Punnett's Notes Printable Version

Punnett Square Notes

Why doesn’t the whole world have brown eyes since it is a dominant trait? That question brought about Punnett squares. Punnett squares are a picture representation for predicting what offspring will look like.

Mendel used peas because they grow fast, little time and investment was involved, and if you totally messed up, you could just throw the peas in a pot and eat them for dinner. Waste not, want not.

To predict offspring, we use letters to represent the alleles, or genes. When Mendel crossed his pure tall pea plants with his pure short pea plants, only tall pea plants were produced.

We call the tall trait dominant, because that trait is always seen.
We use a capital letter to represent dominant alleles.
The traits that are not always seen are called recessive.
We use a lower case letter to represent the recessive alleles. In Mendel’s case, his short pea plants were recessive.

For pea plant height we will use the letter T (for tall because it is dominant).

Let the tall allele be represented with a capital “T.”
Let the short allele be represented with the lower case “t.”
(Please note that we do NOT use an “s” for short. The letters must be the same)

In sexual reproduction an offspring has two parents. They get one allele from each parent. In Mendel’s pure pea patches the tall pea alleles are TT (one T from each parent) and the short peas alleles are represented by tt (they got one t from each parent). Looking at the alleles in this way is called the organism’s genotype. The genotype is the type of genes present in the organism for the trait.

Mendel's First Cross

Mendel pollinated (crossed) a pure tall pea plant TT with a pure short pea plant tt. Another word for “pure” in the world of genetics is homozygous. Homozygous traits are traits that have two of the same alleles, like two tall alleles, TT, or two short alleles, tt.

To make our predictions of this mating, or cross, we first draw a box like the one below and put the parent’s alleles along the side and at the top:

A square divided into four sections. Along the top of the square is two capital T that are labeled tall plant. Along the left side of the square are two lower case t that are labeled short plant. Each section of the main square is empty.

Next we combine the alleles inside the box. The gene pairs in the inner boxes are the possible outcomes for the offspring. You see each parent can donate one allele to the offspring. The Punnett square helps us sort out all the possibilities.

A square divided into four sections. Along the top of the square is two capital T. Along the left side of the square are two lower case t. Each of the four sections of the main square contain Tt.

Well, look at that. All the letters are the same inside the Punnett square - Tt. There is one dominant allele and one recessive allele. We call this a heterozygous. The important thing to note is that because there is one dominant gene all the plants will be tall. We call the description of the trait the phenotype. So the phenotype for height in this Punnett square is 100% TALL.

Mendel’s Second Cross

Mendel’s second experiment was between two heterozygous tall pea plants (genotype Tt) from the first experiment. Try to do this on your own paper and then click on the box to check your answers.

A square divided into four sections. Along the top of the square is a capital T and a lower case t. Along the left side of the square is a capital T and a lower case t. Reading left to right and top to bottom the first section contains TT, the secton section contains Tt, the third section contains Tt, and the fourth section contains tt.

Let’s look at the possible outcomes!

TT – genotype and tall pea plant is phenotype
Tt – genotype and tall pea plant is phenotype
Tt – genotype and tall pea plant is phenotype
tt – genotype and short pea plant is the phenotype

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