How Much DNA Do You Share With a Grandaunt or Granduncle

A full grandaunt or granduncle shares about 12.5% of your autosomal DNA with you, roughly 875 centimorgans. The observed range is wide, from about 251 to 2108 cM, so a 5% match, around 350 cM, is low but not automatically wrong. Treat it as a reason to check the tree before you rewrite the relationship.

A full grandaunt or granduncle is expected to share about 12.5% of your DNA, roughly 875 cM, but real matches can sit anywhere from 251 to 2108 cM. A 5% match is low, not impossible.

How much DNA do you share with a grandaunt? Call her a grandaunt or a great-aunt, the genetics are the same. She is your grandparent's full sibling. You inherit half your genome from your parent, your parent inherits half from your grandparent, and your grandparent shares about half with the grandaunt. Half of half of half is 12.5%. Centimorgans are the unit consumer DNA reports use. At about 70 cM per 1%, 12.5% is roughly 875 cM. The Shared cM Project reports a full great-aunt/uncle to great-niece/nephew range of 251 to 2108 cM, with an average around 877 cM. The same numbers apply to a granduncle and to a grandniece or grandnephew, because the relationship is symmetrical.

Why the range is so wide Full siblings do not share exactly 50% of their DNA with each other. Recombination and random chromosome sorting make that shared amount vary by several hundred cM in either direction. Your parent then inherits a random half of your grandparent's genome, and you inherit a random half of your parent's genome. Those two extra sampling steps widen the spread. A grandparent to grandchild range is already wide, and adding the sibling step makes a grandaunt range wider. Some grandnieces and grandnephews share quite a bit more or less than 12.5% with the same grandaunt. This is why a 5% result is not automatically a sign that the relationship is wrong.

What a 5% match looks like in real data On a testing site, 5% looks like about 350 cM, a total in the low-to-mid hundreds. You would see fewer shared segments than you would for a closer aunt or grandparent, and the predicted relationship may say something like a cousin range rather than grandaunt. Companies combine total shared DNA with segment count, segment size, ages, and any linked tree information, and several relationships still share similar totals. A 12.5% match is often labelled in the first cousin range, while a 5% match can land lower. If the shared total is below about 250 cM, that is below the reported full range for a grandaunt, and a half-grandaunt or another relationship becomes more likely.

When 5% should make you check the relationship - If the grandaunt is a known full sibling of your grandparent, 5% is plausible but on the low side. Check shared matches, tree records, or another relative's test. - If there is a chance the grandaunt is a half-sibling of your grandparent, 5% is close to the expected 6.25% for a half-grandaunt, roughly 440 cM. - A 5% match can also come from a first cousin once removed or a half first cousin. DNA alone cannot always tell them apart. Shared matches can separate a full grandaunt from a half-grandaunt in many cases. A full grandaunt matches relatives through both of the grandparent's parents, while a half-grandaunt matches through only one.

The half-grandaunt possibility If the person is not a full sibling of your grandparent but a half-sibling, the expected share halves again: about 6.25%, roughly 440 cM. That makes a 5% result sit right in the plausible zone for a half-grandaunt. When a known full grandaunt comes in at 5%, one of the first questions is whether the grandparent and grandaunt share one parent or two. Family labels such as great-aunt can be used for both full and half siblings of a grandparent. DNA only sees the biological half.

healthosx.com/dna

Choosing a file. The report is read on your own device.

healthosx.com/dna

The short version: what is worth acting on, first.

What to do next with your own DNA Relationship calculators are useful but they only predict, not prove. The exact thresholds are fuzzy below about 8% shared DNA, and different companies use slightly different totals. If you have raw data from 23andMe, AncestryDNA, MyHeritage or a whole-genome VCF with rsID annotations, the free tool at HealthOS DNA reads it in the browser and reports traits, medication response, carrier-relevant findings and optional ancestry. Raw GATK or Sentieon VCFs with '.' IDs can only do ancestry. It is not a relative-matching database, but it is the obvious next step for seeing your own genetic results in one place.