Punnett Squares

Punnett Square- a diagram that is used to predict the possible genetic outcomes of a particular cross or breeding experiment.

1. Punnett square problems will often read something similar to this:

If a purebred individual with blond hair marries a heterozygous individual with brown hair, what is the likelihood their children will have blond hair?

2. In order to solve this problem, you have to know information about the two parents. You get this information from the word problem itself.

3. Now we have to put this information into the Punnett Square.

PARENT GENOTYPES GO ON THE OUTSIDE OF THE PUNNETT SQUARE. It does not matter which parent goes in which position.

4. Once the parent information is put into the Punnett Square, we can fill in the POSSIBLE OFFSPRING GENOTYPES ON THE INSIDE OF THE PUNNETT SQUARE.

5. With this information, we can determine the POSSIBLE OFFSPRING PHENOTYPES.

The initial question asked: “If a purebred individual with blond hair marries a heterozygous individual with brown hair, what is the likelihood their children will have blond hair?”

The answer is 50%.

Each child who is born has a 50% chance (2 out of 4) of being born with blond hair.

Each child who is born has a 50% chance (2 out of 4) of being born with brown hair.

This means that even if they have 10 children, they can EXPECT that about half of their children will be blond, and half of their children will have brown hair.