Ancestor DNA Percentage by Generation Calculator

How much autosomal DNA did you likely inherit from a specific ancestor?

Select your ancestor

e.g. "great-grandparent" = G 3, "2nd great" = G 4, and so on

or enter directly

G = 1 is a parent; G = 2 is a grandparent; add 1 per "great"

options

Halves the expected percentage

If you have a DNA match result, enter it to compare against expected range

Results

Select a relationship or enter a generation number to see your results.

How to Use This Calculator

  1. Pick a relationship from the dropdown (e.g. "Great-grandparent") or type the generation number directly. G = 1 is a parent, G = 2 is a grandparent, and each additional "great" adds one generation.
  2. If the ancestor is a half-relative (e.g. half-great-grandparent), tick the checkbox.
  3. Optionally enter a shared cM from a real DNA test to see how your result compares to the expected range.
  4. Read the output: expected %, realistic 95% range, estimated cM, detection likelihood, and theoretical ancestor count at that generation.

When would you use this?

Generation Reference Table

Quick lookup for common ancestor relationships and their expected DNA contribution:

G Relationship Expected % Approx. cM Ancestors at this gen. Typical detection

Total autosomal genome ≈ 3,418 cM (Campbell et al.); detection threshold ≈ 7 cM.

The Science Behind the Calculator

How DNA halves with each generation

Every time DNA is passed from parent to child, each chromosome pair is reshuffled — a process called recombination. On average, exactly half of each parent's autosomal DNA (the 22 non-sex chromosomes) is passed to each child. This means the expected percentage from one specific ancestor at generation G is (1/2)G × 100%.

Why real values vary from the average

Because recombination is random, the actual amount inherited from any one ancestor varies around the expected value. The standard deviation of this distribution has been modelled via chromosome simulation studies (Veller et al. 2019, published in Genetics; Philip Gammon's crossover simulations reported by DNAeXplained). Key findings:

The 95% confidence interval shown is mean ± 1.96 × SD, floored at 0% (you can't inherit a negative amount).

Detection thresholds on consumer tests

AncestryDNA, 23andMe, MyHeritage, and FTDNA all have a practical lower limit for reporting shared DNA — typically around 7–10 cM of total shared segments. Below this, matches are filtered out as potential false positives. Since the total human autosome spans roughly 3,418 cM, a match must share at least ~0.2% to appear. This calculator uses 7 cM as the threshold and models detection probability based on how far the expected cM is above that floor.

Source & method

Simulation data: Veller C, Przeworski M, Coop G (2019), Genetics. Cross-over counts: Campbell CL et al. Additional reference: ISOGG Autosomal DNA statistics. Detection probability model and SD estimates align with Philip Gammon's crossover simulations cited by DNAeXplained (Roberta Estes).

This tool estimates genetic inheritance for genealogical planning purposes only. Actual inherited DNA varies due to random recombination. Results are not a clinical or medical determination. Consult a genetic counsellor for health-related interpretations.

Frequently Asked Questions

Why might I inherit 0% DNA from a great-great-grandparent?

At 4 generations back, the expected percentage from any one of your 16 great-great-grandparents is only ~6.25%. Because DNA is inherited in discrete segments reshuffled randomly at each generation, it is statistically possible — though uncommon — to inherit no detectable segments from a specific great-great-grandparent. This is called DNA dropout and becomes more likely beyond 5–6 generations.

How many cM do I share with a 2nd great-grandparent on average?

The human autosomal genome spans roughly 3,418 cM. At 5 generations back (2nd great-grandparent), the expected percentage is about 3.125%, equalling approximately 107 cM. The realistic range is roughly 20–190 cM at the 95% level. Consumer DNA tests typically detect segments above 7 cM, so a match may still appear even if total shared DNA is relatively low.

Does the expected DNA percentage halve for a half-relationship?

Yes — for a half-relationship the expected shared DNA is exactly half the full-relationship value. A half-grandparent shares ~12.5% instead of ~25%. Tick the "half-relationship" checkbox in the calculator to apply this adjustment.

At what generation does autosomal DNA become undetectable?

Consumer DNA tests typically detect shared DNA above about 7–10 cM. Since the total autosome is ~3,418 cM, the expected shared DNA from a single ancestor drops below 7 cM around generation 9 (5th great-grandparent for some relationships), though the range is wide. Beyond generation 7–8, there is a meaningful chance of inheriting no detectable DNA from a specific ancestor — even if that ancestor definitively lived.

How does pedigree collapse affect the theoretical ancestor count?

Theoretically you have 2G ancestors at generation G (e.g., 64 at G = 6). But because all humans share ancient common ancestors, many positions in your pedigree are occupied by the same person — a phenomenon called pedigree collapse. The further back you go, and the more endogamous your ancestry, the larger the collapse. By G = 10 the theoretical count is 1,024, but the actual number of distinct individuals is far lower for most families.

What is the difference between autosomal DNA, mtDNA, and Y-DNA for genealogy?

Autosomal DNA covers all 22 non-sex chromosomes, inherited from both parents — this calculator addresses autosomal DNA only. mtDNA is passed only through the direct maternal line without dilution but traces just one branch. Y-DNA is passed only from father to son along the direct paternal line. For researching all branches of your family tree, autosomal DNA testing is the primary tool; mtDNA and Y-DNA illuminate single specific lineages.