Why You Look Like Your Parents

Lesson Goal

Students will explain heredity as the passing of alleles from parents to offspring, tell dominant from recessive and genotype from phenotype, and use a Punnett square to predict a cross.

Key Concepts

Each parent gives one allele for a trait, so an offspring has two. A dominant allele (a capital letter) shows when it's present, and a recessive allele (lowercase) shows only when both alleles are recessive.

A Punnett square shows what's possible, not what will happen. A cross of two Tt parents has a one in four chance of a short offspring each time, which doesn't guarantee that one of any four offspring is short.

Where Students Get Stuck

Writing the alleles in the wrong places. One parent's two alleles go across the top and the other's down the side, and each box takes one letter from the top and one from the side.

Treating earlobes as a one-gene example. Free and attached earlobes are the classroom hook, but real earlobe shape is messier than a single pair of alleles, which the next lesson comes back to.

Teaching Suggestion

Have the student fill in the Tt by Tt square on paper before reading the answer, and count the boxes by hand: one TT, two Tt, one tt. Counting is what makes 3 to 1 stick.

Key Vocabulary

Heredity

The passing of traits from parents to offspring.

Genetics

The study of how alleles are passed from parents to offspring.

Allele

One of the different forms of a gene for a trait.

Dominant

An allele that shows when it's present and covers up the other one.

Recessive

An allele that's hidden when a dominant allele is present.

Genotype

The combination of alleles an organism carries, written as letters.

Phenotype

How a trait actually looks, the physical result of the alleles.

Homozygous

Having two matching alleles for a trait, like TT or tt.

Heterozygous

Having two different alleles for a trait, like Tt.

Punnett square

A grid that shows every possible pairing of alleles in a cross.