Gr 11 Dihybrid

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Gene loci are said to be linked if on the same chromosome

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)

Review Independent Assortment - video

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

Mendel's Peas - showing a dihybrid cross

Remember to use the FOIL method with the F1 generation, to get the new "parent" gametes for your punnet square

If a Punnett grid is set up for this cross and all the alleles could be shuffled in any random order, we find that there are 16 possible random combinations. As we have seen before with AA and Aa genotypes, some combinations of alleles can generate the same phenotype. If R represents round and Y represents yellow, both the genotype RRYY and RrYy would give seeds that were round and yellow. As long as R and Y are not linked genes, they should segregate independently. This means that they should be able to pass on to the next generation either with or without the other. They show no dependence on each other; no preference one way or the other. During the shuf ing of alleles in meiosis, they are equally distributed between gametes. The result is that, in the offspring, there should be certain predictable ratios. These can be illustrated by Mendel’s dihybrid cross.

Steps to do a DiHybrid Cross

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 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

Try this problem:

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.

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