How to Make a Genotypic Ratio
- 1). Obtain genotypes from both parents. The genotypes may be provided to you, or you may have to determine them from provided information. Since some alleles are dominant (will be expressed) and some are recessive (will only be expressed if there is no dominant allele), you cannot necessarily determine genotype from phenotype. A phenotype that expresses a dominant allele could indicate the presence of two of the same dominant allele (homozygous dominant), or the presence of one dominant allele and one recessive allele (heterozygous), for example. However, if the recessive form of a trait is expressed physically, then the genotype must contain only the recessive allele (homozygous recessive).
- 2). Prepare a Punnett square. Draw a matrix (four squares together) and write the alleles of one parent at the top of the Punnett square, and the alleles of the other parent on one side of the Punnett square (see Resources). The picture is an example of a properly prepared Punnett square, showing the alleles of each parent (along the outside of the grid) for a gene that determines flower color.
- 3). Determine the possible genotypes of offspring. Fill in the Punnett square in a gridlike fashion. That is, write in each square the allele from the top of that column and the allele from the beginning of that row, as in our example. Each combination of alleles in your Punnett square represents a possible genotype for offspring of the two parents.
- 4). Determine the genotypic ratio. Count the number of each genotype represented in the completed Punnett square. Write these numbers down in ratio form, with "/" or ":" between each number. This is the genotypic ratio. You may reduce the fraction or ratio as appropriate (e.g., 2:8 or 2/8 is the same as 1:4 or 1/4). In our Punnett square example, the genotypic ratio is 1RR:2Rr:1rr.
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