Punnett Square
A Punnett square is a diagram used to predict the probability of genotypes and phenotypes in the offspring of two parents, based on the alleles each parent can contribute. Developed by British geneticist Reginald Crundall Punnett in the early 20th century, it arranges parental alleles along the axes of a grid, and each cell represents a possible offspring genotype. It is a foundational tool in Mendelian genetics for calculating ratios such as 3:1 (phenotypic) or 1:2:1 (genotypic) for monohybrid crosses.
Worked Example
Problem
Two heterozygous pea plants (Yy × Yy) are crossed, where Y = yellow seeds (dominant) and y = green seeds (recessive). What are the expected genotypic and phenotypic ratios of the offspring?
Solution
Step 1: Write parent alleles along each axis of the 2×2 Punnett square. Parent 1 (across top): Y y Parent 2 (down side): Y y Step 2: Fill in each cell by combining the allele from the column header with the allele from the row header. Cell 1 (Y × Y) = YY Cell 2 (Y × y) = Yy Cell 3 (y × Y) = Yy Cell 4 (y × y) = yy Step 3: Count genotypes: YY : Yy : yy = 1 : 2 : 1 Step 4: Determine phenotypes: YY and Yy → yellow seeds (3 cells) yy → green seeds (1 cell)
Answer
Genotypic ratio: 1 YY : 2 Yy : 1 yy. Phenotypic ratio: 3 yellow : 1 green.
Punnett Square for Monohybrid Cross Yy × Yy
| Y (Parent 1) | y (Parent 1) | |
|---|---|---|
| Y (Parent 2) | YY (Homozygous dominant) | Yy (Heterozygous) |
| y (Parent 2) | Yy (Heterozygous) | yy (Homozygous recessive) |
| Phenotype | Yellow | Yellow |
| Phenotype | Yellow | Green |
Interactive Tools
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Related Terms
Mendel's Law of Segregation
Mendel's Law of Segregation states that during the formation of gametes (sex cells), the two alleles for each gene separate (segregate) so that each gamete receives only one allele. This segregation is random, and therefore each allele has an equal probability of ending up in any given gamete. Proposed by Gregor Mendel in 1865 based on his pea plant experiments, this law forms the foundation of modern genetics and explains why offspring inherit one allele from each parent.
Homozygous
An organism is homozygous for a particular gene when it carries two identical alleles at that genetic locus — either two dominant alleles (homozygous dominant, e.g., AA) or two recessive alleles (homozygous recessive, e.g., aa). Homozygous individuals breed true for the trait associated with those alleles, meaning all offspring from two homozygous parents with the same alleles will also be homozygous. This condition is significant in selective breeding and genetic disease risk assessment.
Heterozygous
An organism is heterozygous for a particular gene when it carries two different alleles at that genetic locus — one dominant and one recessive (e.g., Aa). The dominant allele is expressed in the phenotype, while the recessive allele is carried but not expressed, making the individual a "carrier" of the recessive trait. Heterozygosity is important in population genetics because it maintains genetic diversity and affects the probability of recessive disorders appearing in offspring.
Named after British geneticist Reginald Crundall Punnett (1875–1967), who developed this diagrammatic method around 1905 while working with William Bateson. The "square" refers to the grid layout of the diagram.