Predicting Inheritance: Dihybrid Crosses (AQA A Level Biology)
Revision Note
Written by: Lára Marie McIvor
Reviewed by: Lucy Kirkham
Dihybrid Crosses
Monohybrid crosses look at how the alleles of one gene transfer across generations
Dihybrid crosses look at how the alleles of two genes transfer across generations
The genetic diagrams for both types of crosses are very similar
There are several more genotypes and phenotypes involved
When writing the different genotypes write the two alleles for one gene, followed immediately by the two alleles for the other gene. Do not mix up the alleles from the different genes
If there was a gene with alleles Y and y and another gene with alleles G and g an example genotype for an individual would be YyGg
Worked Example
Worked example: Dihybrid genetic diagram
Horses have a single gene for coat colour that has two alleles:
B, a dominant allele produces a black coat
b, a recessive allele produces a chestnut coat
Horses also have single gene for eye colour
E, a dominant allele produces brown eyes
e, a recessive allele produces blue eyes
In this example a horse which is heterozygous for both genes has been crossed with a horse that is homozygous for one gene and heterozygous for the other
Parental phenotypes: black coat, brown eyes x chestnut coat, brown eyes
Parental genotypes: BbEe bbEe
Parental gametes: BE or Be or bE or be bE or be
Dihybrid cross punnett square table
Predicted ratio of phenotypes in offspring – 3 black coat, brown eyes : 3 chestnut coat, brown eyes : 1 black coat, blue eyes : 1 chestnut coat, blue eyes
Autosomal linkage
Dihybrid crosses and their predictions rely on the assumption that the genes being investigated behave independently of one another during meiosis
Not all genes assort independently during meiosis
Some genes which are located on the same chromosome display autosomal linkage and stay together in the original parental combination
Linkage between genes affects how parental alleles are passed onto offspring through the gametes
When writing linked genotypes it can be easier to keep the linked alleles within a bracket
For example an individual has the genotype FFGG however if there is linkage between the two genes then it would be written as (FG)(FG)
Worked Example
Worked example: Explaining autosomal linkage
The genes for tail length and scale colour in a species of newt have displayed autosomal linkage
The gene for tail length has two alleles :
Dominant allele T produces a normal length tail
Recessive allele t produces a shorter length tail
The gene for scale colour has two alleles:
Dominant allele G produces green scales
Recessive allele g produces white scales
A newt heterozygous for a normal tail and green scales is crossed with a newt that has a shorter tail and white scales
Parental phenotypes: normal tail, green scales x short tail, white scales
Parental genotypes: (TG)(tg) (tg)(tg)
Parental gametes: (TG) or (tg) (tg)
Dihybrid cross with linkage punnett square table
Predicted ratio of phenotypes in offspring – 1 normal tail, green scales : 1 short tail, white scales
Predicted ratio of genotypes in offspring – 1 (TG)(tg) : 1 (tg)(tg)
Epistasis
In some cases one gene can affect the expression of another gene
Epistasis: when two genes on different chromosomes affect the same feature
If epistasis is present it needs to be taken into account when determining the phenotypes of individuals
The whole combination of alleles from the different genes dictates the phenotype
Worked Example
Worked example: Explaining epistasis
There is a gene that dictates the feather colour of pigeons
The gene has two alleles (R / r) :
Allele R codes for a pigment that produces grey feathers
Allele r doesn’t produce a pigment, resulting in white feathers
Another gene has also been found to have an effect on feather colour
This gene has two alleles (F / f) :
The dominant allele F stops grey feathers being produced even if the allele R is present
These are the possible phenotypes:
RRFF white feathers
RrFF white feathers
rrFF white feathers
RRFf white feathers
RrFf white feathers
rrFf white feathers
rrff white feathers
RRff grey feathers
Rrff grey feathers
Examiner Tips and Tricks
When you are working through different genetics questions you may notice that test crosses involving autosomal linkage predict solely parental type offspring (offspring that have the same combination of characteristics as their parents).However in reality recombinant offspring (offspring that have a different combination of characteristics to their parents) are often produced. This is due to the crossing over that occurs during meiosis. The crossing over and exchanging of genetic material breaks the linkage between the genes and recombines the characteristics of the parents.So if a question comes along that asks you why recombinant offspring are present you now know why!
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