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How to Predict the Genotype Ratio

A genotype is the combination of genes for a particular characteristic. One gene is from one parent and the other from the other. Each gene may be dominant or recessive; dominant genes are written with capital letters and recessive genes with lower case letters. If the two genes are homozygous (that is, they agree) then it doesn't matter if they are dominant or recessive. But if they are heterozygous, then the dominant gene will be expressed.

Instructions

- 1
  Draw a table with nine cells in three rows and three columns.
- 2
  Label the columns with the two genes from one parent. For instance, if it is a gene for tall vs. short, and tall is dominant, then the genes could be TT, Tt or tt. Label one column with the first gene and one with the second; each column in the example would be T or t.
- 3
  Label the rows similarly with the genes from the other parent. Again, in the example, each row will be T or t.
- 4
  Fill in the table by taking one gene from the column and one from the row. So, if the column was t and the row is T, it would be Tt. The order of the genes does not matter.
- 5
  Count each type. There are three possible genotypes: TT, Tt and tt. The genotype ratio is the ratio of these three numbers. Suppose the rows were Tt and the columns were Tt. Then the four cells would be TT, Tt, Tt and tt. The genotype ratio would be 1:2:1. Sometimes one of the numbers will be 0.

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