Transcript Population Genetics

Population Genetics
“The study of genetic variation and its causes in
population”
Suggestion to do well in this class
# Attend every lab session.
# Give your undivided attention.
# Ask the TA to repeat himself.
# Review the power-point presentations before the quiz.
# Hand your assignment in due time.
# Do your best with the lab. reports
Announcements
• Cedo field trip.
• preceptor program
• Home assignment include game 1, 2, and questions 1, 3 and 4
on page 12
•There will be a quiz (8 points) on Thursday, February 3th.
The questions will cover materials from the population
genetics lab and the classification and phylogeny lab.
Population Genetics
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Evolution
Mechanisms of evolution
How to prove it
How to measure evolution
Evolution
• Population is a group of individuals that
coexist in certain place at the same time
and capable of interbreeding with one
another
• Evolution is change over time in the traits
of a population
• Phenotype is the observable properties of
an organism.
• Gene is piece of DNA that codes for a
protein
• Alleles are different forms of a gene
Evolution
• Genotype is the genetic composition the governs
a trait (AA, Aa, aa)
• Evolution is change of the allele frequencies over time.
• Dominant allele, determine the phenotype of
heterozygous individuals.
•Recessive allele, shows its phenotype only in
homozygous individuals
Forces of evolution
• Natural selection
advantage in survival and reproduction.
• Mutations.
random changes in nucleotide sequence.
• Migration (gene flow).
new individuals are introduce to the population.
• Non-random mating.
possession or absence of certain phenotype.
• Genetic drift.
random changes (allele fixation in small
population).
Natural selection
Genetic drift
Genetic drift
• Its effect is clear in small populations
• These populations lose genetic variability rapidly.
• alleles drift to fixation.
• http://darwin.eeb.uconn.edu/simulations/drift.html.
Gene flow
Applied aspects of population
genetics
Cystic fibrosis
Cystic Fibrosis (CF) is the most common AR (autosomal recessive) disorder found in Caucasians
with an estimated incidence of 1 in 2500 in newborns of European ancestry.
About 1 in 25 persons of European ancestry is a carrier.
CF especially affects the respiratory and digestive systems. The CF gene was identified in 1989.
The average life expectancy of a person with CF is now at 30 years, due to advances in medical care.
Evolution
How to prove that evolution
occur
• Hardy-weinberg theory (equilibrium)
A population that is not changing genetically from
generation to generation.(null hypothesis)
• Essential assumptions
1-Mating is random
2-Population size is very large (no effect for genetic
drift)
3-Mutations can be ignored
4-Natural selection has no effect
5-No migration
Hardy-weinberg theory
(equilibrium)
A
A
AA
sperm
a
Aa
Punnett square
eggs
a
Aa
aa
Frequency of event= actual number of occurrence
total number of events
Hardy-weinberg theory
(equilibrium)
 Allele frequency = number of occurrence of that allele
total number of alleles in the population
 Genotype frequency= number of occurrence of that genotype
total number of the population
or product of its alleles frequency= p * q
Note: frequency is just another way to express
(absolute)numbers. It is just the decimal fraction
 p + q =1
 p2 + 2pq + q2 = 1
How to measure evolution
 Calculate allele and genotype frequencies in each generation
and compare it to the previous one.
 are differences in genotype frequency from generation
to generation evident? Are we sure that these differences
have not happened due chance alone?
Significance of the difference in frequency
chi-square analysis ( X2 ) P.200
X2 =  (O # – E# )2
E#
 X2 = 0
it is hardy-weinberg equilibrium ( H0 or null hypothesis)
The higher the value of X2, than that obtained by chance,
the more likely to be significant
How to measure evolution
 Degree of freedom : number of categories that are free to
vary indepenantly .
df = total number of categories - 1
Questions
1a- population in game 2
1b- for game 2, we have high confidence because the probability of
sampling error were less than 5% . For game 1, vice versa..
1c- population in game 1
1d- population in game 2 because evolutionary force (natural selection)
was acting in the population.
3a- by natural selection
3b-heterozygous individuals
4- aa= 0.7x 0.7=0.49
AA=0.3x0.3=0.09
Aa=0.3x0.7=0.21
= 0.21x2=0.42