Yes. Siblings with the same two parents can get quite different ethnicity percentages on a DNA test, and that is normal. Each child inherits their own random selection of each parent’s DNA, so brothers and sisters genuinely carry different shares of their family’s ancestry. Part of the gap also comes from the test, since ethnicity numbers are estimates rather than measurements, and even identical twins don’t always get matching breakdowns.1The Tech Interactive. How can identical twins have different ancestry test results?
Why Siblings Inherit Different DNA From the Same Parents
You get about half your DNA from each parent, but not the same half your sibling got. When a parent makes an egg or sperm, the DNA they received from their own mother and father is cut and swapped. Each egg or sperm ends up with a unique patchwork of the grandparents’ DNA. Full siblings share about 50% of their DNA on average, but the actual figure varies from pair to pair.2PubMed Central. Variation in Genetic Relatedness Is Determined by the Aggregate Recombination Process
Take a dad who is half Irish and half Italian. One child might inherit more of his Irish chunks and another more of his Italian ones. A region that is small in a parent can be passed to one child and missed by another entirely, so a sibling can show a region you don’t have at all.
The farther back an ancestor lies, the smaller and more fragmented their surviving pieces are. That makes distant ancestry the most likely to show up in one sibling and not another. A region in your sibling’s results that is absent from yours is no sign that anyone is not a full sibling.
Many people assume siblings each get an identical 25% from each grandparent. In reality grandparent shares vary by child, sometimes well above or below 25%, so one sibling can carry more of the mother’s side or father’s side than another.
Mothers and fathers also shuffle differently. Men and women tend to make their swaps in different places along the chromosomes, with men’s clustered near the ends and women’s spread more evenly. This male-female difference shows up in humans and in most other mammals.3PubMed Central. Sex differences in the recombination landscape The rate of shuffling differs between mothers, and even between eggs from the same mother.4Nature Genetics. A high-resolution recombination map of the human genome So the shuffle isn’t perfectly even, which adds a bit more spread to how much DNA siblings share.5PubMed Central. Crossover interference and sex-specific genetic maps shape identical by descent sharing in close relatives
How the Test Itself Adds Differences
AncestryDNA splits your DNA into 1,001 small stretches and assumes each piece from mom and from dad comes from one population.6Ancestry. Ethnicity Estimate 2021 White Paper It then compares each stretch with DNA from people whose family origins are well documented.7Ancestry. Ethnicity Estimate 2021 White Paper The test reads your two parents’ copies mixed together, so the software has to consider every possible way they could fit.8Ancestry. Ethnicity Estimate 2021 White Paper Each stretch gets a probability of coming from each region, and these are combined into your overall percentages.9AncestryDNA White Paper. Ethnicity Estimate 2020 White Paper
The software is making a statistical judgment for each stretch, and stretches that resemble several populations can tip either way. Populations that share recent history, such as neighboring European countries, have similar DNA, so the comparison group can’t cleanly separate them. A small change in the software’s settings or comparison data can move a borderline stretch from one region to another.
That is how identical twins who tested with 23andMe have reported confirmed identical DNA but different ancestry breakdowns.10The Tech Interactive. How can identical twins have different ancestry test results? Each company uses its own comparison group, made partly of earlier customers, which helps explain why results differ.11PubMed Central. Consistency of Direct to Consumer Genetic Testing Results Among Identical Twins12PubMed Central. Consistency of Direct to Consumer Genetic Testing Results Among Identical Twins
Large differences in broad regions, like Europe versus Africa, are more likely to be real inheritance. Swings of a few percent between neighboring regions are often estimation noise, and regions under a few percent are the least reliable and most likely to vary. Comparing at the continent level gives a fairer picture than country-level splits.
Testing at different companies makes the gap worse. If you used AncestryDNA and your sibling used 23andMe, you are comparing two different methods, and each company’s region names and borders differ, so the labels themselves don’t line up. For a fair comparison, both siblings should test with the same company.
When you and your sibling disagree, start by lining up the largest regions in both results and ask whether they broadly agree, since that is where real inheritance shows. Treat small, trace-level regions that appear for only one sibling as likely noise unless a parent or grandparent confirms them. Family records or stories for the region in question are stronger evidence than a small percentage.
