Punnett Squares Explained: Mendel’s Rules of Inheritance

Genes, alleles and traits

A trait is something you can observe, such as flower colour in a pea plant. A gene is a stretch of DNA that influences a trait. Most genes come in different versions called alleles. An organism that reproduces sexually inherits two copies of each gene, one from each parent.

Letters make this easy to write down. If a coat-colour gene has a dark allele and a pale allele, we might write them as C and c. An individual could then carry CC, Cc or cc.

Dominant and recessive alleles

When the two alleles differ, one often hides the other. The hiding allele is dominant (capital letter) and the hidden one is recessive (lower case). A Cc individual looks the same as a CC one. The recessive trait only appears when both copies are recessive: cc.

Dominant does not mean better or more common. It only describes which allele shows when both are present. Plenty of harmless traits are recessive, and some dominant ones are rare.

This is the simplest pattern, called complete dominance. Real genes can behave in more complicated ways, which is why the grid below is described as a model for one gene at a time.

Genotype versus phenotype

Two words keep these ideas apart:

• Genotype is the pair of alleles an individual carries: CC, Cc or cc.
• Phenotype is the trait you can actually see.

Different genotypes can give the same phenotype. CC and Cc both show the dominant look. An individual with Cc is a carrier: it shows the dominant trait but can pass the hidden allele on. Words like homozygous (two matching alleles) and heterozygous (two different alleles) describe the genotype.

Mendel’s law of segregation

Here is the key insight. An individual's two alleles separate when sex cells (gametes) form, so each gamete carries only one. Which one ends up in a particular gamete is down to chance. At fertilisation, a gamete from each parent joins with the other and the pair is restored.

So a Cc parent produces two kinds of gamete, C and c, in equal numbers. That is all you need to predict a cross.

How to fill a Punnett square

A Punnett square is a small grid that lists every possible combination. To cross two carriers, Cc × Cc:

1. Write one parent's gametes (C, c) across the top.
2. Write the other parent's gametes down the side.
3. Fill each cell with the pair formed by its row and column.

You get four equally likely cells: CC, Cc, Cc, cc. Read two ratios from them:

• Genotype ratio: 1 CC : 2 Cc : 1 cc.
• Phenotype ratio: 3 dominant : 1 recessive.

So each offspring has a one in four chance of showing the recessive trait. Crossing two true-breeding lines behaves differently: CC × cc gives only Cc, so every offspring is identical (the F₁ generation) and shows the dominant trait.

Probabilities are not promises

A 3 : 1 ratio is an expected average, not a guarantee. Each offspring is an independent event, like a coin toss. A small family can easily show 2 : 2 or even 4 : 0, and that is perfectly normal. Mendel saw clear ratios because he counted hundreds of plants.

Mistakes to avoid

• Assuming that a family of four must contain exactly one recessive child.
• Thinking that a previous result changes the odds for the next offspring. It does not.
• Treating "dominant" as meaning "most common in the population".

How to explore this in the lab

The lab uses invented creatures, so you can practise breeding without any real organisms. It is a simplified teaching model that follows one gene at a time. A good order:

1. What is a trait? to meet the creatures and the vocabulary.
2. Dominant and recessive to see which allele shows.
3. Genotype versus phenotype to separate the code from the look.
4. The monohybrid cross to breed one trait.
5. The Punnett square to predict outcomes before you breed.
6. Homozygous crosses to see how true-breeding lines behave.

Make a prediction with the grid first, breed several litters, and compare. The gap between prediction and result is where the real understanding lives.