Genotype and phenotype ratios
This calculator generates genotype ratios and phenotype ratios from two parental genotypes across
1–3 loci under complete dominance.
It computes exact probabilities (fractions and percentages), simplifies ratios, and provides CSV tables.
Assumptions: independent assortment (unlinked loci), equal segregation, no epistasis, no linkage, no selection,
and complete dominance at each locus.
How genotype strings are interpreted
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Loci must appear in order: A, then B, then C.
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Each locus uses two characters: AA, Aa, or aa.
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For 2 loci, a valid genotype has 4 characters (example: AaBb).
For 3 loci, it has 6 characters (example: AaBbCc).
Uppercase means dominant allele (A), lowercase means recessive allele (a).
Core idea: enumerate gametes, then count offspring classes
Each parent produces a set of gametes. At a locus:
AA produces only A, while Aa produces A and a.
Number of gametes from a parent is the product of allele options across loci.
After listing gametes for Parent 1 and Parent 2, the calculator forms all gamete pairs (each equally likely) and counts outcomes.
From genotype to phenotype (complete dominance)
Under complete dominance, a phenotype at each locus depends only on whether there is at least one dominant allele.
For multiple loci, phenotype classes are combinations (e.g., \(A\_\; bb\; C\_\)). The calculator uses \(A\_\) to mean “at least one A”.
Ratios and simplification
The raw outcome counts are simplified into ratios by dividing all counts by their greatest common divisor (GCD).
The calculator shows both the simplified ratios and the exact probabilities (fraction + percent) for every class.
Classic examples
1 locus: Aa × Aa
Here \(P(A\_)=\frac{3}{4}\) and \(P(aa)=\frac{1}{4}\).
2 loci: AaBb × AaBb
Because loci are assumed independent, phenotype probabilities multiply:
This corresponds to the well-known phenotype ratio \(9:3:3:1\).
Interpreting the calculator outputs
- Genotype table: every distinct multilocus genotype class with probability and percent.
- Phenotype table: classes grouped by dominance pattern (\(A\_\), \(aa\), etc.).
- CSV: downloadable tables for reporting or grading.
- Visualizations: ratio strip, stacked phenotype bar, and mosaic provide a quick view of proportions.
Common input mistakes
- Wrong order (e.g., BbAa instead of AaBb).
- Wrong length for the chosen number of loci (2 loci must be 4 characters, 3 loci must be 6 characters).
- Using the wrong letter at a locus (must be A/a for locus 1, B/b for locus 2, C/c for locus 3).
- Accidentally typing spaces or separators; the calculator ignores spaces but expects the allele letters.
If you want, the next upgrade is to add alternative dominance models (incomplete dominance, codominance) or linkage/recombination.
