A Punnett square is a simple diagram used by geneticists to predict the probable outcomes of a genetic cross. This visual tool organizes the possible combinations of inherited traits from two parents, allowing for a statistical prediction of the offspring’s genetic makeup. This tutorial provides a step-by-step guide on how to calculate the results of these crosses, starting with the basic single-trait analysis.
Foundational Concepts for Genetic Crosses
Before calculating any cross, it is important to establish the language used to describe inherited characteristics. Alleles are different versions of the same gene, and every organism inherits two alleles for each trait—one from each parent. A dominant allele, represented by an uppercase letter like ‘B’, will mask the effect of a recessive allele when paired together. The recessive allele, represented by a lowercase letter ‘b’, only expresses its trait if an individual inherits two copies of it.
The combination of these two alleles makes up the individual’s genotype, which is the precise genetic identity of the organism (e.g., BB, Bb, or bb). The phenotype, by contrast, is the observable physical expression of the genotype, such as having brown or blue eyes. For instance, both BB and Bb genotypes might result in the same brown-eye phenotype, but their underlying genetic codes are distinct.
Performing the Monohybrid Cross
The monohybrid cross focuses on the inheritance pattern of a single trait. The first step is determining all possible single alleles, or gametes, that each parent can contribute. For example, a heterozygous parent (‘Tt’) for plant height can produce two types of gametes: one carrying the ‘T’ allele and one carrying the ‘t’ allele.
The next step involves setting up the 2×2 grid characteristic of the monohybrid cross. The gametes from one parent are placed along the top edge of the square, and the gametes from the second parent are placed down the left side. For a cross between two heterozygous parents (Tt x Tt), the letters T and t are placed individually above and to the left of the four-box grid. This systematic arrangement ensures all pairings are accounted for.
The final step is filling in the resulting offspring genotypes by combining the alleles from the intersecting rows and columns. Combining the T gamete with the T gamete results in the homozygous dominant genotype (TT). Combining T and t results in the heterozygous genotype (Tt), and combining two t gametes results in the homozygous recessive genotype (tt).
In the Tt x Tt example, the completed square contains one TT box, two Tt boxes, and one tt box. These four boxes represent all possible fertilization events, each having an equal 25% chance of occurring. The process demonstrates the random chance of allele combination during reproduction.
Interpreting Genotypic and Phenotypic Ratios
After filling the Punnett square, the resulting genotypes are translated into probability statements, known as ratios. The Genotypic Ratio describes the proportion of each specific genotype found in the offspring. For the Tt x Tt cross, counting the boxes yields one TT, two Tt, and one tt offspring.
This count is expressed as the genotypic ratio 1:2:1, representing the proportions of homozygous dominant, heterozygous, and homozygous recessive individuals. This translates to a 25% chance for TT, a 50% chance for Tt, and a 25% chance for tt. The sum of these probabilities must always equal 100%.
The Phenotypic Ratio is derived by grouping the genotypes that produce the same observable trait. Since the T allele is dominant, both TT and Tt genotypes result in the tall plant phenotype. Only the tt genotype results in the short phenotype.
Therefore, three of the four boxes result in a tall plant, and one results in a short plant. The resulting phenotypic ratio is 3:1, indicating a three-to-one probability of the dominant phenotype over the recessive phenotype. This ratio provides a statistical prediction of the offspring’s observable traits.
Expanding to Dihybrid Crosses
The dihybrid cross simultaneously tracks the inheritance of two different traits, such as plant height and seed color. This requires a 4×4 grid, resulting in 16 possible offspring combinations. This expansion is based on the principle of independent assortment, which states that the alleles for one trait separate into gametes independently of the alleles for another trait.
A parent with the genotype RrYy (R for round seed, Y for yellow seed) must combine the R/r alleles and the Y/y alleles systematically to determine all possible gametes. This process generates four distinct gametes: RY, Ry, rY, and ry. These four gametes are then placed along the top and side of the 16-box square.
The process of filling the 16 boxes remains the same: combining the alleles from the top and side. However, the final interpretation requires careful counting of all possible genotypic and phenotypic combinations. The phenotypic ratio for a dihybrid cross between two double-heterozygous parents (RrYy x RrYy) typically results in a 9:3:3:1 ratio, predicting the proportions of the four possible phenotype combinations.