How to Do a Punnett Square

Punnett squares contain information about the probability of inheriting certain traits.
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In 1905, Reginald Punnett published Mendelism, the first textbook of modern genetics. In the course of his studies, Punnett developed a graphical method for predicting outcomes of genetic crosses.

Now referred to as a Punnett square, this graphic organizer provides a relatively simple method to predict the probability of genotypes and phenotypes.

Genetic Vocabulary

Understanding how Punnett squares work requires familiarity with some specialized terms. Before moving ahead in this Punnett square tutorial, let's review some vocabulary.

Traits, Genes and Alleles

Traits are inherited characteristics. Genes carry traits from one generation to the next. An organism has two genes for each trait, inheriting one gene from each parent. Alleles are variants of a gene.

For example, a person may inherit the gene for blue eyes from one parent and the gene for brown eyes from the other parent. The person has inherited two different alleles for eye color.

Genotypes and Phenotypes

The genotype definition in biology means an organism's combination of genes. The phenotype is the physical expression of the genotype.

The person inheriting one allele for brown eyes and one allele for blue eyes has a hybrid or heterozygous genotype, meaning two different versions or alleles for eye color. All other factors aside, this person's phenotype will be brown eyes.

If both inherited genes are the same or homozygous, the phenotype will show that trait.

Dominant, Recessive and Co-dominant Genes

Traits can be carried by dominant, recessive or co-dominant genes.

Dominant traits mask or hide recessive traits, meaning that while a person may inherit two different alleles for a trait, the phenotype or physical expression of the trait will be for the dominant allele. In the case of brown and blue eye color alleles, the brown eye allele is dominant over the recessive blue eye allele.

Blood types A and B are co-dominant genes so a person inheriting a gene for type A blood and a gene for type B blood will have type AB blood.

Standard notation uses capital letters to represent dominant traits and lowercase letters to represent recessive traits.

Caveat: Hypothetical Outcomes vs. Reality

Inherited genes interact and influence each other. These interactions mean that phenotypes don't always match the expected outcome of the dominant allele vs. recessive allele model predicted by Punnett squares.

How to Use a Punnett Square

    Before a Punnett square can be used, the genotypes for each parent must be determined.

    If the parent genotype isn't known, the grandparent genotypes can be used. If a parent has brown eyes, one allele in the genotype will be for brown eyes.

    The other allele could be for brown eyes, green eyes or blue eyes. If one grandparent has brown eyes and the other grandparent has blue eyes, the second allele may be for blue or green eyes but not brown eyes.

    In general, dominant traits appear more often in the phenotype than recessive traits. Brown hair, in general, is dominant over blonde or red hair and is much more common in the world's population.

    Local populations, however, may not reflect this dominance because the gene pool might include a large number of people with blonde or red hair.

    The graphic organizer called a Punnett square may be drawn as a square divided into fourths or like a standard tic-tac-toe frame.

    Sometimes the tic-tac-toe frame is drawn with a right side and base added, but these aren't necessary.

    The genotype for parents contains two alleles for each trait. To calculate the probability of offspring receiving an allele, both alleles must be placed in the Punnett square. Place alleles from one parent on the upper edge of the Punnett square and alleles from the other parent on the left side of the Punnett square.

    There should be one allele symbol over each column of squares and one allele symbol on the left side of each row of squares.

    Copy the symbol from the top of each column into each square of that column. Copy the symbol from the left side of the row into each square of that row. Each of the squares should now have two symbols.

    For example, if the top of the column has a capital B and the left end of the row has a lowercase b, then the square should have the symbol pair Bb.

    Each of the four squares now contains two allele symbols. These are the possible genotypes. If the two symbols are the same, the genotype is homozygous.

    If the two symbols are different, such as Bb, the genotype is heterozygous. If both symbols are capital letters, like BB, then the genotype is homozygous dominant. If both symbols are lowercase, like bb, then the genotype is homozygous recessive.

    Presuming no other genetic factors, the dominant allele controls the physical expression of each genotype. In other words, if there is a dominant gene (shown with a capital letter) in the genotype pair, that trait will show in the offspring.

    For eye color, if B represents brown eyes and b represents blue eyes, then an offspring that inherits the gene pair BB or the gene pair Bb will have brown eyes. To have blue eyes, both inherited genes must be the homozygous recessive bb.

    In a simple or monohybrid cross to evaluate one trait, there are four possible outcomes. If the genotypes in the squares are from a cross of a homozygous dominant BB and a heterozygous Bb, then the four possible outcomes are BB, BB, Bb and Bb.

    Two of the four possible outcomes or 50 percent of the offspring have the homozygous dominant genotype BB and two of the four possible outcomes or 50 percent of the offspring have the heterozygous genotype Bb.

    Calculating the phenotype probabilities means looking for the dominant gene. In the eye color example, count every square with a capital letter. In the example with outcomes BB, BB, Bb and Bb where B represents brown eyes and b represents blue eyes, all four squares contain a dominant B gene.

    All four possible outcomes or 100 percent of the offspring would therefore have brown eyes.

Punnett Square Calculator

Online Punnett square calculators are available. To use these calculators, input the genotypes for the parents and the calculator will generate the resulting genotype and phenotype combinations.

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