A Punnett square is a visual tool in genetics that predicts the probability of offspring inheriting specific genetic traits from their parents. It helps visualize all possible combinations of alleles from a genetic cross. This diagram aids in understanding the principles of inheritance for a single trait.
Understanding Key Genetic Terms
Understanding basic genetic terms is helpful for using a Punnett square. Alleles are different forms of a gene, such as the gene for pea plant height having an allele for “Tall” or “short.” A dominant allele expresses its trait even when only one copy is present, represented by a capital letter (e.g., ‘T’ for Tall). Conversely, a recessive allele only expresses its trait when two copies are present, symbolized by a lowercase letter (e.g., ‘t’ for short).
The genotype refers to an organism’s specific genetic makeup, represented by the combination of alleles (e.g., TT, Tt, or tt). The phenotype is the observable physical characteristic resulting from the genotype, such as a pea plant being Tall or Short.
An individual is homozygous if they have two identical alleles for a trait (e.g., TT or tt). When an individual has two different alleles for a trait (Tt), they are considered heterozygous. In a heterozygous individual, the dominant allele’s trait will be expressed, while the recessive allele’s trait remains hidden.
Constructing a Punnett Square
Constructing a Punnett square begins by identifying the genotypes of the parent organisms. For instance, consider pea plant height where Tall (T) is dominant and short (t) is recessive. If both parent plants are heterozygous for height, their genotype would be Tt.
The next step involves determining the possible gametes each parent can produce. A heterozygous parent (Tt) can produce two types of gametes: one carrying the ‘T’ allele and another carrying the ‘t’ allele.
After identifying the gametes, a 2×2 square grid is drawn for a monohybrid cross. The possible gametes from one parent are placed along the top of the grid, and the gametes from the other parent are placed along the left side. Finally, the squares are filled by combining the alleles from the corresponding row and column. For example, a ‘T’ from the top parent combined with a ‘t’ from the side parent results in ‘Tt’. Filling the squares completes the Punnett square, mapping all potential allele combinations in the offspring.
Deciphering Punnett Square Outcomes
Once a Punnett square is constructed, its results are interpreted by analyzing the combinations within the grid. Using our example of two heterozygous pea plants (Tt x Tt), the filled Punnett square shows one TT, two Tt, and one tt combination. These represent the possible genotypes of the offspring.
The genotypic ratio is stated as 1 TT : 2 Tt : 1 tt. This ratio indicates the relative proportions of each genotype expected among the offspring.
Next, determine the phenotypes corresponding to each genotype. Since ‘T’ (Tall) is dominant over ‘t’ (short), both TT and Tt genotypes result in a Tall phenotype, while only the tt genotype results in a short phenotype. This leads to a phenotypic ratio of 3 Tall : 1 Short. These ratios represent the probability of offspring inheriting specific traits. For our example, there is a 75% probability of a Tall plant and a 25% probability of a short plant from this cross.