12.2: Punnett Squares

A Punnett square displays the possible genotypes offspring can inherit from two parental genotypes. If a trait’s inheritance pattern (e.g., dominant or recessive) is known, Punnett squares can also be used to determine the probability of inheriting a phenotype. Punnett squares are applicable in situations where trait inheritance is determined by a single gene locus and traits are independently inherited. However, they cannot predict trait probabilities for more complex genetic inheritance scenarios.

Punnett Squares Display the Likelihood of Inheriting One or More Traits

Punnett squares are visual representations that display possible offspring genotypes resulting from a cross between two parental genotypes. They can depict inheritance of one or multiple phenotypes, or traits, although other tools are more appropriate for investigating the inheritance of more than two traits.

Punnett squares can be used to determine the likelihood of offspring inheriting a specific genotype, or pair of alleles causing a particular characteristic (i.e., phenotype or trait), provided that the phenotype is caused by a single gene locus and is independently assorted during meiosis. In other words, Punnett squares are useful for determining inheritance probabilities in scenarios where the likelihood of inheriting one trait does not affect the probability of inheriting another. Although there are many exceptions to these assumptions (e.g., traits encoded by neighboring genes on the same chromosome), these qualifiers hold true for many plant and animal traits.

Mendel’s Experiments Inspired the Creation of the Punnett Square

Punnett squares were created in the early 1900s by Reginald Punnett, several decades after Gregor Mendel’s ground-breaking pea plant experiments revealed the fundamental laws of inheritance. Today, Punnett squares are often used to illustrate the principles underlying Mendel’s experiments.

Mendel studied the inheritance of several pea plant characteristics, including pea and pod shape and color, flower color and position, and plant size. Mendel also examined inherited traits for each characteristic. For example, purple and white are possible traits for the flower color characteristic. In pea plants, purple and white flowers are determined by distinct gene variants, or alleles, at the flower color gene locus.

Punnett Squares Are Grids That Organize Genetic Information

Each box in a Punnett square represents a possible fertilization event, or offspring genotype, arising from two parental gametes. Punnett squares are typically arranged in 2x2 or 4x4 configurations to visualize inheritance of one or two traits, respectively.

Regarding nomenclature, alleles are indicated by the first letter of the trait caused by the dominant allele. For example, because yellow is the dominant pea color trait, alleles encoding pea color are denoted with the italicized letter ‘y.’ Uppercase and lowercase letters represent dominant and recessive alleles, respectively. Thus, Y represents the dominant yellow allele and y denotes the recessive green allele.

To create a 2x2 Punnett square examining one trait, one parental genotype is listed above the diagram, with one allele over each column. The other parental genotype is displayed vertically to the left of the diagram, with one allele next to each row. Each Punnett square box contains the two parental alleles (one from each parent) corresponding to the box’s row and column, representing one possible fertilization outcome. The full Punnett square contents can be used to determine the likelihood of offspring inheriting a particular trait.

Punnett Squares Are Informative Tools for Genetic Counselors and Breeders

Despite being created over 100 years ago, Punnett squares still have several relevant applications. For a couple receiving genetic counseling, Punnett squares can help determine their child’s risk of an inherited disease. For example, if one parent has cystic fibrosis (two recessive, causal alleles) and the other neither has nor carries it, their child will be a carrier (i.e., have one causal allele) but have no risk of cystic fibrosis. Punnett squares can also help animal and plant breeders select organisms with specific traits for continued breeding.

Although Punnett squares are useful in many contexts, they cannot accurately depict complex genetic inheritance. For example, traits encoded by neighboring genes on the same chromosome are often inherited together from one parent, a phenomenon called linkage. These traits are not independently assorted, so a Punnett square cannot accurately predict their inheritance patterns. Some traits, like height, are inappropriate for Punnett squares because they are determined by several genes and affected by environmental conditions (e.g., diet). Punnett squares are also ineffective at predicting heredity of traits acquired from only one parent.

The Punnet square, a simple diagram named after its inventor, is a graphical way to predict the likelihood that offspring may inherit a single gene trait, like lactose intolerance, given the parental genotypes.

Using this grid, the genetic contributions of one parent, who in this case, is homozygous recessive with alleles denoted as two lowercase L's are placed at the top. Those of the other, heterozygous with mixed alleles, represented by a dominant capital L along with a recessive lowercase L, are written on the left side.

Combining these row and column headers into the boxes, results in two possible genotypes and both phenotypes for a child. A 50% chance of being heterozygous and able to digest lactose. And a 50% chance of being homozygous recessive and lactose intolerant.