A Punnett square is a visual tool in genetics that predicts the genetic outcomes of offspring from two parents. It illustrates how different versions of genes, known as alleles, combine during reproduction. This diagram determines the likelihood of an offspring inheriting specific traits, showing potential genotypes and phenotypes. It simplifies genetic inheritance for a particular cross.
Essential Genetic Terms
Understanding basic terms is helpful before constructing a Punnett square. An “allele” refers to a specific version of a gene, like an allele for brown or blue eyes. Alleles are either “dominant” or “recessive,” describing how their traits are expressed. A dominant allele expresses its trait even if only one copy is present, represented by a capital letter. A recessive allele expresses its trait only if two copies are present, represented by a lowercase letter.
The combination of alleles an individual inherits for a specific gene is called their “genotype.” For example, an individual might have two identical dominant alleles, two identical recessive alleles, or one dominant and one recessive allele. The “phenotype” is the observable characteristic or physical expression of these alleles, such as brown or blue eyes. Genotype is inherited, and phenotype is influenced by the genotype.
Constructing the Punnett Square
Constructing a Punnett square involves steps to predict the offspring’s genetic makeup. First, draw a square and divide it into four smaller squares, forming a grid. For a monohybrid cross, which analyzes a single trait, a 2×2 grid is standard.
Next, identify the genotypes of the two parents involved. For instance, if crossing two parents heterozygous for a trait (e.g., ‘Tt’), place their individual alleles along the top and side of the square. One parent’s alleles (T and t) go above the columns, and the other parent’s alleles (T and t) go to the left of the rows. It does not matter which parent’s alleles are placed on the top or side.
Finally, fill each inner box by combining the allele from its corresponding row and column. For example, the top-left box combines ‘T’ from the top and ‘T’ from the side, resulting in ‘TT’. The top-right would be ‘Tt’, the bottom-left ‘Tt’, and the bottom-right ‘tt’. Each filled box represents a possible offspring genotype, with each box representing a 25% chance of that combination.
Interpreting the Probabilities
Once the Punnett square is completed, interpret the results by determining the genotypic and phenotypic ratios of potential offspring. In our example of two heterozygous parents (Tt x Tt), the filled Punnett square shows one ‘TT’, two ‘Tt’, and one ‘tt’ combination. This translates to a genotypic ratio of 1:2:1 (homozygous dominant:heterozygous:homozygous recessive). ‘TT’ has a 25% chance, ‘Tt’ has a 50% chance (2 out of 4 boxes), and ‘tt’ has a 25% chance.
To find the phenotypic ratio, consider which trait each genotype expresses. Assuming ‘T’ (tall) is dominant and ‘t’ (short) is recessive, both ‘TT’ and ‘Tt’ genotypes result in a tall phenotype. Only ‘tt’ results in a short phenotype. Thus, three out of four outcomes will be tall (TT, Tt, Tt) and one will be short (tt), giving a phenotypic ratio of 3:1 (tall:short). A Punnett square predicts probabilities, not guaranteed outcomes. Like flipping a coin, observed ratios approach predicted probabilities with a larger number of offspring.