In: Biology
What is Linkage Disequilibrium?
What will happen if Linkage Disequilibrium is high?
And what is the correlation of Linkage Disequilibrium with positive
or negative selection?
1. The non random association of alleles at two or more loci is called linkage disequilibrium (LD). When two alleles are linked they have equal frequency in a population and they are said to be in equilibrium. It may also be present at different loci between two unlinked alleles. It happens when recombination happens and breaks the association between the alleles and many factors such as recombination, mutation, genetic drifts are responsible for LD.
Let's take an example
Suppose there are two alleles P and B at different loci. The frequencies of these two alleles are fP and fB. The frequency fPB is the frequency of both alleles coming together. When these alleles are assorting independently and do not have effect on each others frequency the product of there frequencies will be fPfB. The expected frequency is changed in linkage disequilibrium.
In case where fPB≠fPfB linkage disequlibrium can be defined calculated as:
dPB= fPB-fPfB
Where d is the is the coefficient of linkage disequilibrium.
When dPB=0 the alleles are said to be in equilibrium with fPB=fPfB
But when dPB ≠0 they are said to be in disequilibrium.
2. A high LD signifies that factors like positive selection, random mating are present in the population and there is higher rate of variations and the distribution of alleles is unique.The frequency of two alleles will not be as expected.
3. A advantageous mutation is positively selected it results in higher LD. For example suppose an ancestral haplotype gains a mutation and this mutation gets established in the population, so now two haplotypes will be present in the population. Negative selection on the other is a selection process which deletes an allele no longer required thus negative linkage disequilibrium.