Punnett squares were developed after Mendelian genetics took root in the scientific community as a way to quickly assess a predicted outcome between two organisms with either know genotypes or phenotypes. In the case of the flower punnett square below, it is know that the purple flower is the dominant coloration for the particular species. This was figured out by crossing two homozygous purple, “PP,” flowers with each other and noting that the offspring was always purple. When a true-breeding homozygous purple flower was crossed with a white flower, the offspring was predominately purple with little white. Therefore, the “P” allele is know to be the dominant allele and the “p” allele that represents the white color is recessive. If two of the heterozygous purple flowers generated from the cross of the true-breeding purple flower and white flower are crossed, the probability of the offspring’s coloration percentages can be seen in the punnett square below that depicts a standard monohybrid cross (cross of one trait).
To construct a Punnett square, first draw a box and divide it into four smaller boxes. Then take the alleles from one parent, in this case “Pp,” and put them above the two boxes (columns) of the chart. Then take the alleles from the other parent, also “Pp,” and put them on the left side (rows) of two boxes. From that arrangement, you can then predict the ratio of offspring by combining the alleles from the parents. Simply pull the allele from the top of the column of the chart into the boxes and pull the allele from the rows of the chart into the boxes. For the punnett square below, the boxes would read “PP” in the upper left box, “Pp” in the upper right box, “Pp” in the lower left box, and “pp” in the lower right box. From our knowledge of dominant and recessive alleles, we would see that there would be a phenotypic 3:1 ratio of purple flowers : white flowers. The offspring produced would have a genotypic ratio of 1:2:1 of homozygous purple : heterozygous purple : homozygous white.
This phenotypic 3:1 and genotypic 1:2:1 outcome is the usual pattern of a heterozygous cross using Mendelian genetics. Several other variations of allele arrangement can be tested using Punnett squares, but as long as you use the basic principles of constructing and completing the chart, all outcomes you will see in basic genetics are possible to figure out. The picture below is an example of a dihybrid cross in which two traits are examined and the outcome is predicted. The first circles are the parent generation with the individuals being a homozygous dominant “BBSS” and a homozygous recessive “bbss.” The first generation offspring produced a heterozygous “BbSs” that was then used to cross and determine the second generation offspring’s phenotype and genotypic ratios. A dihybrid cross most generally yields a phenotypic ratio of 9:3:3:1.
Check out some Punnett square practice below!