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Day 9
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Lecture 9:Mutation and selection and genetic load.
In most cases selection leads to loss of variation, except:
- Frequency dependent selection
- Heterozygote advantage
So how can variation of alleles that affects fitness be maintained
- Migration
- Mutation
Some history:
- Jean-Baptiste de Lamarck (1744-1829): mutations are acquired. Environment influences
genetic constitution: experience is heritable
- Francis Galton (1822-1912): a quantitative model of inheritance and acquisition of new
mutations.
- Charles Darwin (1809-1882): blending inheritance. Traits were intermediate between the
parent traits.
o A problem with this is that variation disappears
- After mendel we know better
o All genetic material of an individual is present in one..
So we know that variation is discrete, but were does mutation come from. Mutation
Mutation:
- Mutation is the ultimate source of variation. Mutation theory was developed by Hugo de
Vries
- Evening …….
Luria and Delbrcuk (1943) demonstrated that mutations are random.
In animals, mutations in germline count for evolution, and somatic mutations do not.
Mutation rates are low, but vary between organisms.
,Mutation-rate in human germline:
- Recently, mutation rate has been estimated directly by comparing offspring with the two
parents
o 1.2*10-8
o …..
Mutation rates for a gene:
- Typical nuclear gne has 500 codons, so 1500 nucleotides. About 70% of nucleotide changes
leads to change in amino acid per gene mutation rate: 0.70 * 1500 * 1*10 -5 per generation
- Most mutations are (slightly deleterious)
- The mutation rate is not the same as substitution rate. This is the rate at which mutations
accumulate (go to fixation) in organisms
Mutation selection base:
- Why are deleterious alleles not completely removed by natural selection?
o Recurrent mutation
-
- Deleterious allele rare, so reverse mutation can be neglected.
- Selection pressure on mutant:
o Recessive: mutant is hidden in
, -
Mutation-selection balance for non-recessive mutations:
Genotype AA Aa aa
Frequency P2 2pq q2
Fitness 1 1-hs 1-s
- Δqmu =
-
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