflect your genealogical ancestry. For example, you could have a grandparent who is 100% Irish, but inherit only 40-60% of their DNA.
Ancestry is Complex
Most people have ancestry from multiple countries and populations. When all of your ancestors are mixed together, it becomes impossible to precisely unravel what percentage came from where. Your DNA is made up of many small segments, each telling a story of one ancestor. But those stories combine in a way that can’t be simplified to a whole number percentage.
What is the range of possible percentages?
Although exact whole number percentages are unlikely, DNA tests can still give you a reasonable estimate by providing a range. Here are some examples of the ranges you might see for different ancestry combinations:
One population
For ancestry from just one population, such as 100% Irish, the range is usually around 80-95% for that population. A full Irish ancestor 4 generations back contributes about 12.5% of your DNA. Due to randomness, you might inherit anywhere between 0-25% of that ancestor’s DNA, leading to the wide range.
Two populations
For a 50/50 split between two populations, such as Irish/German, the ranges tend to be 40-60% or 35-65% for each population. The range widens as more populations mix together.
Three or more populations
As you get more mixed, precision declines. You might see ranges like 15-35% for one population, 10-30% for a second population, etc. The ranges start to overlap more significantly.
Why not a single percentage?
Given the limitations described earlier around reference populations, inheritance randomness, and ancestral complexity, single point estimates of ancestry are highly unlikely to be accurate. Ranges better capture the inherent uncertainty in genetic ancestry estimates. Here are some examples:
Example 1
If your ancestry is 50% Irish and 50% German, you would not expect a test to say “50% Irish, 50% German.” It is much more likely you would see ranges like “40-60% Irish, 35-65% German.” Those wider ranges better represent the many possible outcomes given randomness in inheritance.
Example 2
Even if you have a parent who is 100% Irish, your results would probably not come back as “50% Irish.” More likely you would see a range like “45-55% Irish” reflecting the fact that you only inherit a random subset of your parent’s DNA.
Example 3
As ancestry mixes over generations, precision declines further. If you had an Irish grandparent, Irish great-grandparent, and German great-grandparent, seeing precise percentages like “25% Irish, 12.5% Irish, 12.5% German” would not capture the complexity of your ancestry. Broad overlapping ranges would be more accurate.
What factors influence the size of the range?
A few key factors impact how wide or narrow the range is for a particular ancestral population in your results:
Number of generations since ancestry
The range narrows as ancestry is more recent. For example, you share about 50% of your parent’s DNA but only 12.5% of a 4th great-grandparent. More recent ancestry is better represented in your genes.
Sample size of reference population
Larger reference panels lead to more precision. The ranges for your Irish ancestry, for example, are likely to be more refined as the Irish reference panel includes more samples.
Genetic distance between populations
It is easier to distinguish ancestry from two very genetically distinct populations than two closely related ones. The range for Scandinavian versus Irish ancestry, for instance, will be narrower than trying to distinguish German versus French ancestry.
What are some common misconceptions?
There are a few common misconceptions people have around the precision and accuracy of genetic ancestry estimates:
Precision vs. accuracy
Many people focus heavily on getting an exact percentage result like “32% German” but do not think about whether that number is truly reflective of their ancestry. Ranges better capture accuracy along with uncertainty. A result of “35-55% German” may in fact be much more accurate for someone with mixed German ancestry than “46% German” even though it is less precise.
Recent immigrant ancestry
Some people assume that having a parent or grandparent who immigrated means their ancestry from that population should be “50%” or “25%.” But DNA inheritance is random, so the ranges could still be quite wide. Those recent ancestors are also just two out of dozens in your family tree.
Understanding ranges
People sometimes misinterpret a wide range like “10-35%” to mean the test has no idea what the ancestry is. In reality, wide ranges just reflect the inherent limitations in pinpointing the percentages. The ancestry is still detected and identified, just not with unrealistic precision.
One updated number
Ancestry percentages are often cited as if they are a single fixed number. But they can shift by a few percentage points between testing companies or updates. The ranges capture this uncertainty better than focusing on any one number.
How are ancestral percentages calculated?
DNA testing companies use statistical algorithms and modeling techniques to translate your DNA data into ancestral percentage estimates. Here is an overview of the general process:
1. DNA matching
Your DNA is scanned for thousands or millions of genetic variants and compared to the reference populations. Algorithms assess which variants you share in common with each population.
2. Admixture modeling
Statistical models estimate ancestry proportions that are most consistent with your DNA matches to each reference population. The models consider linkage patterns and other factors in determining the percentages.
3. Range determination
Based on the statistical uncertainty levels in the models, a plausible range is calculated around each ancestral percentage estimate. Conservative ranges capture the inherent ambiguity.
