What does heterozygous mean?

Heterozygous means an organism has two different alleles for a trait, like $Aa$. Homozygous means they have two of the same alleles, like $AA$ (homozygous dominant) or $aa$ (homozygous recessive).

Does a 25% chance mean exactly 1 in 4 offspring will have the trait?

No. A Punnett square predicts probabilities for each independent event, just like flipping a coin. If you flip a coin four times, you aren't guaranteed exactly two heads. Similarly, every single offspring has that same 25% chance, regardless of what its siblings look like.

Why do we put the capital letter first?

Writing the capital (dominant) letter first is just a standard scientific convention to make the genotypes easier to read. It quickly shows you that the dominant trait will dictate the phenotype.

How do you use a Punnett square?

A Punnett square is a simple grid used to predict the possible genetic outcomes of a cross between two parents. Think of it like a multiplication table for biology, where you match up the traits from each parent to see what combinations their offspring might inherit. <br><br> To use it, you write the genotype (the letter combinations representing genes) of one parent across the top and the other down the left side. Then, you fill in the empty boxes by combining the row and column letters. The resulting grid shows you all the possible genetic combinations and helps you calculate the probability of each trait appearing in the next generation.

What are alleles?

Before drawing the square, you need to understand alleles. Alleles are different versions of a gene. We represent them with letters. A capital letter stands for a dominant allele, which means it will mask the other version. A lowercase letter stands for a recessive allele, which only shows up if there is no dominant allele present. For example, if $T$ is tall and $t$ is short, a plant with $Tt$ will be tall.

Setting up the grid

Start by drawing a two-by-two grid for a single-trait cross (a monohybrid cross). Take the genotype of parent one and write its two letters above the two columns. Take the genotype of parent two and write its two letters beside the two rows. Since each parent passes on only one allele per gene to their offspring, separating the letters this way represents the sex cells (sperm and egg) carrying those individual alleles.

Filling in the boxes

To fill out the square, simply bring the top letters down into the boxes below them, and bring the side letters across into the boxes next to them. Each box inside the grid will end up with two letters, representing the genotype of a potential offspring. By convention, if a box gets a capital letter and a lowercase letter, you always write the capital letter first (like $Aa$ , not $aA$ ).

Reading the results

Once the grid is full, count the different types of genotypes inside the four boxes. Each box represents a 25% chance of that specific outcome. If two boxes have the genotype $Aa$ and two have $aa$ , there is a 50% chance the offspring will be $Aa$ and a 50% chance they will be $aa$ . Remember to distinguish between genotype (the letters) and phenotype (the physical trait you actually see).

Worked through

Cross a heterozygous tall pea plant ( $Tt$ ) with a homozygous short pea plant ( $tt$ ). What is the probability that their offspring will be short?

First, identify the parents: Parent 1 is $Tt$ and Parent 2 is $tt$ . Draw a 2x2 grid. Put the $T$ and $t$ of Parent 1 over the two columns. Put the $t$ and $t$ of Parent 2 next to the two rows. Now fill in the boxes. The top row of boxes gets a $t$ from the side; combined with the top, they become $Tt$ and $tt$ . The bottom row also gets a $t$ from the side, resulting in $Tt$ and $tt$ . Out of the four boxes, two contain $Tt$ (tall) and two contain $tt$ (short). Therefore, there are 2 out of 4 chances to get a short plant. The probability of the offspring being short is 50%.

Questions students ask

Ask about this topic

Explain at

Where this comes from: OpenStax Biology 2e, Chapter 12: Mendel's Experiments and Heredity · Khan Academy, High School Biology: Classical Genetics