Transcript Population Genetics

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Population Genetics
Learning Objectives
Define a population, a species, microevolution and population
genetics.
What is the population gene pool and what is a fixed allele?
Learn how to calculate the allelic frequencies in a population from
the genotypic frequencies (same as lab exercise).
What is the Hardy-Weinberg theorem and what is its formula or
equation?
How is the equation for the Hardy-Weinberg equilibrium used to
calculate allelic and genotypic frequencies?
Using the Hardy-Weinberg equation, calculate the frequency of
carriers of a particular disease within a population when given the
numbers of affected individuals with a recessive disease.
What are the population conditions under which the HardyWeinberg equilibrium applies, and how are they related to the
evolutionary process?
Define genetic drift and its two causes?
What are the four factors of microevolution?
How do new alleles originate?
A population is a localized group of
individuals that belong to the same
species.
A species is a group of populations whose
individuals have the potential to interbreed
and produce fertile offspring in nature.
Population genetics
Study of the extensive genetic variation
within populations
Recognizes the importance of quantitative
characters
The population’s gene pool consists of
all alleles at all gene loci in all
individuals of a population at any one
time
Microevolution:
• Defined as a change in the allele
frequencies in the gene pool of a
population from generation to generation
• Populations not individuals are the units
of evolution
- If all members of a population are homozygous
for the same allele, that allele is said to be fixed
Calculating the allelic frequencies from the genotypic frequencies
What is the allelic frequency (of R and r) in this population?
Do not confuse allelic frequency with
genotypic frequency Genotypic frequency:
RR= 320/500 = 0.64
Rr = 160/500= 0.32
rr = 20/500 = 0.04
What is the allelic frequency in a population of 500
flowers?
How many total alleles are there?
500 X 2 = 1000
Frequency of R allele in population
RR + Rr = 320 X 2 + 160= 640+160= 800
800/1000 = 0.8 =80%
Frequency of r allele = 1- 0.8 = 0.2 =20%
or
rr +Rr = 20 X 2+ 160= 200
200/1000 = 0.2
- The gene pool of a non-evolving population
remains constant over the generations
- The shuffling of alleles that accompanies
sexual reproduction does not alter the
genetic makeup of the population
- Meiosis and random fertilization do not
change the allele and genotype frequencies
between generations
The frequencies of alleles and genotypes in
a population’s gene pool will remain
constant over generations unless acted
upon by factors other than Mendelian
segregation and recombination of alleles
The Hardy-Weinberg theorem describes
the gene pool of a nonevolving population
Theorem assumes that fertilization is
completely random and all male-female
mating combinations are equally likely.
Because each gamete has only one allele for
flower color, we expect that a gamete drawn
from the gene pool at random has a 0.8
chance of bearing an R allele and a 0.2
chance of bearing an r allele.
Population geneticists use p to represent the
frequency of one allele and q to represent
the frequency of the other allele.
The combined frequencies must add to 100%;
therefore p + q = 1.
If p + q = 1, then p = 1 - q and q = 1 - p.
It is possible to calculate the genotypic
frequencies of RR, Rr, rr in next
generation based on allelic frequency of p
= 0.8 and q =0.2
Let’s watch the following video segment and
see how…
Using the allelic frequencies and rule of
multiplication, the probability for genotypic
frequencies is:
RR = (0.8 x 0.8) = 0.64
Rr or rR = (0.8 x 0.2) + (0.2 x 0.8) = 0.16 + 0.16
= 0.32
rr = (0.2 x 0.2) = 0.04
Given R = p and r = q
The genotype frequencies should add to 1:
p2 + 2pq + q2 = 1
0.64 + 0.32 + 0.04 = 1.
In the wildflower example, p is the frequency of red
alleles (R) and q of white alleles (r).
– The probability of generating an homozygous
dominant offspring is p2 (an application of the rule of
multiplication).
In this example, p = 0.8 and p2 = 0.64.
– The probability of generating an homozygous
recessive offspring is q2.
In this example, q = 0.2 and q2 = 0.04.
– The probability of generating heterozygous offspring
is 2pq.
In this example, 2(0.8 x 0.2) = 0.32.
This general formula is the Hardy-Weinberg
equation and it is used to calculate:
- frequencies of alleles in a gene pool if we
know the frequency of genotypes
or
- the frequency of genotypes if we know the
frequencies of alleles
Estimate the percentage of the human
population that carries the allele for the
inherited disease, phenylketonuria (PKU) - in
other words, what is the genotypic frequency
of heterozygote carriers?
First, we need some background info:
– About 1 in 10,000 babies born in the United States is
born with PKU, which results in stunted mental
development and other problems if left untreated.
– The disease is caused by a recessive allele.
http://www.ygyh.org/pku/whatisit.htm
- The frequency of homozygous recessive
individuals = q2 = 1 in 10,000 or 0.0001.
- The frequency of the recessive allele (q) is
the square root of 0.0001 = 0.01.
- The frequency of the dominant allele (p) is
p = 1 - q or 1 - 0.01 = 0.99.
Is this what we’re looking for? No, we are looking for the
percentage of carriers (a.k.a. heterozgotes):
The frequency of carriers (heterozygous
individuals) is 2pq = 2 (0.99 x 0.01) = 0.0198 or
about 2%.
• About 2% of the U.S. population carries the
PKU allele.
Populations at Hardy-Weinberg equilibrium must
satisfy five conditions.
(1) Very large population size. In small populations,
chance fluctuations in the gene pool, genetic drift, can
cause genotype frequencies to change over time.
(2) No migrations. Gene flow, the transfer of alleles due
to the movement of individuals or gametes into or out
of our target population can change the proportions of
alleles.
(3) No net mutations. If one allele can mutate into
another, the gene pool will be altered.
(4) Random mating. If individuals pick mates with
certain genotypes, then the mixing of gametes will not
be random and the Hardy-Weinberg equilibrium does
not occur.
(5) No natural selection. If there is differential survival
or mating success among genotypes, then the
frequencies of alleles in the next variation will deviate
from the frequencies predicted by the HardyWeinberg equation.
Evolution usually results when any of these five
conditions are not met - when a population
experiences deviations from the stability
predicted by the Hardy-Weinberg theory.
Genetic Drift
changes allelic frequencies in populations
2 causes of Genetic Drift:
1. The bottleneck effect
2. The founder effect Isolation event from a larger
poulation (e.g.
colonization)
Microevolution is the generation-to-generation
change in a population’s frequencies of alleles.
Caused by four factors:
1. genetic drift – due to sampling/ bottleneck and
founder effects
2. natural selection- accumulates and maintains
favorable genotypes in a population
3. gene flow- genetic exchange due to migration
of fertile individuals or gametes between
populations
4. Mutation- transmitted in gametes can
immediately change the gene pool of a
population
New alleles originate only by mutation
– rare and random.
– mutations in somatic cells are lost when the
individual dies.
– Only mutations in cell lines that produce
gametes can be passed along to offspring.
Diversity within a population
Humans have relatively little genetic
variation
- Gene diversity- average # of heterozygous
loci
about 14% in humans
- Nucleotide diversity- difference in
nucleotide sequences is only 0.1%.
Any two people have the same nucleotides at 999
out of every 1,000 nucleotide of their DNA.
Macro-evolution reflects the changes within
a species that take place over a long
period of time as a result of natural
selection