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Flashcards in Recombination and Multigene Evolution Deck (38)
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How is genetic variation maintained in a population

Balancing selection (overdominanc and frequency dependent selection)
Mutation-drift balance
Migration-drift balance (and hybridisation, genetic introgressions)


What dos MHC stand for

Major Histocompatibility Complex


What is the MHC

The MHC is a gene dense region that is present in all jawed vertebrates which harbours immune gene that encode for proteins involved in antigen presentation


What does the MHC play important biological functions in

resistance to infectious,
individual odours, mating preferences, kin recognition and reproductive success


Where are the most polymorphic genes in the vertebrate genome



What is the main characteristic of the MHC

strong linkage disequilibrium


What is linkage disequilibrium

Linkage disequilibrium refers to the non-random association of alleles at two or more loci in a general population. When alleles are in linkage disequilibrium, haplotypes do not occur at the expected frequencies.


What two types of balancing selection have been proposed to explain the MHC

Overdominance model (heterozygous superiority)
Negative frequency dependent selection model


What are the 3 main question surrounding the MHC

How can selection maintain high levels of polymorphism in small populations?
How can balancing selection maintain MHC alleles over long evolutionary time (trans-species polymorphism)?
Why is the MHC is linked to so many diseases?


How does the MHC present itself in small, bottlenecked populations

MHC has a high level of diversity which neutral genetic marker loci have very low diversity


What is trans-species polymorphism?

is the occurrence of similar alleles in related


What dos polymorphic mean?

discontinuous genetic variation


What pairs share identical MHC alleles

Guppies and P. picta and humans and chimps


Why is it strange that identical MHC sequences have been found in distantly-related species

because host-parasite coevolution and Red Queen arms race predict a fast turnover of alleles


What pathlogies are associated with the MHC in humans

psoriasis, rheumatoid arthritis, insulin-dependent diabetes, autoimmune disorders


What can you conclude from the observation that 'MHC has a strong haplotype block structure'

Epistasis must be strong
Positive epistasis between immune genes that operate well together
Negative epistasis between bad mutations


What can you conclude from the observation that 'many disease mutations in the MHC

Faster mutation rate
More important genes that can cause disease
Less efficient selection against mutations


What could cause the less efficient selection against mutations in the MHC

There must be benefits from those mutations (antagonistic pleiotropy)
Mutations do not get often expressed (are rare and recessive, which mean they are ‘sheltered’ by a healthy wild type allele)


What is Muller's ratchet and what does it cause

the accumulation of mutations in asexuals. Mutations can become fixed because without recombination, purifying selection is inefficient.


What does genetic hitchhiking do to genetic variation

reduces genetic variation because the ‘fate’ of polymorphisms depends on selection on nearby genes.


What is epitasis

The masking of the phenotypic effect of alleles at one gene by alleles of another gene e.g albinism


What dos ABC stand for

Associated Balanced Complexes


What are ABCs

Recessive deleterious mutations accumulate in linkage blocks in haplotypes


In ABC theory, what type of selection is occuring on the MHC

Selection on the MHC is overdominance (on the immune genes) and purifying selection (on the linked mutations)


What do the current models say about the MHC

Current models (overdominance, frequency dependence) suggest that balancing selection acts on a single gene only


Key features of ABC evolution

Accumulation of recessive deleterious mutations in MHC haplotype blocks in a process similar to Muller’s ratchet
These mutations are potentially disease causing
They furthermore define the haplotype block structure by epistatic selection against recombinants
Selection against mutations is strong when haplotype block becomes common, which retains homogeneous allele frequency (and polymorphism) in the population
This selection can also work without selection on the actual immune gene


What was Aguilar et al 2004 theory about MHC diversity

They argued the MHC diversity is maintained by balancing selection, with MHC loci having a selection coefficient greater than 0.5


what did the original over dominance model state

overdominance model states that polymorphism is
maintained because heterozygous individuals are able to recognize a wider variety of parasites


What was originally thought to maintain the high level of polymorphsim in the MHC

Selection by parasites and sexual


what does negative frequency-dependent
selection model assumes that

The negative frequency-dependent
selection model assumes that, because selection favours parasites that can avoid recognition by the most common MHC variants, rare MHC alleles offer better parasite