Punnett Squares Explained: Predicting Inherited Traits (Grade 8 Science)

Hi, I'm Luna from DoodleBeez. Why do some children have their mother's eyes or their grandfather's curly hair? Traits are passed down through genes. Today we will use a tool called a Punnett square to predict inherited traits.

Genes come in different versions called alleles. You inherit two alleles for each gene, one from each parent. Scientists use letters to show them, a capital letter for one allele and a lowercase letter for another.

A dominant allele shows its trait even if there is only one copy. A recessive allele only shows when there are two copies. In pea plants, purple flowers, big P, are dominant over white flowers, little p.

The letters are the genotype. The trait you can see is the phenotype. A plant with big P big P or big P little p looks purple. Only little p little p looks white.

To build a Punnett square, draw a grid of four boxes. Write one parent's alleles across the top and the other parent's down the side. Then fill each box with one letter from the top and one from the side.

Let's cross two purple plants that are both big P little p. The boxes show big P big P, big P little p, big P little p, and little p little p. So three out of four are purple, and one out of four is white.

Now it's your turn. Cross a big P big P plant with a little p little p plant. What fraction of offspring will be purple? Pause and try. All four boxes are big P little p, so all offspring are purple.

Great job! Remember, each parent gives one allele. Dominant alleles show with one copy, recessive alleles need two, and Punnett squares predict the chances. Practice Punnett squares in the Science Lab at doodlebeez.com. See you next time!