Autosomal Linkage
This lesson covers:
- How autosomal linkage affects inheritance patterns
- An example of autosomal linkage in fruit flies
- How crossing over can affect linked genes
- Calculating recombination frequencies
What is autosomal linkage?
Autosomes are chromosomes that do not determine the sex of an organism. When genes are located on the same autosome, they are described as linked genes. Linked genes tend to be inherited together in offspring, rather than assorting independently.
This means that:
- There is a non-random association of alleles at different loci.
- The phenotypic ratios observed in dihybrid crosses are different from those expected from independent assortment.
- Parental allele combinations are preserved across generations.
How crossing over affects autosomal linkage
During meiosis, crossing over can exchange genetic material between homologous chromosomes at regions called chiasmata. Offspring that have different combinations of alleles from their parents due to crossing over are called recombinant offspring.
The effects of crossing over on autosomal linkage:
- Crossing over potentially separates linked genes.
- However, when genes are linked, fewer recombinant offspring tend to be produced.
- This indicates less genetic variation being introduced from crossing over when genes are linked.
- The probability of linked genes being separated during crossing over is inversely proportional to how close they are on a chromosome i.e., the closer the genes, the more likely they will be inherited together.
An example of autosomal linkage in fruit flies
In fruit flies, body colour and wing length are determined by genes that are linked on the same autosome.
The alleles for these genes are:
- B - Brown body colour (dominant)
- b - Black body colour (recessive)
- V - Long wings (dominant)
- v - Short wings (recessive)
For instance, if a brown, long-winged fly (BbVv) is crossed with a black, short-winged fly (bbvv):
- Autosomal linkage results in mainly parental phenotypes among offspring (BbVv and bbvv).
- This is because the linked alleles for body colour and wing length are usually inherited together.
- This produces very few recombinants with new genetic combinations.
Calculating the recombination frequency
The recombination frequency is a measure of how often recombinant offspring are produced as a result of crossing over.
It is determined using the following formula:
recombination frequency = total number of offspringnumber of recombinant offspring
Interpreting the recombination frequency:
- A 50% recombination frequency indicates that there is no linkage between the genes.
- Frequencies below 50% signify some degree of autosomal linkage.
- The lower the frequency, the closer the genes are located to each other on the chromosome, so they have a lower chance of being separated during crossing over.
This frequency helps to determine the relative positions of genes on chromosomes.
Worked Example - Calculating the recombination frequency
In a fruit fly cross, 42 offspring showed the parental phenotypes while 8 recombinant offspring were produced.
Calculate the recombination frequency and interpret what this indicates about the proximity of the linked genes on the chromosome.
Step 1: Equation
recombination frequency = total number of offspringnumber of recombinant offspring
Step 2: Substitution and correct evaluation
number of recombinant offspring =8
total number of offspring =42+8=50
recombination frequency = 508 = 0.16
Step 3: Interpretation of the result
a recombination frequency of 0.16, or 16%, indicates that the genes are linked, as the frequency is significantly less than 50%
this suggests the genes are moderately close together on the chromosome