Punnett Squares

1

2

3

Bb

4

bb

x

b

b

B

b

Bb

bb

Bb

bb

Bb

Punnett Squares

50% Bb

50% bb

Dominant Allele: B -> brown eyes

Recessive Allele: b -> blue eyes

Alleles: Different versions of genes

An allele is one of two or more versions of a gene. There are two kinds of alleles for each trait. You inherit two alleles for each gene - one from each parent. The dominant allele is represented with a capital letter. The recessive allele is represented with a lowercase letter.

For example:

Brown eyes are dominant. Brown eyes are represented with a B

Blue eyes are recessive. Blue eyes are represented with a b

Your alleles can be homozygous dominant (both dominant), heterozygous (one dominant, one recessive), or homozygous recessive (both recessive).

Homozygous dominant: BB

A person with homozygous dominant alleles would have brown eyes.

Heterozygous: Bb

A person with heterozygous alleles would have brown eyes since they carry the dominant brown eye allele.

Homozygous recessive: bb - blue

A person with homozygous recessive alleles would have blue eyes.

Punnett squares measure the probability of a genetic outcome based on a genetic cross. They can't tell you what what will be, but they can tell you what could be. A simple Punnett square like the one to the right crosses two genotypes.

Genotypes: the alleles carried by an organism

As you can see in this example, we get two possible genotypes from this cross. Each square represents a 25% chance of the result occurring, but in this case we only get two different results. Therefore, there is a 50% chance of a Bb result, and a 50% chance of a bb result. These genotypes give us two possible phenotypes. Phenotypes are observable characteristics in an individual. In this particular cross, the two phenotypes we get are a 50% chance of the offspring having brown eyes, and a 50% chance of the offspring having blue eyes.