Punnett Square Calculator

How a Punnett Square Works

1. Write each parent’s gametes. Each gamete gets one allele of every gene: Yy makes Y and y; YyRr makes YR, Yr, yR and yr (independent assortment).
2. Put one parent’s gametes across the top and the other’s down the side, then fill each box with the two gametes combined.
3. Count genotypes. For Yy × Yy the four boxes are YY, Yy, Yy, yy: a 1 : 2 : 1 genotype ratio.
4. Count phenotypes. With complete dominance YY and Yy look alike, so the phenotype ratio is 3 : 1. “Y_” means at least one dominant allele.

For more than one gene, multiply instead of drawing: each heterozygous gene pair crossed with another heterozygote contributes 3/4 dominant and 1/4 recessive, so the dihybrid gives 9/16, 3/16, 3/16 and 1/16. OpenStax’s Table 12.5 sums it up: n heterozygous gene pairs give 2ⁿ gamete types, 3ⁿ genotypes and 2ⁿ phenotypes; the trihybrid phenotype ratio is 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1.

Common Crosses

The test cross is how geneticists tell a homozygous dominant from a heterozygote that looks the same: cross it with a homozygous recessive. All dominant offspring means YY; a 1 : 1 split means Yy.

Mendel’s Real Numbers

Ratios are probabilities, so real counts wobble around them. Mendel’s F2 counts from OpenStax Table 12.1, with a chi-square test against 3 : 1 (1 degree of freedom):

None of the p-values is below 0.05, so every trait fits 3 : 1. Plant height comes closest to rejecting it, which is what a sample of about a thousand plants can do by chance.

Common Mistakes

• Putting two alleles of the same gene in one gamete. A gamete carries one allele per gene: YR, not YY.
• Confusing genotype and phenotype ratios. Yy × Yy is 1 : 2 : 1 by genotype but 3 : 1 by phenotype.
• Expecting the exact ratio in a small family. Each child is an independent 1-in-4 or 3-in-4 draw; four children can all be recessive.
• Assuming independent assortment for linked genes. Genes close together on one chromosome don’t give 9 : 3 : 3 : 1.
• Using complete-dominance ratios for incomplete dominance. Red × white snapdragons give pink heterozygotes, so the F2 is 1 : 2 : 1 by phenotype too.

What This Calculator Does Not Cover

It assumes two alleles per gene, independent assortment and either complete or incomplete dominance for all genes. Sex-linked traits, multiple alleles such as ABO blood groups, epistasis and linkage need adjustments the calculator does not make.

Frequently Asked Questions

How to Use This Calculator

1. Enter parent 1’s genotype — letter pairs, one pair per gene: Yy, YyRr or YyRrTt.
2. Enter parent 2’s genotype — same genes in the same order.
3. Choose the dominance type — complete for Mendel’s peas; incomplete for blended traits like pink snapdragons.
4. Read the ratios — gametes, genotype ratio, each phenotype’s probability and the number of genotypes and phenotypes.