Multiple loci Flashcards
Continuous traits are polygenic. What does this mean?
Multiple loci are involved.
For polygenic traits, the effects of all the alleles are additive. True or false?
True.
For polygenic traits, the alleles can vary in fitness. True or false?
True.
Define an epistatic allele.
One that prevents expression of a different allele at another locus.
Are all polygenic interaction additive?
No, epistasis is not.
Say an organisms possesses an advantageous allele but it is not expressed. Why might this be?
It is masked by an epistatic allele.
Epistasis only reduces fitness by masking other alleles. True or false?
False - epistasis can also increase fitness by masking deleterious alleles.
There are 2 categories of epistasis. What are they?
- Magnitude
2. Sign
What are the 2 types of magnitude epistasis?
- Synergistic
2. Antagonistic
Define synergistic epistasis.
A double-mutant results in a larger mutational effect than expected.
What kind of effect does synergistic epistasis have on beneficial mutations?
There are even greater improvements to function.
What kind of effect does synergistic epistasis have on deleterious mutations?
It protects against the negative effects, creating a less severe drop in fitness.
What kind of effect does antagonistic epistasis have on beneficial mutations?
There is a reduction in expected fitness.
What kind of effect does antagonistic epistasis have on deleterious mutations?
It causes a greater than expected fitness drop.
Define sign epistasis.
The mutation has the opposite effect than expected when in the presence of another mutation.
What is meant by a polymorphism?
Variation.
Sign epistasis can stabilise polymorphism. What does this mean?
The expression of an allele at one locus can be altered by expression at another locus (as opposed to masked like in traditional epistasis).
How can sign epistasis constrain evolution?
It can prevent new/beneficial traits from being expressed.
What are fitness landscapes used for?
To visualise the relationship between genotypes and reproductive success.
Epistasis can shape the fitness landscape for an organism. How?
It confines evolution to fewer trajectories because it prevents the expression of particular alleles.
Define linkage.
Alleles are physically linked as they are situated on the same (section of) chromosome.
Define linkage disequilibrium.
The random association of alleles due to their physical location, that results in a deviations between the observed and expected frequencies.
What reduces linkage and how?
Recombination, the chromosomes exchange equivalent portions and alleles are spit apart.
If loci are further apart, what effect does this have on linkage disequilibrium?
It is lowered, as linkage is more likely to be broken at recombination.
If loci are close together, what effect does this have on linkage disequilibrium?
It is increased, as there is less chance alleles will be split by recombination.
What is the Hill-Robertson effect?
A principle that states a finite population subject to natural selection will undergo random linkage disequilibria, which will slow evolution.
Why does linkage disequilibrium slow evolution?
It conserves disadvantageous alleles.
Why does linkage conserve disadvantageous alleles?
Because they are linked to beneficial ones that are selected for.
‘If there is strong selection at one locus, there will be drift in neighbouring loci’. What does this mean?
Alleles that are selected for are often linked to others. As a result these linked alleles will also be selected for and conserved, reducing allelic variation at that locus.
Define a selective sweep.
The reduction or elimination of variation in DNA neighbouring a mutation.
What causes selective sweeps?
Linkage disequilibrium.
What is a haplotype?
A block of genes that are all conserved together.
How are haplotypes created?
By selective sweeps.
Separate populations display unique haplotypes. Give an example of this.
Humans are the only animals that drink milk as adults and thus require lactase. African populations have problems digesting milk as the haplotype containing the lactase enzyme has not been highly conserved.
