A Punnett square is a visual tool used in genetics to predict the genetic outcomes of a cross or breeding experiment. It systematically organizes parental contributions, providing a clear representation of genetic inheritance patterns and the probabilities of offspring inheriting specific traits.
Key Terms for Understanding
Understanding genetic terms is important before constructing a Punnett square. An allele refers to different forms of a gene, such as those responsible for tall or short plant height. Organisms inherit two alleles for each gene, one from each parent. The combination of these alleles in an individual is its genotype. For instance, a plant might have a genotype for tallness represented as ‘TT’, ‘Tt’, or ‘tt’.
The observable physical or biochemical characteristics of an organism, which result from its genotype and environmental influences, are called its phenotype. While ‘TT’ and ‘Tt’ might both result in a tall phenotype, ‘tt’ would produce a short phenotype. Alleles are represented by letters, with a capital letter denoting a dominant allele (like ‘T’ for tall) and a lowercase letter representing a recessive allele (like ‘t’ for short). A dominant allele expresses its trait even when paired with a recessive allele, while a recessive allele only expresses its trait if two copies are present.
Constructing the Punnett Square
Setting up a Punnett square begins by drawing a grid, typically a 2×2 square for analyzing a single trait. This grid houses the potential genetic combinations from the parents. Determine the alleles each parent can contribute to their offspring. Each parent contributes one allele for a given trait to each gamete (reproductive cell).
For example, consider pea plant height where ‘T’ represents the dominant tall allele and ‘t’ represents the recessive short allele. If one parent plant has a genotype of ‘Tt’ (heterozygous tall) and the other parent also has a genotype of ‘Tt’, each can produce gametes containing either ‘T’ or ‘t’. These parental alleles are placed along the top and left sides of the square, with one allele per row or column.
Filling the Grid
Once the Punnett square is set up with the parental alleles, combine these alleles into the inner squares. To do this, bring the allele from the top of each column down into the squares below it, and bring the allele from the left of each row across into the squares to its right. Each of the four inner squares will contain two alleles, representing a possible offspring genotype.
Continuing the example of two ‘Tt’ pea plants crossing, the top-left square would be ‘TT’ (combining ‘T’ from the top and ‘T’ from the left). The top-right square would be ‘Tt’ (combining ‘t’ from the top and ‘T’ from the left, conventionally written with the dominant allele first). The bottom-left square would also be ‘Tt’, and the bottom-right square would be ‘tt’. Each inner square represents one of the four equally probable genetic combinations for the offspring.
Decoding the Results
Interpreting the filled Punnett square involves calculating the genotypic and phenotypic ratios of potential offspring. First, count the occurrences of each unique genotype within the square. For the ‘Tt’ x ‘Tt’ cross, there is one ‘TT’, two ‘Tt’, and one ‘tt’. This gives a genotypic ratio of 1 ‘TT’ : 2 ‘Tt’ : 1 ‘tt’, which can also be expressed as percentages: 25% ‘TT’, 50% ‘Tt’, and 25% ‘tt’.
Next, translate these genotypes into observable phenotypes, remembering that ‘T’ (tall) is dominant over ‘t’ (short). Both ‘TT’ and ‘Tt’ genotypes will result in a tall phenotype, while only the ‘tt’ genotype will produce a short phenotype. Therefore, three out of the four squares (one ‘TT’ and two ‘Tt’) represent tall plants, and one square (‘tt’) represents a short plant. This yields a phenotypic ratio of 3 tall : 1 short, or 75% tall and 25% short offspring. These ratios represent the probability of offspring inheriting specific traits.