crosscalc

Genotypic and phenotypic ratios as percentages and fractions

Published

Table of contents
  1. Ratio, fraction, percentage, probability
  2. How to find genotypic and phenotypic ratios
  3. Example 1: Aa × Aa
  4. Example 2: Aa × aa (a test cross)
  5. Example 3: AaBb × AaBb (two genes)
  6. Do you simplify genotypic ratios?
  7. What if every box is the same?
  8. Expected vs observed results
  9. Common mistakes
  10. Practice
  11. Related
  12. Sources

Level: School and high school · You’ll need: a basic Punnett square (try the monohybrid calculator)

A Punnett square gives you a set of equally likely boxes. To report the result, you count the boxes and write them as a genotypic ratio (which allele combinations) and a phenotypic ratio (which visible traits). Each ratio can then be written as fractions or percentages. They’re all the same information in different forms.

Ratio, fraction, percentage, probability

Take a 2 × 2 square where 3 boxes show the dominant trait and 1 shows the recessive trait.

FormHow it’s writtenWhat it means
Ratio3 : 1For every 3 dominant there is 1 recessive
Fraction¾ and ¼Part ÷ total boxes (3/4 and 1/4)
Percentage75% and 25%Fraction × 100
Probability0.75 and 0.25The chance that one offspring has that trait

The model behind these numbers: complete dominance, alleles separating equally into gametes, and (for two or more genes) independent assortment. Real traits don’t always follow it. See genotype vs phenotype for why.

How to find genotypic and phenotypic ratios

  1. Count the boxes for each genotype. Write them in a fixed order: homozygous dominant, heterozygous, homozygous recessive (AA : Aa : aa).
  2. Group the genotypes by phenotype. Under complete dominance AA and Aa look the same, so they join one class.
  3. Simplify by dividing every number by the largest number that goes into all of them.
  4. Convert. Fraction = class ÷ total. Percentage = fraction × 100.

Example 1: Aa × Aa

GenotypeBoxesFractionPercentage
AA1¼25%
Aa2½50%
aa1¼25%
  • Genotypic ratio 1 : 2 : 1
  • Phenotypic ratio 3 : 1: 75% dominant (AA + Aa), 25% recessive

Example 2: Aa × aa (a test cross)

Two boxes are Aa and two are aa. That’s 2 : 2, which simplifies to 1 : 1. The genotypic and phenotypic ratios are the same here: 50% dominant, 50% recessive.

Example 3: AaBb × AaBb (two genes)

A dihybrid cross has 16 boxes and four phenotype classes.

PhenotypeBoxesFractionPercentage
Both dominant (A_B_)99/1656.25%
First dominant, second recessive (A_bb)33/1618.75%
First recessive, second dominant (aaB_)33/1618.75%
Both recessive (aabb)11/166.25%

The 9 : 3 : 3 : 1 ratio is just these four fractions with the “/16” dropped. The _ means “either allele”, so A_ covers both AA and Aa.

Do you simplify genotypic ratios?

Yes, usually. Divide every number by their greatest common factor, so 2 : 2 becomes 1 : 1 and 4 : 8 : 4 becomes 1 : 2 : 1. Two things to keep:

  • Keep the order. Simplifying never reorders the classes. AA : Aa : aa stays in that order.
  • Don’t simplify across different crosses. Each cross has its own ratio.

Some ratios are already as simple as they get. The dihybrid genotypic ratio 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 can’t be reduced, because 1 appears in it.

What if every box is the same?

Cross AA × aa and every box is Aa. There’s only one genotype class, so:

  • Genotypic result: all Aa, or 100% Aa
  • Phenotypic result: all dominant, or 100% dominant

Write this as “all Aa” rather than forcing it into a ratio. It’s clearer and is what most teachers expect.

Expected vs observed results

A 3 : 1 ratio does not mean that out of four offspring exactly three will show the dominant trait. Each offspring is a separate event with a ¾ chance. With only four offspring you could easily get 4 : 0 or 2 : 2.

The more offspring you count, the closer the observed ratio usually gets to the expected one. Mendel counted 7,324 F2 pea seeds for seed shape and found 5,474 round to 1,850 wrinkled, a ratio of about 2.96 : 1.

This is why ratios describe expected proportions across many offspring, not a guarantee for one family or individual.

Common mistakes

  1. Mixing up the two ratios. 1 : 2 : 1 is genotypic and 3 : 1 is phenotypic for the same Aa × Aa cross.
  2. Forgetting to add AA and Aa when counting dominant phenotypes.
  3. Reordering classes when simplifying.
  4. Treating percentages as promises. 25% recessive is a probability per offspring.
  5. Dividing by the wrong total. A dihybrid square has 16 boxes, not 4.

Practice

  1. Bb × bb. Give the genotypic ratio, phenotypic ratio and the percentage of recessive offspring.
  2. AaBb × aabb. What fraction of offspring show both recessive traits?
  3. A cross gives 6 TT, 12 Tt and 6 tt offspring. Write the simplified genotypic ratio.
Answers
  1. Genotypic 1 Bb : 1 bb; phenotypic 1 dominant : 1 recessive; 50% recessive.
  2. ¼. Each gene alone gives ½ recessive, and ½ × ½ = ¼ (the 1 : 1 : 1 : 1 test cross).
  3. 6 : 12 : 6 simplifies to 1 : 2 : 1 (divide by 6).

Check your own crosses: the monohybrid, dihybrid and trihybrid calculators show ratios and probabilities for every genotype.

Sources

Try it: check any cross instantly with the dihybrid cross calculator, or use the monohybrid and trihybrid versions.