How Much DNA Do Grandparents and Grandchildren Share?

A grandparent and grandchild share about a quarter of their DNA on average, the same as an aunt or a half-sibling. That quarter is only an average, though, and real grandparent pairs can land well above or below it.1My Forever DNA. What Percentage of DNA Do Grandparents Share with Their Grandchildren?

The Numbers You’ll See on a DNA Test

Here are the reference values to compare your result against:

Test sites report shared DNA in centimorgans (cM), not just percentages. A cM measures the length of shared DNA stretches, weighted by how often DNA gets shuffled in that spot. The parts of the genome being compared total roughly 7,000 cM, so 1,750 cM works out to about a quarter. You’ll see the figure on your match page, right beside the predicted relationship.

Those figures come from thousands of people who reported their known relationships along with their test results.5Journal of Genetic Genealogy. The Shared cM Project: A Demonstration of the Power of Citizen Science They describe typical results, not hard limits every family must fall inside.6My Forever DNA. What Percentage of DNA Do Grandparents Share with Their Grandchildren? A result near the edge of the range is unusual, but it isn’t proof that something is wrong.

About 1 in 100 real pairs will land outside the range by chance. Landing inside it doesn’t prove the relationship either, since aunts, uncles and half-siblings fill much the same range.

Your site may predict several relationships for a single number. A match of 1,750 cM often appears as “grandparent, aunt/uncle, or half-sibling.” The cM number alone can’t separate them. Age and what you know of your family tree usually settle which one it is.

To check your own match, compare the shared cM figure with the 984 to 2,462 cM range, not the site’s percentage. Sites round percentages and compute them differently from the chart, so a percentage that looks a little off from 25% isn’t a sign of an error. If you and the other person tested at different companies, compare each site’s number separately, because each uses its own matching method and the totals can differ somewhat.

Why It Isn’t Exactly 25 Percent

Every parent carries one set of chromosomes from their mother and one from their father. As eggs and sperm form, the paired chromosomes swap matching stretches of DNA.7My Forever DNA. What Percentage of DNA Do Grandparents Share with Their Grandchildren? So a chromosome passed to a child can hold pieces from both of that parent’s parents. Which whole chromosome of each pair gets passed on is also a matter of chance.8My Forever DNA. What Percentage of DNA Do Grandparents Share with Their Grandchildren? The result is not a measured quarter from each grandparent.9My Forever DNA. What Percentage of DNA Do Grandparents Share with Their Grandchildren?

Sometimes more chunks come from one grandparent by chance. Across all the chromosomes the share evens out toward a quarter, but not exactly.

How much grandparent sharing varies depends on how much shuffling happens.10PubMed Central. Variation in Genetic Relatedness Is Determined by the Aggregate Recombination Process The number of chromosomes, their sizes, and how many swaps occur all affect the spread.11PubMed Central. Variation in Genetic Relatedness Is Determined by the Aggregate Recombination Process Small chromosomes vary more in what they pass on because they get fewer swaps.12gcbias. How much of your genome do you inherit from a particular grandparent?

A parent’s chromosomes are cut into a limited number of chunks, so the child’s half is made of a limited number of pieces. A few extra pieces from one grandparent tilt the total. The tilt is usually modest, which is why most pairs stay in the same broad band around a quarter. Each grandchild gets a different random mix, so one may share 30% with a grandmother while a sibling shares 20%.

Maternal Versus Paternal Grandparents

Egg formation averages about 42 DNA swaps, while sperm formation averages about 27.13DNAeXplained – Genetic Genealogy. DNA Inherited from Grandparents and Great-Grandparents So DNA from your mother’s parents arrives in about 64 pieces.14DNAeXplained – Genetic Genealogy. DNA Inherited from Grandparents and Great-Grandparents DNA from your father’s parents arrives in about 49 larger pieces.15DNAeXplained – Genetic Genealogy. DNA Inherited from Grandparents and Great-Grandparents You get the same total from each side, just in bigger chunks from your father’s side.16DNAeXplained – Genetic Genealogy. DNA Inherited from Grandparents and Great-Grandparents That’s why amounts shared with paternal grandparents swing more widely than with maternal ones.17DNAeXplained – Genetic Genealogy. DNA Inherited from Grandparents and Great-Grandparents18gcbias. How much of your genome do you inherit from a particular grandparent?

Many small pieces average out more evenly, while a few large pieces leave more room for one grandparent to dominate. Both sides still average to about a quarter.

When you compare results, sharing with a paternal grandparent far above or below 25% is less surprising. Larger chunks also mean fewer but longer matching segments on your match page.

Can You Share No DNA With a Grandparent

The chance a mother passes on none of a particular grandparent’s regular DNA is about 1 in 10 followed by 28 zeros.19gcbias. How much of your genome do you inherit from a particular grandparent? For a father it is about 1 in 10 followed by 21 zeros.20gcbias. How much of your genome do you inherit from a particular grandparent? A true grandchild will therefore, in practice, always show substantial sharing with each grandparent.

Each chromosome has to pass entirely from the other grandparent, and with 22 pairs of regular chromosomes all needing to do so, the chances multiply into a vanishingly small number. A very low number should make you question the relationship rather than put it down to luck.

If a grandparent match is missing or very low, check that both people tested with the same company or uploaded raw data, and confirm the right kit is linked to the right person. A result far below about 984 cM suggests a different relationship. A professional relationship test is worth considering before you draw conclusions.

