PHENOTYPIC RATIO
The gamete combinations predict which offspring (F₂ generation) can be produced from these F₁ parents. The phenotypic ratio (relative count of trait combinations) can now be determined. It is recommended that phenotypes be written in each square when working a Punnett square problem.

Remember that phenotype is determined by genotype. Therefore, the phenotypes can be determined by the alleles in each offspring square: an uppercase letter represents a dominant allele. Any offspring with one uppercase letter for a particular trait will have that trait.

For example, a B means the guinea pig will have black fur, and an S means the guinea pig will have short fur. Any trait represented by two lowercase letters, ss (long fur) and bb (brown fur) in this case, will be expressed.

The purpose of determining the phenotypic ratio is to reveal an inheritance pattern. If the pattern is known, then the type of inheritance can often be determined. For example, the phenotypic ratio for this cross (two heterozygous individuals) is 9:3:3:1. This pattern is characteristic of a cross between two heterozygous parents with traits that exhibit complete dominance. The 9:3:3:1 ration would NOT be produced by any other combination of alleles, nor would this ratio result from traits exhibiting incomplete or codominance.