How to Calculate Phenotypic Ratio

By Andrew Youngker; Updated April 25, 2017
Physical characteristics like flower color are a specific phenotype.

Mendel, the father of genetics, conducted observations that contributed to genetic principles still used today. In Biology, the physical trait a living organism exhibits is referred to as the phenotype. The alleles, or genes for a trait, are known as the genotype. A phenotypic ratio represents a relationship between the different physical characteristics and how often they occur. Ratios are typically done in relationship to a single trait among the individuals.

Ratio by Observation

Make a frequency chart by labeling the desired traits in columns and placing a tally mark to count the number of subjects with that trait. Count the individuals in the group only once.

Rank the frequencies from smallest to largest by writing a number next to each of the categories.

Divide each frequency by the smallest one, and note the answer in the margins of the table. For example, if there are 10 in category one and 30 in category two, 10 divided by 10 equals 1 and 30 divided by 10 equals 3.

Write the phenotypic ratio using rounding when appropriate. So a ratio of 8.7, 3.1 and 1 would be written as 9:3:1.

Punnet Square Ratio

Make a Punnet, for one trait, by drawing a two-by-two block of squares.

Label the possible alleles from one parent across the top of the squares. The possible alleles from the other parent are labeled across the left side of the block. There should be only one allele per column or row.

Fill in the Punnet square by crossing one column and row and writing the result in each square. So a cross of “A” and “a” should be written as “Aa.”

Write the amount of homozygous dominant (AA) and heterozygous (Aa) squares as one phenotypic group. Count the amount of homozygous recessive (aa) squares as another group.

Write the result as a ratio of the two groups. A count of 3 from one group and 1 from the other would give a ratio of 3:1.

Incomplete Dominance

Complete the first three steps from “Punnet Square Ratio” from above.

Count the number of homozygous squares in their own group. For example, each of “aa” and “bb” would be in their own respective group.

Count the number of heterozygous squares as a separate group.

Write the phenotypic ratio as a relationship of the physical traits in each group. Three distinct traits are common with incomplete dominance.

About the Author

Based out of Reno, Nev., Andrew Youngker has been writing since 2007. He writes articles for various websites, covering cooking and education. Youngker is pursuing a Bachelor of Science in biology from the University of Nevada, Reno.