Population genetics Flashcards
population
clusters of people sharing common gene pool
populations characteristics
age structure
geography
birth and death rates
allele frequencies
population genetics
study of genetic variation and how gene/ geneotypes are maintained/ change in populations, not indicidual matings
population diversity
populations are more diverse than individuals
all alleles in a population are the gene pool
gene pool
set of genetic info carried by the members of a sexually reproducing population
allele frequency
frequency with which alternate forms of a particular gene are in the population
-freq of alleles in a population can change from generation to generation
chagne in alele frequency= change in phenotype frequency
-change in the gene pool= evolution in population
human genetic diversity
polymorphisms–> rep DNA seq variants that have a freq in the population of 1%<
how can allele freq be meaured in populations
- sometimes allele frquencies can be meausred directly
- otherwise HW law has to be used to est allele freq with in populations
codominant allele frquency i
measured diresctly
both allleles are expressed
measured by counting the phenotype
recessive alle frequency
cant be measured directly
A/a= masked can count homo recessive
Hardy-Weinburg Law (equilibrium)
Relationship btw allele freq and genotype freq on a population
Allele and genotype freq is constant from generation to generation … Population meets certain assumptions
Resistance to human immunodeficiency virus
CCRS gene- receptor HIV uses to enter cell
Change in CCRS mutant recessive allele= HIV CANT ENTER RESSISTAN TO HIV
Homozygotes= resistant to HIV infection= lack of receptors
Assumptions HW law
1) population= big= no errors in measuring allele crews
2) all genotypes= equally reproduced
3) mating is random
4) no migration in/out of population
5) no new mutations
6) no motifs btw different generations
7) all matings produce same # offspring who are equally fertile
Equations
p+q=1
p=A
q=a
p^2 + 2pq + q^2 = 1
1=100% genotypes in new generation
p^2= AA q^2= aa
pq= Aa
Genetic equilibrium
Allele freq for a certain gene stays constant from generation to gen
Equilibrium in population explains why dominant alleles don’t replace recessive alleles
HW Law Human Genetics
- Establish freq auto dom and recessive alleles in population
- detect when allele frequency shifts in a population
- measure frequency of heterozygous carriers of deleterious recessive alleles in a population
Calculate frequency of autosomal dominant and AutoSomal recessive alleles
- Count frequency of individuals with recessive phenotype which is also homozygous recessive for aa
-frequency genotype aa= q^2
Solve for a
Freq of dominant allele A=p=1-q
Calculate frequency of alleles for X-linked traits
Female=2/3 of alleles
Males=1/3 of alleles
The number of males with the mutant phenotype =to the allele frequency for the recessive trait
Freq of x-linked train in males=q
Freq of trait in females is q^2
Frequency of heterozygotes
Disease causing alleles carried by heterozygotes
Frequency of heterozygous carrier= used to calculate the risk of having an affected child
Count homozygous (q^2) then calculate q
Calculate dominant allele p(p=1-q)
Calculate heterozygous=2pq
Factors that disturb Hardy Weinberg
1) nonrandom matings in humans
-associative mating
Consanguinity and inbreeding
2) founder effects
- population small number of individuals. (Founder) or drastic decrease I. Population (bottleneck)
3) genetic drift
- random flux of allele frequency from generation to generation (happens in small isolated populations)
Founder is a type of genetic drift
4) migration/gene flow
So diffusion of genrs across a barrier= merger of different gene population into larger populations results in a change in allele frequency
5) selection
Increase reproductive success of fitter genotypes
Natural selection
Acts on genetic diversity in populations and major driving force of evolution
Fitness
Better adapted individuals result in increased chance of more offspring
Heterozygote advantage
Increase freq of genetic disorders and some populations= do too selection= increased fitness
Natural selection affects frequency of genetic disorders
Rare lethal or deleterious recessive alleles survive because many carried in heterozygous conditions
Factors that affect distribution of alleles in human population
Migration
Founder effects
Mutations
Selection
Lethal alleles
Duchene muscular dystrophy DMD die before reproducing
Mutation rate increases result in lots of DMD alleles
Frequency of DMD alleles in a population is often the balance between alleles introduced by mutation and that was removed by deaths
Mutation generates
All new alleles but drift migration and selection deter the frequency of alleles in population
Evidence for genetic contribution
Clustering of affected individuals and families
Risk of relatives = based on relatedness to affected individual
Shared phenotype can be due to a common environment
Multifactorial inheritance
Single gene disease = predictable
Precise genotypes are unknown and environmental effects are variable
Multifactorial inheritance the risk to relatives
Empiric risks: but you observed from data from families with an infected individual
ER specific for each multifactorial disease can change from population size /geographic area to another
ER for MF traits
ER not calculated but is an observation population stat
ER is used to predict reoccurrence of the MF trait in a family
ER is affected by a number of factors
MFI
Many factors involved in causing birth defects genetic and environmental factors combined genes from environmental factors and both parents
ER birth defects
Cleft palate Deafness Club foot Congenital heart Neural tube defect
ER common diseases
Asthma Diabetes type one Diabetes type two I blood-pressure Rheumatoid arthritis Alzheimer's disease late onset above 60 Psoriasis
Heritability of disorder
Proportion of liability given to genetic factors versus environmental factors
H2=100% traits=fully genetic
H2=0% traits= fully environmental
Heritability of a traits equation
# of relatives that share trait ------------------------------------- # expected to share if trait 100% genetic
Heritability (H2)= variation (DZ-MZ)
———————
Variation DZ
DZ NOT EQUAL TO DZ= genetic and environment= MF
MZ=MZ THEREFORE GENETIC
MZ (100%)= 2xDZ(50%) MZ NOT = to MZ ENVIRONMENTAL DIFFERENCES
DZ=MZ ENVIRONMENT = if same environment any different= genetic factors
Problems with heritability
Families share genetics and environment
Separate genes from the environment
Adopted children
Compare birthparents to adopted parents
Sharing the environment only not genes
Twins
Compare monozygotic versus dizygotic twins
Difference in Twins = difference between amount of Genes shared environment is constant
Twin studies
Monozygotic twins have the same genotype and equal environment
Dizygotic twins have a different Genotype and equal environment
Concordance
If one twin is affected how often is the other affected?
Concordance rates
-compare percent monozygotic twins concordant for traits versus percent of DZ TWINS concordant
If monozygotic twins are more concordant than differences are due to genetics
Autism= 90% MZ VS 2% DZ DEPRESSION = 46% MZ VS 9% DZ
Genetics
MZ 80 DZ 16 disease is?
Genetic AR
MZ 30 DZ 4 disease is?
Mutifactorial
MZ 95 DZ 48 disease is?
Genetic AD
MZ 7 DZ 7 disease is?
Environmental
MZ 100 DZ 12.5 disease is?
Genetic xL Recessive males affected
Sex ratio
Birth defects/ MF traits= difference in sex ratio
-risk to a relative increase if a proband is of a rarely where affected sex
Quantitative traits
QT Loci: chromosomal regions associated with a complex trait present in individuals who have been affected
Q TL is right then one of the genes in this region should be directly involved in causing the trait
Q TL mapping
1) complex treat that show strong genetic contribution
2) phenotype large group of individuals That have the same trait
3) genotype everyone
Complexity of treat
More complex the trait harder it will be to identify QTL Or genes involved Need an increased number of individuals to have power to identify contributing Loci
Heritability
Measure of the relative contribution of nature versus nurture
Obesity
QL= obese overweight and normal
Quantitative= weight in pounds
MF= controlled by the environment
Controlled by genes protein that has an effect on appetite
