Dihybrid Crosses

A dihybrid cross determines the allele combinations of offspring for two particular genes that are unlinked (not on the same chromosome)

Because there are two genes with two alleles per gene (multiple alleles not required), there can be up to four different gamete combinations

Performing a Dihybrid Cross

To work out gamete combinations remember FOIL:  

• First  (AaBb = AB)                    • Outside (AaBb = Ab)                    • Inside (AaBb = aB)                    • Last (AaBb = ab)

When calculating genotype, always pair alleles from the same gene (e.g. ABab should be AaBb) and always write capitals first

Linked Genes

  • When two genes are linked, they do not follow the expected phenotypic ratio for a dihybrid cross between heterozygous parents
  • Instead the phenotypic ratio will follow that of a monohybrid cross as the two genes are inherited together
  • This means that offspring will tend to produce the parental phenotypes
  • Recombinant phenotypes are those combinations of genes not found in parents and will only be evident if crossing over occurs in prophase I and would thus be expected to appear in low numbers (if at all)
  • An example of a cross between two linked genes is the mating of a grey bodied, normal wing fruit fly with a black bodied, vestigial wing mutant

The Formation of Recombinant Chromosomes via Crossing Over

In a test cross of a heterozygous fruit fly (grey bodied, normal wings) with a homozygous recessive mutant (black bodied, vestigial wings), the recombinants would be the grey bodied, vestigial winged offsprings and the black bodied, normal winged offspring

Linked genes that have undergone recombination can be distinguished from unlinked genes via a test cross because the frequency of the recombinant genotypes will always be less than would occur for unlinked genes (crossing over does not happen every time)

  • For example:
    • Heterozygous test cross of unlinked genes =  1 : 1 : 1 : 1 phenotypic ratio
    • Heterozygous test cross of linked genes = 1 : 1 : 0.1 : 01 phenotypic ratio (uncommon phenotypes are recombinants)

Linked versus Unlinked Genes