X, Y and Mitochondrial DNA From Grandparents

Only males have a Y chromosome, and a boy gets it from his father.21The Tech Interactive. How much DNA do you inherit from each grandparent? So a grandson’s Y comes from his father’s father, and the two share essentially the same Y chromosome.22The Tech Interactive. Do women share more than 25% DNA with a paternal grandmother?

The X works differently. A son gets his only X from his mother, usually a mix of her two parents.23The Tech Interactive. How much DNA do you inherit from each grandparent?24DNA Science. Modeling the Inheritance of X Chromosome DNA A daughter gets one X from her mother too. A daughter also gets her father’s X, which came whole from his mother.25DNAeXplained – Genetic Genealogy. Grandparent Inheritance Chart – DNAeXplained – Genetic Genealogy26DNA Science. Modeling the Inheritance of X Chromosome DNA So a granddaughter carries her paternal grandmother’s X intact.27DNAeXplained – Genetic Genealogy. Grandparent Inheritance Chart – DNAeXplained – Genetic Genealogy Neither grandchild gets any X DNA from the paternal grandfather.28The Tech Interactive. How much DNA do you inherit from each grandparent?

Mitochondrial DNA normally passes from mother to child.29My Forever DNA. What Percentage of DNA Do Grandparents Share with Their Grandchildren? You carry the version your mother’s mother passed down.30My Forever DNA. What Percentage of DNA Do Grandparents Share with Their Grandchildren? It usually passes unchanged across several generations, with rare new changes.31PubMed Central. Whole Mitochondrial Genome Detection and Analysis of Two- to Four-Generation Maternal Pedigrees Using a New Massively Parallel Sequencing Panel

These routes can explain odd-looking results. A granddaughter may share more with her paternal grandmother partly because of the whole X. Many consumer tests leave X out of the main cM total, so check it separately. Y and mitochondrial tests can confirm a paternal-line or maternal-line link, but they can’t measure how much total DNA you share.

Each test answers a different question:

  • Y test: for a grandson and his paternal grandfather.
  • Mitochondrial test: for the maternal grandmother line, any grandchild.
  • Standard autosomal test: works for all four grandparents, and it’s the one that gives the cM numbers above.

When Families Share More Than Expected

In families where ancestors married relatives or come from small closed communities, relatives share more DNA than usual.32Journal of Genetic Genealogy. The Shared cM Project: A Demonstration of the Power of Citizen Science Grandparent pairs in such families can sit above the typical average.33Journal of Genetic Genealogy. The Shared cM Project: A Demonstration of the Power of Citizen Science

If your grandparents were themselves cousins, some of the DNA you inherit from each can come from the same distant ancestor, adding to what you share. The matching stretches then count both through the direct line and through the shared ancestry, pushing totals up.

To see whether family history explains a high number, look for cousins marrying in your tree. Check whether you match many relatives on both parents’ sides. High sharing with several relatives, not just one, points to related ancestors, and predicted relationships will tend to look closer than they are.

A high total could also mean the real relationship is closer, such as a parent and child or a full sibling. If the other person is much older than your parent, a parent relationship is unlikely, so age helps narrow it.

Testing sites also report differently. AncestryDNA’s processing lowers the total shared DNA it reports and can break long segments into pieces.34The DNA Geek. Escape from the Overlap Zone The same pair may therefore show a lower number there than elsewhere.

Using DNA to Confirm a Grandparent

Because grandparent sharing varies, it overlaps with other relationships like aunts or half-siblings. Grandparent results are not judged by a single pass-or-fail percentage.35My Forever DNA. What Percentage of DNA Do Grandparents Share with Their Grandchildren? Labs compare how likely your DNA is under the claimed relationship versus being unrelated, and set cutoffs based on error rates.36PubMed. Evaluating the statistical power of DNA-based identification, exemplified by ‘The missing grandchildren of Argentina’37PubMed. Making decisions in missing person identification cases with low statistical power The result is often expressed as how many times more likely the DNA is under the claimed relationship than under no relationship. Testing more relatives, such as both grandparents, gives a stronger answer.38PubMed. Prioritising family members for genotyping in missing person cases: A general approach combining the statistical power of exclusion and inclusion

A strong match far above unrelated levels supports the relationship. A borderline result may fit a grandparent or another close relative, and adding a second relative often resolves the doubt. An unclear result is not proof of no relationship.

Consumer tests give cM and a predicted relationship, not legal proof. Legal tests need documented sample collection. If you need one, bring both grandparents or other relatives if possible, and ask the lab what result would count as confirmation.

Can Grandparents Pass On More Than Genes

Grandchildren of women who were undernourished during pregnancy have shown more body fat as newborns.39PubMed Central. Transgenerational Epigenetic Inheritance: myths and mechanisms A grandparents’ food supply has also been linked to grandchildren’s health.40PubMed Central. A critical view on transgenerational epigenetic inheritance in humans But it’s hard to rule out other causes, such as the child’s own diet after birth.41PubMed Central. Transgenerational Epigenetic Inheritance: myths and mechanisms Many family patterns once thought to be inherited this way turned out to be ordinary gene variants.42PubMed Central. A critical view on transgenerational epigenetic inheritance in humans These findings still need confirming.

This differs from shared DNA, which tests measure directly. These effects concern how genes are switched on, not the cM total, and your DNA test does not measure them. Family health history still matters for your doctor.