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Chapter
26. Population Genetics • •
Population
Geneticists want to understand the extent of genetic variation within a
population. –
The
why and how it occurs over many generations. •
Emerged
in the 1920’s and 30’s as a mathematical extension of the principles
of Mendel and Darwin. –
Formulae
are used to explain genotype occurrences in a population. •
Genes in
Populations. –
In
population genetics the focus is shifted away from the individual and
towards the population. – •
Therefore it is
defined as the totality of genes within a particular population. –
Therefore
a member of the population receives its genes from the pool and
contributes genes to the pool by reproducing. –
Population
geneticists study the genetic variation in the gene pool and how it
changes from one generation to the next. –
A
population is a group of interbreeding individuals of the same species
that share a gene pool. •
A large population
can be composed of smaller groups called subpopulations, local populations
or demes. •
Subpopulations are
often separated by moderate geographic barriers and are more likely to
breed with other members of the subpopulation than with members of the
population at large. •
Populations are
dynamic changing geographical location, size and genetic composition. • All these changes can effect the gene pool. •
Population
geneticists study how the gene pool changes in response to these
fluctuations. – •
Polymorphism
(many forms) in population genetics refers to the idea that traits show
many variations in a population. –
Historically
these are things that can be visualized with the naked eye. –
Polymorphisms
in coloration and markings have long been studied by population
geneticists. –
At
the level of DNA trait polymorphism is due to different alleles that
affect a trait. –
These
alleles will affect the traits of those who inherit them. –
This
is genetic variation. •
A gene is considered
monomorphic when it exists as only
one allele in 99% of the population. •
John Hubby and
Richard Lewontin (1966) were the first to study genetic variation with
molecular genetic techniques. –
Studied
variation in the amino acid sequence of enzymes in the fruit fly Drosophila
pseudoobscura. –
Found
that these slight amino acid differences would change the mobility of the
enzyme during gel electrophoreses. –
Two
enzymes with small differences in their gel mobility due to slight amino
acid changes are called allozymes (alleles of the same enzymes). –
Studied
18 different enzymes in five different populations and found that 30% had
two or more allozymes. –
Interpretation:
30% of the enzymes had alleles of the gene that lead to amino acid
differences in the enzyme (allozyme) the gene encoded for. »
There
methods actually underestimate the actual number since many amino acid
changes will not affect gel mobility of the enzyme. –
These
studies indicate that genetic variability is quite high. –
Around
the same time it was found that approximately 30% of human enzymes have
are polymorphic. –
Most
natural populations a substantial percentage of genes are polymorphic. –
Small
populations near extinction do not have much genetic variation. »
Ex:
African cheetah genetic variation is near 0. –
Since
30% of human genes are polymorphic and we have approximately 35,000 genes
that means 10,500 have polymorphisms (over the population). •
One example would be
ABO blood type, there are three alleles (IA, IB and
i). –
Population
geneticists are concerned with allele and genotype frequencies. •
In order to evaluate
the prevalence of genes in a population they use calculations. •
•
Allele frequency is
defined as: •
Genotype frequency
is defined as: •
These to frequencies
are related and distinct. –
Ex:
population of 100 pea plants with the following phenotypes and genotypes: »
64
tall plants with the genotype TT. »
32
tall plants with the genotype Tt. »
4
dwarf plants with the genotype tt. – –
For
the t allele there are 32 copies from the heterozygotes and 8 copies from
the 4 homzygous recessive plants. –
The
allele frequency for the t allele is: –
20%
of the alleles for this gene in the population are the t allele. –
If
we calculate the genotype frequency we get a different percentage. –
4%
of the population are dwarf plants. •
Allele or genotype
frequencies are always less than or equal to 1 (100%). •
•
For polymorphic
genes if you add up all the frequencies for all the alleles in a
population it should = 1.0. –
For
our example if you calculate the allele frequency for T it = 0.8 and 0.8 +
0.2 = 1.0. •
Hardy-Weinberg
Equilibrium
(H-WE). –
An
equation derived independently by Godfrey Hardy and Wilhelm Weinberg
(1908) that predicts the stability of allele and genotype frequencies from
one generation to the next, the Hardy-Weinberg equation. • This equation relates allele and genotype frequencies within a population. •
Called an
equilibrium because the allele and genotype frequencies will not change
over time (generations). •
The H-WE established
a framework to understand genetic stability. •
We know that in a
population genetics are not stable. –
Population
geneticists study the H-WE and the conditions that must be met for it to
be valid. –
Then
they study the reasons that natural populations violate the H-WE and lead
to changes in genotype and allele frequencies. –
The H-W
equation calculates genotype frequencies based on allele frequencies. –
EX: If we
are considering a gene with two alleles A and a where the variable p=A and
q=a then p + q = 1. •
If p = 0.8 then q
must = 0.2; total = 1. •
Can be said the
allele frequency of A = 80% then the allele frequency of a = 20%; total =
100%. •
The Hardy-Weinberg
equation states that: p2+2pq+q2=1. – Where p2 = – Where 2pq = – Where q2 = •
If p = 0.8 and q =
0.2 then: –
AA
= p2 = (0.8)2 = 0.64. –
Aa
= 2pq = 2(0.8)(0.2) = 0.32. –
aa
= q2 = (0.2)2 = 0.04. •
Therefore if the
allele frequency of A is 80% and a is 20% then the genotype frequencies
are: –
AA
= 64%. –
Aa
= 32%. –
aa
= 4%. •
You get the same
result if you use the product rule: –
Ex:
Frequency of producing an AA = 0.8 X 0.8 = 0.64 or 64%. •
The H-W equation
predicts and equilibrium in large populations if the following criteria
are met: –
1)
Population so large allele frequencies do not change due to random
sampling error. –
2)
Members of a population mate with one another without regard for
phenotypes or genotypes. –
3)
No migration between populations. –
4)
No survival or reproductive advantage for any of the genotypes (no natural
selection). – 5) • Provides for a quantitative relationship between allele and genotype frequencies. •
This is extremely
unlikely to occur in a natural population. •
However, extremely
large populations with little migration or natural selection may
approximate H-W equilibrium. •
The H-W equilibrium
gives a point for comparison when we encounter populations that do change
from generation to generation. –
Many
things may occur to upset the conditions needed for the H-WE. •
•
Factors that change
allele frequency include neutral and adaptive forces. –
Neutral
forces include random genetic drift and migration. »
Random
genetic drift:
Allele frequency changes due to random (chance) from one generation to the
next. »
Migration: Populations with different allele frequencies may have movement of
individuals from one population to the next may lead to a change in allele
frequency for those populations. –
Adaptive
forces include natural selection. »
Natural
selection:
Beneficial phenotypes resulting from certain genotypes leading to enhanced
survival and/or reproduction. »
Directional
(Survival of one phenotypic extreme) , Disruptive (two different classes
survive) and Stabilizing (survival of individuals with intermediate
phenotypes) selection can play a role. –
Random
mutation
can also provide genetic variation. »
May
be neutral advantages or detrimental either way this could lead to a
change in allele frequency.
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