Punnett Square Basics (simple) — free printable clipart

How to read a Punnett square

A Punnett square is a grid for working out what offspring a cross between two parents could produce, and in what proportions. One parent's alleles go along the top, the other's down the side, and each cell is filled in with the pair of alleles an offspring would inherit — one from each parent. A two-by-two grid handles one gene, which is the standard case.

What it is for

Predicting the possible offspring of a genetic cross

Set-up

One parent's alleles along the top, the other's down the side

A 2×2 grid

One gene — a monohybrid cross

Allele

A version of a gene, written as a letter

Dominant allele

Capital letter — shows in the offspring if present at all

Recessive allele

Lower-case letter — shows only if both copies are recessive

Genotype

The pair of alleles an individual carries, such as Bb

Phenotype

The characteristic you can actually observe

Bb × Bb gives

1 BB : 2 Bb : 1 bb — a 3:1 ratio of phenotypes

• Each parent passes on one allele of its pair, so the four cells of a 2×2 grid are simply every possible combination of one allele from each side. Nothing more complicated is happening.
• The capital and lower-case convention carries the genetics. BB and Bb both show the dominant characteristic because one dominant allele is enough; only bb shows the recessive one.
• Genotype and phenotype are different answers to different questions, and the grid gives both. Bb × Bb produces three genotypes in a 1:2:1 ratio but only two phenotypes, in a 3:1 ratio.
• A cross between a heterozygous and a homozygous recessive parent — Bb × bb — gives a 1:1 phenotype ratio instead. Those two crosses cover most of what is asked at this level.
• The result is a probability, not a promise. A 3:1 ratio means each offspring has a three-in-four chance of showing the dominant characteristic; four actual offspring can easily come out 4:0.
• A four-by-four grid does the same job for two genes at once, a dihybrid cross. The method is identical — every combination of one allele per gene from each parent.