Brent Cornell

    
According to the law of independent assortment, pairs of alleles are inherited independently of one another if their gene loci are on separate chromosomes – these genes are said to be unlinked

• This is due to the random orientation of homologous pairs during metaphase I of meiosis

The independent segregation of unlinked genes results in a greater number of potential gamete combinations, as well as a greater variety of possible phenotypes

• This also results in more complex inheritance patterns (e.g. monohybrid versus dihybrid crosses)

Inheritance of a Single Gene versus Two Unlinked Genes

    
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

If you are having difficulty remembering the FOIL method, an alternative way to identify potential gamete combinations in a dihybrid cross is to remember the claw

The CLAW – Calculating Dihybrid Gamete Combinations

How to Complete a Dihybrid Cross

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

Step 1:  Designate characters to represent the alleles

• Capital letter for dominant allele, lower case letter for recessive allele

Step 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)

Step 3:  Write down all potential gamete combinations for both parents

• Use the FOIL method (or the claw) to identify all possible combinations

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

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

Step 5:  Write out the phenotype ratios of potential offspring

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

Example of a Typical Dihybrid Cross

    
Below are two examples of questions involving dihybrid crosses – click on the question to show / hide the solution

Problem 1:

In sheep, the allele for black wool (B) is dominant over the allele for white wool (b).
Similarly, the allele for horns (H) is dominant over the allele for being hornless (h).
Pure breeding horned sheep with black wool were crossed with pure breeding hornless sheep with white wool.
(a) State the genotype and the phenotype of the F1 individuals produced as a result of this cross.
(b) Two F1 offspring were mated together. Calculate the expected ratio of phenotypes in the F2 generation.

Problem 2:

In cats, the allele for grey fur (G) is dominant over the allele for beige fur (g).
The allele for a solid coat (S) is dominant over the allele for a striped coat (s).
A pure breeding solid, beige cat is crossed with a pure breeding striped, grey cat.
(a) State the genotype and the phenotype of the F1 individuals produced as a result of this cross.
(b) Calculate the phenotypes resulting from a cross between a pure breeding solid, beige cat and an F1 offspring.