BIOLOGY 1M03 Lecture Notes - Lecture 8: Foxp2, Heterozygote Advantage, Ke Family
29 Jun 2018
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Patterns of genetic variation/skin colour Evolutionary Psychology
Lactase persistence
Directional selection in populations with history of dairying may have driven the
frequency of lactase persistence up.
“Selective sweep” allows molecular evidence of this.
Detecting Selective Sweeps
Beneficial mutation favoured by natural selection appearing on front
Imagine this situation if there were no crossing over and recombination – then the
whole chromosome would spread through the population and genes on that
chromosome would hitch-hike with the preferred allele. But, there is crossing over,
so if beneficially chromosome is on blue, it could eventually appear on other. Nature
of grouping of genes can be broken up by crossing over, can’t expect for it always
stay together.
When a new favourable mutation initially spreads, it will be surrounded by a large
chunk of with the same sequence. This sequence is linked with the mutation and is
called a haplotype. A section of DNA that is clumped together so that loci that are
close together on a section of DNA – sections called a haplotype, this on one
chromosome may be different from haplotype from another chromosome, they
would be the same genes but different alleles, different patterns on those genes.
What we see here we have a mutation within this linked sequence, a haplotype,
over time this can be transferred by crossing over events.
The haplotype will remain intact until recombination breaks the linkage -- this takes
around 10,000 years depending on the size of the haplotype and generation time. If
you are talking about linkage of whole chromosome, quick but small section of
chromosome if genes are very close together, that break up by crossing over wont
occur right away, recombination will take a lot longer to break up a small section of
DNA, it will travel together for a longer period of time.
This figure: this is referred to as 16 different chromosomes, you can think of them as
a chunk of a chromosome. Very small, 10 genes, in this example at the beginning,
16 chromosomes have equal alleles. Each column repreents a different haplotype.
Kinds of chromosomes, kinds of chunks of chromosome. The grey, blue, red, all
different type of chromosomes or different haplotype. Haplotype refers to
organization of gens and patterning of them. You get a favourable mutation, you will
notice that this haplotype has a lot of these black alleles, these are even number,
this haplotype has this mutation, natural selection will favour that mutation along
with haplotype. Natural selection occurring, that haplotype, one wit mutation is
favoured. And in this next, more of those 16 carry the mutation and as well carry
those same pattern of genes. We could look at a population and recognize that
there is a pattern of genes that are on the same, and the argument with why you
would have this pattern that is the same but different when carrying this mutation is
selectively sweep happened. That haplotype is kept together and is increasing in
population, that haplotype, that particular series of genes have been favoured. It is
predominating in this pool of DNA, with recombination then the pattern, some
remain same but some get broken up. We don’t see a parituclar haplotype that has
find more resources at oneclass.com
find more resources at oneclass.com
become common in population, so we cant tell by this point that there has been a
selective sweet. The frequency of white and black alleles ahs changed, its not
because black are favoured, black happen to be this proffered haplotype so they
also increased, favoured by selection, changed the frequency of black. Idea of
selective sweep – idea of haploid that are selected together because of a beneficial
mutation.
With a small selective advantage, the frequency of the allele can sweep to
prominence within 7000 years (300 to 350 generations since dairying began)
Convergent adaptation of human lactase in Africa and Europe
These people have looked for molecular evidence in these two different locations
and have found that there is evidence of a
selective sweep over past 7000 years but they
are using single nucleotide polymorphism to
detect this and what they detected is that the two
populations that have had this selective sweep
originated on different haplotype background.
Natural selection over past 7000 years lead to
lactase persistence in tow different populations in
two different ways
Bloom and Sherman – Dairying barriers affect the distribution of lactose
malabsorption – lack of lactase persistence, you cant digest lactose well. And what
they were looking at rather than just trying ot look historically for dairy, they asked
what shaped where dairying popped up, does it explain these patterns of lactose
malabsorption. Opposite of lactase persistnace.
Frequency of “Lactose Malabsorption” in 39 countries
If you look at a bunch of different populations, this shows proportion of populations
that have lactose malabsorption and fraction of lactose malabsorption. In some
cases, all in population have lactose
malabsorption, 10% of population. IN some
cases, 90% of people have lactase
malaborption, 23% of populations. Spread
across map, some populations about 7%
that have very low fractions of the population
with lactose malabsorption.
Reflected in the world map where all of
these populations with low frequency of
malabsorption, can digest lactase well, occur
in Northern Europe, Northern Africa and
large dots in through Africa and in warm
countries where lactase malaborption is very
high.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Directional selection in populations with history of dairying may have driven the frequency of lactase persistence up. Beneficial mutation favoured by natural selection appearing on front. Imagine this situation if there were no crossing over and recombination then the whole chromosome would spread through the population and genes on that chromosome would hitch-hike with the preferred allele. But, there is crossing over, so if beneficially chromosome is on blue, it could eventually appear on other. Nature of grouping of genes can be broken up by crossing over, can"t expect for it always stay together. When a new favourable mutation initially spreads, it will be surrounded by a large chunk of with the same sequence. This sequence is linked with the mutation and is called a haplotype. What we see here we have a mutation within this linked sequence, a haplotype, over time this can be transferred by crossing over events.
