Lecture 17 Flashcards
Human Genomes
-There is no single
‘human genome’
* People’s genomes
differ on average by
millions of genetic
differences
* Study of population
genetic variation of
genomes helps us
understand past,
present, and future
evolutionary potential
What factors influence patterns of
genetic diversity and evolution?
Mutation
Recombination
Random Genetic Drift
Natural Selection
-negative (purifying) selection
-Positice (directional) selection (adaptation)
-selection favouring diversity
-Migration(gene flow)
Heterozygosity (H)
Fraction of individuals that are heterozygous
averaged across gene loci
* Recall from Mendelian genetics:
* Heterozygote individuals have both alleles
Polymorphism (P)
Proportion of gene loci that have 2 or more alleles
in the population
* A locus can be polymorphic without being
heterozygous
What Maintains Genetic Variation?
- Mutation-selection balance
* Less fit types reintroduced by mutation
* Followed by selection acting to remove them - Selection maintaining variation
* Heterozygote advantage
* Frequency-dependent selection
* Fitness varies in space or time
* Umbrella term “balancing selection
Classical School
-Wild type is normal
genotype
* Low polymorphism
* Low heterozygosity
* Negative selection
* Morgan, Muller
Balance
- Selection favours diversity
- High polymorphism
- High heterozygosity
- Heterozygote advantage
- Dobzhansky, Ford
Studying Genetic Diversity:
Genetic “markers” pre-1966
- Morphological
– e.g. snail colour polymorphism - Cytological
– e.g. chromosome inversions
Early Quantitative Genetic Evidence
for the Existence of Genetic Variation
- Rather than focus on Mendelian discrete
traits, focus on continuous polygenic traits - Selection experiments on different groups
of organisms - Involves controlled breeding of individuals
with particular traits for many generations
— Artificial Selection
Evolutionary responses of continuous traits
- Demonstrates existence of heritable variation in fitness-
related phenotypes - Due to many underlying genes
Results of artificial selection experiments
on quantitative traits
- Selection responses demonstrate that
abundant genetic variation exists for
polygenic quantitative traits - BUT often no information on P & H as
key population genetic parameters - Also: comparative studies difficult
as traits studied often are group specific - Still no solution to the question:
What maintains genetic variation?
Richard Lewontin
and the electrophoresis revolution
- Allozyme* gel electrophoresis provided a way to ask:
– “What proportion of genes are variable (P & H)?”
– Answering it addresses a fundamental dispute between
the classical and balance schools - Initiated large scale surveys of electrophoretic variation in
enzymes & proteins in diverse organisms
R.C. Lewontin - Allozymes = different allelic forms of the same protein
Advantages of studies of
enzyme polymorphism
Many loci can be examined
* Can be used in nearly any organism
* Loci co-dominant, heterozygotes can be identified
* Variation examined close to DNA level
* Provides genetic marker loci for other studies
- Mutation-selection balance
Less fit types maintained by repeated mutational input
- Selection maintaining variation
Heterozygote advantage
* Frequency-dependent selection
* Fitness varies in space or time
