Genotype Ratio Calculators

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Genotype ratio is the relative frequency of different genotypes expected among offspring from a genetic cross. Genotype ratios are calculated using Punnett squares and the principles of Mendelian segregation. The classic monohybrid cross (Aa × Aa) yields a 1:2:1 genotype ratio (AA:Aa:aa) and a 3:1 phenotype ratio under complete dominance. Genotype ratios are fundamental to genetics — used to predict inheritance patterns, calculate probabilities of specific offspring genotypes, and design breeding programs.

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Monohybrid Cross Genotype Ratios

Single gene, two alleles (A dominant over a):

Aa × Aa (Heterozygote × Heterozygote)

Punnett square: AA (25%), Aa (50%), aa (25%) → Genotype ratio 1 AA : 2 Aa : 1 aa
Phenotype ratio: 3 dominant : 1 recessive

Aa × aa (Testcross)

Offspring: Aa (50%), aa (50%) → Genotype ratio 1 Aa : 1 aa
Phenotype ratio: 1 dominant : 1 recessive

AA × aa

Offspring: all Aa → Genotype ratio 1 Aa (all heterozygous)

Dihybrid Cross Genotype Ratio

Two independently assorting genes (AaBb × AaBb):
16 possible genotype combinations in Punnett square.
Phenotype ratio (complete dominance): 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb = 9:3:3:1

Genotype Probability Calculations

For independent genes, multiply individual probabilities:
P(AABB) = P(AA) × P(BB) = 1/4 × 1/4 = 1/16
P(AaBb) = P(Aa) × P(Bb) = 2/4 × 2/4 = 4/16 = 1/4

Chi-Square Test for Mendelian Ratios

χ² = Σ[(O−E)²/E] tests whether observed offspring counts fit expected Mendelian ratios. df = classes − 1. If p > 0.05, observed ratios are consistent with Mendelian expectation.

Glossary

Genotype Ratio
The relative frequency of different genotypes among offspring from a cross. Calculated using Punnett squares and Mendelian segregation. Classic monohybrid: Aa × Aa → 1 AA : 2 Aa : 1 aa (1:2:1). Dihybrid: 9:3:3:1 phenotype ratio.
Testcross
A cross between an individual of unknown genotype and a homozygous recessive (aa). Offspring ratio reveals whether the unknown parent is homozygous dominant (all offspring dominant) or heterozygous (1:1 dominant:recessive ratio).
Dihybrid Cross
A cross involving two genes, each with two alleles. Standard cross: AaBb × AaBb. Produces 9:3:3:1 phenotype ratio under complete dominance and independent assortment. 16-cell Punnett square gives all genotype combinations.

Frequently Asked Questions

Crossing two heterozygotes (Aa × Aa) gives: AA (25%), Aa (50%), aa (25%) → genotype ratio 1:2:1. With complete dominance: phenotype ratio 3 dominant (AA + Aa) : 1 recessive (aa). The 1:2:1 genotype ratio is always produced by a heterozygote × heterozygote cross for a single gene with two alleles.

A testcross crosses an individual of unknown genotype with a homozygous recessive (aa). Aa × aa gives: 1/2 Aa (dominant phenotype) + 1/2 aa (recessive phenotype) → genotype ratio 1:1. AA × aa gives: all Aa — only dominant phenotypes, no recessive offspring. The ratio of dominant to recessive offspring in a testcross reveals the unknown parent's genotype.

Crossing two dihybrid parents (AaBb × AaBb) for two independently assorting genes with complete dominance gives the 9:3:3:1 phenotype ratio: 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb. This ratio assumes: complete dominance at both loci; independent assortment (genes on different chromosomes or far apart); and a sufficiently large sample. Departures from 9:3:3:1 suggest epistasis, linkage, or other non-Mendelian interactions.

List one parent's gametes along the top and the other parent's gametes along the left side. Fill in each cell with the genotype produced by that gamete combination. Count the frequency of each genotype. For Aa × Aa: four cells give AA, Aa, Aa, aa → 1:2:1. For dihybrid crosses, list all four gamete types (AB, Ab, aB, ab) for each parent to create a 4×4 grid of 16 cells. Count each genotype class to determine ratios.