Why DNA Results Are Either Above 99.99% or Below 0.1% With Nothing in Between

Open any paternity test report and the headline number is either very high (99.99% probability of paternity, often with more 9s after it) or very low (0%, or something like less than 0.01%). Results in the middle, like 50% or 75% or 92%, essentially never happen on standard paternity testing. Many families expect their result to land somewhere in between and find the all-or-nothing nature surprising.

This bimodal pattern is not an accident of lab reporting. It is a direct consequence of how the math works when multiple independent markers are combined. This article explains why the result is either very high or very low, when the middle range actually does appear (hint: kinship tests, not paternity trios), and what to think about if a paternity result lands in an unusual place.

The math is multiplicative, not additive

A paternity test does not score each marker on a sliding scale and then average the results. Each marker is a yes/no question with a numerical weight attached. The question at each marker is: given the alleles the mother contributed and the alleles the tested man carries, is the child's genotype at this marker consistent with the tested man being the biological father?

If the answer at a marker is yes, that marker contributes a likelihood ratio (how much more likely the observation is if the tested man is the father versus a random man). The ratio is usually between 2 and 50 depending on how common the specific alleles are in the general population. Rare alleles produce higher per-marker ratios because they are less likely to appear by chance in an unrelated man.

If the answer at a marker is no (there is a mismatch that cannot be explained by inheritance from the mother), that marker contributes effectively zero to the paternity side of the ratio. The ratio for that marker is near zero, not just small.

The key: when you combine 23 markers, you MULTIPLY their individual ratios, not add them. Multiplication is why the final number is either extremely high or extremely low.

The arithmetic of a true inclusion

Imagine every marker on a 23-marker test contributes a ratio of just 2 (a conservative lower bound; real values are often higher). Multiplying 2 by itself 23 times gives 2 to the 23rd power, which is 8,388,608. The Combined Paternity Index would be over 8 million. In practice, many markers contribute ratios of 10 to 50, so real CPI values on confirmed paternity trios often land in the hundreds of millions or billions.

Convert that to probability of paternity using the standard formula (CPI divided by CPI plus 1): a CPI of 8 million gives a probability of 99.999987%. The lab rounds this to 99.99% for display, but the number is far above the reporting threshold.

There is no way to end up in the middle. Even if a few markers contribute weaker ratios, the overall product is still astronomical.

The arithmetic of a true exclusion

Now imagine a true exclusion. The tested man is not the biological father, so some markers show allele mismatches that cannot be explained by maternal inheritance. Even one marker with a zero contribution multiplies the whole product by zero.

In practice, labs do not call an exclusion on a single marker mismatch because of the small background rate of STR mutations (approximately 0.1 to 0.3% per marker per generation). A single-marker inconsistency can be a rare mutation rather than a true exclusion, and the lab re-runs that specific marker or adds additional markers to confirm. When two or more markers show inconsistencies, the mutational explanation becomes mathematically implausible, and the result is called an exclusion.

Once the Combined Paternity Index drops near zero, the derived probability of paternity rounds to 0%. The report may show "0%" or "less than 0.01%" depending on lab conventions, but biologically the question is settled.

The NIST STR DNA Database publishes the per-marker mutation rates and the technical details labs use for these interpretations.

Why middle-range results require a specific kind of situation

For a Combined Paternity Index to land somewhere in the middle (say, between 10 and 10,000), something unusual has to be happening. In practice, this means one of three scenarios:

Partial marker data

If the lab could only read 10 or 12 markers reliably instead of all 23 (because one or more samples produced low DNA yield), the combined ratio is based on fewer data points and is smaller. A fully consistent read of 10 markers produces a CPI in the hundreds or low thousands rather than millions. This is a sample-quality issue, not a biologically ambiguous result. The fix is a fresh collection with cleaner samples.

Closely related participants

If the mother and the alleged father share recent biological lineage (first cousins, uncle-niece, more rarely siblings), many alleles are shared between them before testing even begins. The paternity math can misread these shared alleles, producing a middle-range result that reflects the shared lineage rather than true parentage. Labs that detect this pattern flag it and recommend additional markers or a lineage test.

Legacy small-panel tests

Older or budget paternity tests using only 13 to 16 markers produce weaker signals than modern 23-marker tests. On a 13-marker test, even a true inclusion can produce CPI values in the thousands rather than millions, which translates to probabilities in the 99.9% range (lower than modern 99.99%+ reports) that may feel like a middle-ground result. Modern 23-marker tests effectively eliminate this.

Kinship tests actually do produce middle-range results

Everything above applies to standard paternity trios. Kinship tests, where the question is grandparent, aunt or uncle, half-sibling, or similar relationships, behave differently.

A grandparent shares about 25% of autosomal DNA with a grandchild on average, with wider natural variance than the 50% parent-child relationship. A 23-marker test on a single grandparent gives the lab less decisive data to work with, and Combined Relationship Index values between 0.1 and 10 (the inconclusive range) show up regularly on single-relative kinship tests.

This is the biological reality of kinship testing, not a lab limitation. The USDC protocol runs additional autosomal markers on inconclusive first-pass kinship results (reflex up to 28 markers total) to push borderline cases out of the inconclusive range. The guide on kinship DNA testing when only one relative is available covers the kinship-specific decision framework in detail.

What to do if a paternity result unexpectedly lands in the middle

A paternity result between 90% and 99.9% (or between 0.1% and 10%) is unusual and worth investigating. The sequence to work through:

1. Check the panel size used. If the test ran 13 to 16 markers, retest on a modern 23-marker panel.
2. Check the per-marker breakdown on the report. If some markers read as inconclusive or returned partial data, the issue is sample quality. A fresh collection resolves this.
3. Confirm the mother and alleged father are not closely related. If they share recent biological lineage, flag this to the lab and ask about additional markers or a lineage test.
4. Verify sample handling. Mislabeled envelopes, cross-contamination during drying, or wrong-person swabs can produce unusual patterns.

A clean 23-marker test on properly collected samples essentially never lands in the middle range on a paternity trio. If a result does, something upstream of the lab math is responsible and is usually fixable with a fresh collection.

Why the bimodal nature is actually a feature

Families sometimes feel uncomfortable with the all-or-nothing nature of paternity results. A 99.99% result sounds decisive but leaves no room for the "99.5% confident" feeling many people would prefer. A 0% result allows no softening. The lack of middle-ground results can feel starker than other scientific tests families are used to.

The bimodality is not a bug. It is what makes the test useful. If paternity results could land anywhere between 0% and 100%, families would be left to interpret ambiguous signals themselves, and the test would resolve far fewer questions than it does. The multiplicative math forces the result into a decisive bucket, and that is why 23-marker paternity testing has become the standard for both personal-knowledge and legal applications.

USDC's home paternity test ($79) uses the standard 23-marker panel. For an overview of how to interpret the specific numbers on your report, see the guide on how to read paternity test results.