4. Sample size influence
For populations with larger reference panels, more precision may be possible in narrowing the range further. But wide ranges generally persist for most ancestry, especially as generations mix.
How accurately can I trace specific ancestors?
While DNA tests can often accurately identify regions and populations of the world your ancestors came from, pinpointing specific ancestors usually requires traditional genealogical research to build your family tree. Here are some key limitations:
No family tree from DNA alone
Your DNA results alone cannot reliably determine relationships and ancestors more recent than approximately 5th cousins. Closer relatives and specific ancestors must be identified through historical records, family histories, and genealogical research.
False leads from inherent ambiguity
Since genetic ancestry estimates provide wide ranges across many populations, it is easy to see false leads about specific ancestors that cannot be verified genealogically. For example 10-30% Finnish ancestry does not necessarily mean you had an ancestor from Finland.
No precise snapshot from a single point in time
Your DNA reflects the blend of all your ancestors over generations, not just those living at one point in time you may be interested in researching (e.g. the 1880 census). DNA cannot isolate ancestors just from a particular decade without additional genealogical context.
Limitations tracing female ancestors
Since mitochondrial DNA is passed down only from mother to child, it is difficult to trace female ancestors through your father’s side using your DNA alone. Genealogical records tracing all branches of the family tree are essential.
How reliable and accurate are ancestry DNA tests?
Ancestry DNA testing has become extremely reliable for identifying regional ancestry, but limitations lead to uncertainty about more precise and recent ancestry:
Regional ancestry highly reliable
The latest ancestry tests can accurately identify genetic ancestry from broad regions at a continental level with high reliability for most people. Results successfully distinguish European, African, Asian, and American ancestry, for example.
Caution interpreting smaller regions
Genetic differentiation decreases for more narrow regions within continents. While tests can often accurately identify some subregions like Scandinavia or Iberia, smaller details should be interpreted more as clues requiring additional genealogical research rather than definitive proof.
Limitations increase for recent ancestry
Due to randomness in inheritance and limitations of reference populations, precision declines rapidly as you try to identify ancestors more recent than great-grandparents. Published estimates of accuracy for identifying ancestry in the last 100 years are as low as 10-20% from DNA alone.
Results differ between companies
Due to differences in the reference panels and algorithms used, your ancestry results can shift noticeably between testing companies. These differences in the percentages reveal the inherent uncertainty and limitations to pinpointing precise ancestry from DNA alone.
How are ancestry tests regulated?
There is minimal government regulation around DNA ancestry tests. Key aspects include:
No FDA approval
DNA ancestry tests are not approved by the U.S. Food and Drug Administration. The FDA previously tried asserting regulatory authority but ultimately chose not to regulate ancestry tests.
General FTC requirements
The U.S. Federal Trade Commission requires DNA testing companies to comply with truth-in-advertising standards. This includes ensuring any claims are supported by scientific evidence.
Transparency reports
Some DNA testing companies voluntarily release transparency reports detailing their accuracy statistics. But reporting methods are inconsistent and standards are not uniform across the industry.
Professional guidelines
Groups like the American Society of Human Genetics have published recommendations around reporting of ancestry estimates and explaining uncertainties, but there is no mechanism to enforce adherence.
State consumer protection laws
General state consumer laws prohibiting false or deceptive advertising provide some backstop against grossly misleading claims about ancestry tests. But specifics are untested.
What questions should I ask ancestry DNA testing companies?
Here are some key questions to ask when considering purchasing an ancestry DNA test:
How are reference populations constructed?
Ask for details on the populations used for comparison, including sample sizes and geographic sources. Larger and more representative reference panels lead to more accurate results.
How are ancestral percentages calculated?
Inquire about the statistical and computational methods used to estimate ancestry, and how ranges are determined around those estimates. Look for tests using rigorous statistical modeling approaches.
How often are estimates updated or changed?
Ask whether your results are expected to change over time as reference panels expand. Changing estimates can reveal the inherent uncertainty in the process.
Can results be verified through genealogical research?
A good DNA ancestry test will emphasize traditional genealogical research using records and family trees is still essential, especially for recent ancestry. DNA alone cannot prove specific ancestors.
What populations have less certainty?
Find out if any reported ancestries, especially smaller regions, are known to have higher uncertainty due to limitations in reference data, genetic differentiation, or other factors.
Conclusion
DNA ancestry tests rely on comparing your genetic variants to sample reference populations. While these tests provide useful estimates of your regional ancestry, the percentages are unlikely to be precise for more recent ancestry due to the inherent randomness in inheritance and limitations of reference panels. Ancestry DNA testing can guide genealogical research, but it is essential families also work with historical records, documents, and family narratives to fill out the family tree. Any DNA results must be interpreted with an understanding of the limitations and uncertainties involved in testing.