A dihybrid cross determines the genotypic and phenotypic combinations of offspring for two particular genes that are unlinked

• Because there are two genes, each with two alleles, there can be up to four different gamete combinations
  

The easiest way to work out potential gamete combinations in a dihybrid cross is to use the FOIL method:

• FOIL = First / Outside / Inside / Last

Completing Dihybrid Crosses

The inheritance of dihybrid traits can be calculated according to the following steps:

1. Designate characters to represent the alleles

   • A capital letter is used for the dominant allele, lower case letter for the recessive allele

2. Write down the genotype and phenotype of the parents (P generation)

   • Always pair alleles from the same gene and always write capitals first (e.g. AaBb, not ABab)

3. Identify all potential gamete combinations for both parents

   • Use the FOIL method to identify all possible combinations
     

4. Use a Punnett square to work out potential genotypes of offspring

   • Only include different gamete combinations for each parent (e.g. AaBB has only two combinations = AB and aB)
     

5. Write out the phenotype ratios of potential offspring

   • Phenotypic ratios reflect mathematical probabilities only and may not necessarily reflect actual offspring ratios