Punnett Square

What are the chances a child inherits a trait? Pick the genotypes of both parents and see every possible combination of the alleles they pass on — as Gregor Mendel first worked out with pea plants.

GeneticsInheritanceMendel

Inputs

Input examples

Each parent passes on one of their two alleles, each with a 50% chance. A capital A is the dominant allele, a small a the recessive one.

Parents

Parent 1

Parent 2

The trait

Inheritance

Trait shown by A (AA, and Aa if dominant)

Trait shown by aa

Trait shown by Aa when blended — incomplete dominance only

Number of children — for the family odds

How a Punnett Square Works

Every person carries two copies (alleles) of each gene, one from each parent. When eggs and sperm form, each gets just one of the two, at random. The square lists the possible eggs along one side and sperm along the other; each box is one equally likely child.

A dominant allele shows its trait even in a single copy, so a recessive trait only appears in children with two recessive copies. Two healthy carriers of a recessive condition (Aa × Aa) have a 1 in 4 chance per child of an affected child — independently for every pregnancy.

3 : 1Aa × Aa, dominant trait

1 : 2 : 1incomplete dominance

1 : 1test cross Aa × aa

each childodds reset every time

Punnett Square

parent 1 × parent 2

Result

Enter the values above

Key Relations

Law of Segregation

Each parent passes on either of their two alleles with equal chance, independently of the other parent. That gives four equally likely combinations.

Example:

Probability per Child

Multiply the chances that each parent passes on the allele. For two carriers that is ½ · ½ = ¼.

Several Children

Children are independent, so the chance that none of n children shows the trait is (1 − p)ⁿ.

Example:

Sources & Verification

References

G. Mendel, “Versuche über Pflanzen-Hybriden”, Verhandlungen des naturforschenden Vereines in Brünn 4, 3–47 (1866).