Download Population Genetics (Learning Objectives)

Population Genetics (Learning Objectives)
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Define the terms population, species, allelic and genotypic frequencies,
gene pool, and fixed allele, genetic drift, bottle-neck effect, founder
effect.
Explain the difference between microevolution and macroevolution.
Review how genotypic and allelic frequencies are calculated.
Given the appropriate information about a population you should be
able to calculate the genotypic and allelic frequencies of homozygous
dominant, recessive, or heterozygous individuals (following the example
discussed in class).
Visit this website to learn the factors that lead to changes in genotypic
and allelic frequencies between generations:
http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15-6OSU.swf
What is the Hardy-Weinberg Equilibrium and what are its conditions.
What are the factors that lead to microevolution?
What is the source of new alleles within any population?
Definitions
A population is a localized group of
interbreeding individuals in a given
geographic area
A species is a group of populations who
interbreed and produce fertile offspring
Definitions
• Gene pool = The collection of all alleles in the
members of the population
• Population genetics = The study of the genetics
of a population and how the alleles vary with time
• Gene Flow = Movement of alleles between
populations when people migrate and mate
Changes allelic frequencies in populations
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
Allele Frequencies
Allele frequency =
# of particular allele
Total # of alleles in
the population
Count both chromosomes of each individual
Allele frequencies affect the frequencies of the three
genotypes
Phenotype Frequencies
Frequency of a trait varies in different
populations. Example: PKU an autosomal recessive
trait
Table 14.1
Evolution
Microevolution small changes due to changing
allelic frequencies within a population from
generation to generation
Macroevolution large changes in allelic
frequencies over 100’s and 1000’s of
generations leading to the formation of new
species
Calculating the allelic frequencies from the genotypic frequencies
What is the allelic frequency (of R and r) in this population?
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
- Meiosis and random fertilization do not
change the allele and genotype frequencies
between generations
- The shuffling of alleles that accompanies
sexual reproduction does not alter the
genetic makeup of the population
The Hardy-Weinberg theorem describes
the gene pool of a non-evolving population
Hardy Weinberg animation
http://zoology.okstate.edu/zoo_lrc/biol1114/t
utorials/Flash/life4e_15-6-OSU.swf
practice questions
http://science.nhmccd.edu/biol/hwe.html
Hardy-Weinberg Equation
p = allele frequency of one allele
q = allele frequency of a second allele
All of the allele frequencies
together equals 1
p+q=1
p2 + 2pq + q2 = 1
p2 and q2
2pq
All of the genotype frequencies
together equals 1
Frequencies for each homozygote
Frequency for heterozygotes
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 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
The bottleneck effect
The founder effect
Microevolution
Caused by four factors:
1. Non-Random mating
2. Genetic drift – due to sampling/ bottleneck &
founder effects, geographic & cultural
separation
3. Migration- of fertile individuals
4. Mutation- in germline cells transmitted in
gamete
5. Natural selection- accumulates and maintains
favorable genotypes in a population
Source of the Hardy-Weinberg
Equation
Figure 14.3
Figure 14.3
Solving a Problem
Figure 14.4
Solving a Problem
Figure 14.4
Calculating the Carrier Frequency
of an Autosomal Recessive
Table 14.3
Calculating the Carrier Frequency
of an Autosomal Recessive
Figure 14.5
Figure 14.3
Calculating the Carrier Frequency
of an Autosomal Recessive
What is the probability that two unrelated
Caucasians will have an affected child?
Probability that both are carriers =
1/23 x 1/23 = 1/529
Probability that their child has CF = 1/4
Therefore, probability = 1/529 x 1/4 =
1/2,116
Figure 14.3
Calculation of % PKU carriers from screening
About 1 in 10,000 babies in US are born with PKU
- 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.
The frequency of carriers (heterozygous individuals) is
2pq = 2 x 0.99 x 0.01 = 0.0198 or about 2%.
About 2% of the U.S. population carries the PKU allele.
Question
What is the chance or probability that two
unrelated US individuals will have an
affected child?