Why Your Results Change After an Update
Companies grow their comparison groups over time and re-process customers’ results.13Living DNA. Family Ancestry Results In one update, AncestryDNA went from comparing customers with 3,000 reference samples to 16,000.14CBC News. Did your Ancestry.com DNA results change dramatically? Here’s why New regions can appear, and low-percentage ones can disappear entirely.15CBC News. Did your Ancestry.com DNA results change dramatically? Here’s why If one sibling looked at results before an update and the other after, they may be seeing different versions.
With more reference samples, the software can recognize finer differences between populations, so a broad region may split into smaller named areas. A region that once absorbed some of your DNA may now be assigned to a neighboring region that is better represented. A small percentage that vanished may have been the software’s best guess under older data, not evidence that an ancestor was misidentified.
Your raw DNA stays the same through all of this; only the interpretation shifts. Your relative matches stay far more stable than ethnicity percentages.
To compare fairly, both siblings should open their results on the same date in the same app and check whether the site shows an update notice or version year. Older screenshots or printouts may not match today’s estimate, so use today’s version for both rather than memory.
Does a Big Difference Mean You Aren’t Full Siblings
Different ethnicity results are expected for full siblings, so they’re poor evidence about parentage. When siblings test at the same company, though, the site reports how much DNA you share, measured in centimorgans (cM), the unit DNA sites use to measure shared DNA. Full siblings typically share about 2,600 cM, half siblings about 1,750 cM, though ranges overlap with other relationships. Full siblings also share stretches where both copies match, which half siblings mostly don’t, so sites often label the match “full sibling” directly. Half siblings get their shared DNA through one parent, and the number of pieces they share can vary unusually widely.16PubMed Central. Crossover interference and sex-specific genetic maps shape identical by descent sharing in close relatives
To check, open your DNA matches list and find your sibling. Look at the shared cM figure and the relationship label shown. The cM number matters more than the label, since labels are guesses based on ranges that can overlap. A figure near the full-sibling range with a “full sibling” label is strong evidence you share both parents. A figure near the half-sibling range is a signal to look further.
That is because half siblings, grandparent-grandchild pairs, and aunt-nephew pairs can all share similar amounts of DNA, so the number alone can be ambiguous. Age gaps and family history often settle the question: a grandparent is a generation older, while a half sibling is usually close in age. Shared matches help too, because half siblings share relatives on only one side of the family. The pattern of which cousins you both match points to which parent you share. If it reads as half sibling, check matches on each parent’s side to see which side is missing. Asking a known cousin from each side to test can confirm which parent is shared.
Surprises do happen. Many people who test learn that the man they believed was their father isn’t their biological father.17PubMed Central. Discovering your presumed father is not your biological father: Psychiatric ramifications of independently uncovered non-paternity events resulting from direct-to-consumer DNA testing If the answer is unexpected, take time to absorb the news privately before sharing it.
Check that both siblings are looking at the right accounts and the right people before drawing conclusions. Sample mix-ups at the lab are rare but possible, which is why retesting is a sensible step before acting. A parent’s test is the most direct way to confirm whether that parent is shared with each sibling.
Getting the Most Reliable Comparison
A parent’s DNA test shows what was actually available for siblings to inherit. If a parent tests, sites can sort your matches into mom’s and dad’s sides, and the sibling you test with will share some of those matches. Every region in your result should trace back to one parent or the other, so a region that neither parent shows is more likely estimation error than inheritance, though parents’ own estimates carry the same noise. If a parent isn’t available, an aunt, uncle, or the parent’s full sibling can stand in partially, since they share part of that side’s DNA.
More siblings testing gives a fuller picture too. Each sibling reveals a different slice of the parents’ DNA, so combining several results covers more of each parent’s ancestry.
Many sites show a range behind each percentage, and it’s worth checking. A wide range means the estimate is shaky. Sibling numbers that fall within each other’s ranges are effectively the same.
Very small shared segments can be misleading. In one large family study, over two-thirds of shared stretches of 2 to 4 cM were false.18PubMed Central. Reducing Pervasive False-Positive Identical-by-Descent Segments Detected by Large-Scale Pedigree Analysis They often occur by chance or from software errors rather than from a recent common ancestor. A match made up of only small segments is much less likely to be a real close relative than one with a long segment. When judging relationships, look at the total shared amount and the longest single segment, not a list of many tiny pieces. If you are building a family tree from matches, set aside the weakest ones until a larger match supports them.