Test whether observed offspring counts fit an expected Mendelian ratio, such as 3:1, 1:1, 9:3:3:1, or a custom expected count set.
χ² = Σ((O - E)² / E)A genetics chi-square test compares observed offspring counts with expected counts from a hypothesis. The calculator sums the squared difference between observed and expected counts divided by expected counts, then estimates a p value from the chi-square distribution.
| Symbol | Variable | Description |
|---|---|---|
| O | Observed count | The number actually counted in each phenotype or genotype class. |
| E | Expected count | The count predicted by the Mendelian ratio or genetic model. |
| df | Degrees of freedom | Number of categories minus one for a simple goodness-of-fit test. |
Enter observed counts
Type the counts you measured, separated by commas or spaces.
Enter expected counts
Use the same category order. Convert ratios to counts before entering them.
Read the p value
If p is below 0.05, the observed data deviate significantly from the expected ratio at the 5% level.
You can enter any expected counts as long as they match the observed categories. For a 3:1 ratio with 100 offspring, enter expected counts of 75 and 25. For a 9:3:3:1 dihybrid ratio with 160 offspring, enter 90, 30, 30, and 10.
A high p value does not prove the expected ratio is true; it means the observed deviation is not large enough to reject it with this test. A low p value suggests the model, counts, assumptions, or experimental setup should be reviewed.
Before relying on a chi-square genetics result, check that categories are independent, expected counts are not too small, and no phenotype class was accidentally combined or omitted.