Mendelian Ratio + Chi-Square Calculator

Classic Mendelian Cross Patterns

Two heterozygotes for a single gene. Three dominant phenotypes for every one recessive. The classic Mendel pea cross: tall $\times$ tall → 75% tall, 25% dwarf.

Two heterozygotes for two unlinked genes. Nine show both dominants, three show one dominant + one recessive (each way), one shows both recessives. Mendel's second law of independent assortment.

A heterozygote crossed with a homozygous recessive. Used to confirm whether a dominant-phenotype individual is heterozygous (1:1 expected) or homozygous (no recessives).

When the heterozygote shows an intermediate phenotype (e.g., red $\times$ white snapdragons → pink). Three distinguishable categories instead of two.

• 9:3:4 — recessive epistasis (one gene masks another)
• 12:3:1 — dominant epistasis
• 9:7 — complementary gene action
• 13:3 — dominant suppression
• 15:1 — duplicate dominant genes

When Deviations Are Biologically Real

Most chi-square deviations are not random noise — they're the start of an interesting biological story. The five most common explanations:

Two genes on the same chromosome don't segregate independently. A dihybrid cross will deviate from 9:3:3:1 toward the parental combinations. Use a recombination frequency calculator to estimate the genetic distance.

Some genotypes die in utero or during development. A 1:2:1 incomplete dominance cross becomes 0:2:1 (or 2:1) if the homozygous dominants are lethal. Yellow mouse coat color is the classic example.

Genes on the X or Y chromosome give sex-specific ratios. A monohybrid for an X-linked recessive shows 50% affected males but 0% affected females in the F1 of a heterozygous mother $\times$ wild-type father.

Some genotypes don't always express the expected phenotype. The cross deviates from the Mendelian ratio in a consistent direction.

Always rule this out first. If your deviation is huge and unexpected, audit the scoring before invoking biology.