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Recall, Gregor Mendel experimented on pea plants to understand how traits are inherited. From his experiment, Mendel knew that offspring inherit a set of instructions for each trait through DNA. Scientists now call these instructions genes.

Through sperm or egg, each parent passes on 1 gene for every trait to the offspring. The offspring receives 1 gene from the female's egg and 1 gene from the male's sperm. The offspring has 2 genes for every trait.

Genes take on different forms, called an allele. The alleles for hair color are brown, blonde, red, black, and so on. Some alleles are dominant and others are recessive.

Reinforce your understanding of genes and alleles using the video linked above.

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A gene specifies a trait, such as eye color. The allele specifies the form of the trait, such as brown eyes or blue eyes.

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Imagine a coin toss. There is always a 50-50 chance that a coin will land on heads, and a 50-50 chance it will land on tails. This means, half the time it will land on heads and half the time it will land on tails. What is the chance of it landing heads twice in a row? Or three times? These rules of probability that we use to predict the outcome of a coin flip also apply to genetics.

A Punnett square is a special tool used to predict the possible genes of an offspring based on those of the two parents. In a Punnett square, the possible genes are represented with letters. Dominant genes are uppercase letters, while recessive genes are lowercase letters.

To understand Punnett squares, there are a few more vocabulary words you need to know. A parent has two copies of a gene, but only gives one copy to an offspring. When an individual has two different copies of a gene, it is called heterozygous. Having one dominant gene and one recessive gene is heterozygous. Think of it like being a hybrid. When an individual has two identical copies of a gene, it is called homozygous. Having two dominant genes or two recessive genes is homozygous. Think of it like being purebred.

If a parent has one dominant gene and one recessive gene for a trait (or it heterozygous) then about half the time the dominant gene will be passed on, and half the time the recessive gene will be passed on. If a parent has two dominant genes for a trait (or is homozygous), then every time the dominant gene will be passed on. If a parent has two recessive genes for a trait (or is homozygous), then every time the recessive gene will be passed on.

A Punnett square shows us how parents can pass on their genes. An example of a Punnett square is shown in the image below and is recapped in the video linked above. Make sure you examine the image and read it thoroughly. You can review the video if needed.

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Why do we use Punnett squares in genetics?

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In this example, black fur is the dominant trait and brown fur is the recessive trait. Black fur is represented with B, and brown fur is represented with b.

One parent is homozygous for black fur (represented with BB) and the other is homozygous for brown fur (represented with bb). Each parent passes on one gene to the offspring.

The offspring are all heterozygous for black fur (represented with Bb).

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When interpreting Punnett squares, scientists look for two things called the genotype and phenotype.

Genes determine the physical traits of offspring. The combination of genes each individual has is called the genotype. A genotype can be homozygous or heterozygous. In the sample Punnett square picture above, both parent dogs were homozygous.

The physical appearance of the genes is called the phenotype. We can observe the phenotype in the organism. In the example picture above, you can observe that one dog has black fur, while the other has brown fur.

When interpreting the Punnett square, remember that the recessive trait "hides" when paired with a dominant one. Since there are 4 boxes, each box represents 1/4 or a 25% chance.

If needed, review the video from class today using the link above.

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Genotype, Phenotype and Punnet Squares Made EASY!

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A Punnett square predicts the outcome of offspring using probability. Punnett squares show the genes of parents combining to form the genes of offspring. From a Punnett square, you can predict the genotype and phenotype of offspring.