Monohybrid Cross Calculator

Calculate offspring genotype and phenotype ratios for monohybrid crosses.

Parent 1 genotype

Parent 2 genotype

Dominant trait name

Recessive trait name

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Why the 3:1 Ratio Appears

Mendel's monohybrid ratio follows directly from two facts: each parent carries two alleles, and gametes receive one at random. A heterozygous cross produces four equally likely combinations, three of which show the dominant phenotype.

Cross	Genotype ratio	Phenotype ratio
Aa × Aa	1 AA : 2 Aa : 1 aa	3 : 1
Aa × aa	1 Aa : 1 aa	1 : 1 — test cross
AA × aa	All Aa	All dominant
AA × Aa	1 AA : 1 Aa	All dominant

Note that the genotype ratio 1:2:1 and phenotype ratio 3:1 describe the same cross — the heterozygotes are simply indistinguishable from the dominant homozygotes by appearance.

The Test Cross

An organism showing the dominant phenotype may be AA or Aa, and no amount of looking will tell you which. Crossing to a homozygous recessive resolves it: AA gives all dominant offspring, while Aa gives a 1:1 split. Any recessive offspring at all proves the parent was heterozygous.

Why Real Ratios Deviate

Cause	Effect on ratio
Small sample size	Random deviation — use chi-square to test
Incomplete dominance	1:2:1 phenotypes, no 3:1
Codominance	Both alleles expressed; 1:2:1
Lethal alleles	2:1 instead of 3:1 — one class dies
Linkage	Applies to dihybrid crosses, not monohybrid

A 2:1 ratio is the classic signature of a recessive lethal allele: the homozygous recessive class dies before scoring, leaving only the two surviving genotypes. Yellow coat colour in mice is the textbook example.

With small samples, observed ratios scatter around expectation. A chi-square test determines whether a departure from 3:1 is real or simply sampling noise — a 3:1 expectation on 20 offspring can easily produce 13:7 by chance.

Worked Examples

Example 1: Aa x Aa (classic 3:1)

Offspring: AA=25%, Aa=50%, aa=25%

Result: 75% dominant : 25% recessive

Mendel's pea plants experiment

Example 2: AA x aa (all dominant)

All offspring Aa

Result: 100% dominant phenotype

F1 all heterozygous

Example 3: Test cross logic

Dominant-phenotype parent crossed to aa, 40 offspring all dominant

Result: Parent was almost certainly AA

If the parent were Aa, half the offspring would be recessive. Forty consecutive dominant offspring makes that vanishingly unlikely.

Example 4: The 2:1 lethal signature

Cross of two heterozygotes yields 2 dominant : 1 recessive consistently

Result: Recessive lethal allele

The homozygous recessive class dies in utero, removing one quarter of the expected offspring and converting 3:1 into 2:1 among survivors.

Common Mistakes

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Expecting exact ratios in small samples

A 3:1 expectation is a probability, not a guarantee. With 20 offspring, 13:7 is entirely consistent with 3:1 by chance — use chi-square to judge.

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Confusing genotype and phenotype ratios

Aa × Aa gives genotypes 1:2:1 but phenotypes 3:1, because heterozygotes look identical to dominant homozygotes.

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Assuming dominance is always complete

Incomplete dominance and codominance both give 1:2:1 phenotype ratios. Seeing three phenotypes rules out simple dominance.

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Overlooking lethal alleles

A persistent 2:1 ratio suggests the homozygous recessive class is dying before it can be scored, not that Mendelian inheritance has failed.

Frequently Asked Questions

Mendel's laws?

Law of Segregation: alleles separate during gamete formation. Law of Independent Assortment: genes on different chromosomes segregate independently. These are the basis of all Mendelian genetics.

Incomplete dominance?

Heterozygote shows intermediate phenotype. Cross Aa x Aa: 25% AA (red), 50% Aa (pink), 25% aa (white). Ratio: 1:2:1 phenotype instead of 3:1.

Why does Aa × Aa give 3:1?

Four equally likely allele combinations arise, and three of them contain at least one dominant allele. Only the aa combination shows the recessive phenotype.

What is a test cross?

Crossing an organism of dominant phenotype to a homozygous recessive. All-dominant offspring indicate AA; a 1:1 split indicates Aa.

Why might a real cross not give 3:1?

Small sample size, incomplete dominance, codominance, or a lethal allele. A consistent 2:1 ratio strongly suggests a recessive lethal.

How do I know if a deviation is significant?

Use a chi-square test comparing observed to expected counts. Small samples deviate substantially by chance alone.