BIO 370 Lecture Notes - Lecture 23: Linkage Disequilibrium, Haplotype, Mutation
Evolutionary Processes Create Linkage Disequilibrium
Now we look at how associations between alleles at different loci arise and what happens to
these associations over time.
Linkage disequilibrium can arise from the evolutionary processes we have been studying –
mutation, selection, migration, and drift.
Linkage Disequilibrium Via Mutation
Suppose a population is initially polymorphic at the A locus, with both A and a alleles present,
but is monomorphic at the adjacent B locus on the same chromosome, with only B alleles
present.
Because only one of the two loci is polymorphic, there is no linkage disequilibrium. There are
no A|b or a|b haplotypes; therefore:
D = (hAB x hab) – (hAb x haB)
= (hAB x 0) – (0 x haB)
= 0 – 0 = 0
Now suppose a new b allele is formed by mutation at the B locus ad, for this eaple, let’s sa
that b arises adjacent to an a allele.
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In this case, the presence of the b allele at the B locus guarantees the presence of the a allele at
the A locus.
This means that the A and B loci are now in linkage disequilibrium:
D = (hAB x hab) – (hAb x haB)
= (hAB x hab) – (0 x haB)
= (hAB x hab) – 0
= hAB x hab
Over time, this linkage disequilibrium may break down because of recombination, but for now
the important point is that simple evolutionary history – the mutations that occur and the
genetic background on which the mutations arise – generates linkage disequilibrium among
loci.
Natural selection can generate linkage disequilibrium, but only between alleles with epistatic
interactions.
Epistasis is typically a result of linked or overlapping biochemical pathways.
As shown in the next slide, either of two biosynthetic pathways is sufficient to produce an
essential product from precursor raw materials. In this example, an A allele or a B allele – but
not both – is needed to produce the essential product.
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Document Summary
Now we look at how associations between alleles at different loci arise and what happens to these associations over time. Linkage disequilibrium can arise from the evolutionary processes we have been studying mutation, selection, migration, and drift. Suppose a population is initially polymorphic at the a locus, with both a and a alleles present, but is monomorphic at the adjacent b locus on the same chromosome, with only b alleles present. Because only one of the two loci is polymorphic, there is no linkage disequilibrium. There are no a|b or a|b haplotypes; therefore: = (hab x 0) (0 x hab) Now suppose a new b allele is formed by mutation at the b locus a(cid:374)d, for this e(cid:454)a(cid:373)ple, let"s sa(cid:455) that b arises adjacent to an a allele. In this case, the presence of the b allele at the b locus guarantees the presence of the a allele at the a locus.
