Exam 5: Population Genetics Flashcards Preview

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Flashcards in Exam 5: Population Genetics Deck (30):


genome-wide SNP analysis can be used to calculate accurate parameters of genetic ancestry (membership to certain population)
data reflects human migration and admixture - more accurate than physical appearance & race


epidemiological studies

measure contributions of certain genetic polymorphisms (which underlie susceptibility) to disease
uses genome-wide association studies (GWAS) to compare prevalence of certain SNPs


SNPs & diseases

SNPs not necessarily disease-causing mutations
often lie outside coding regions - show strong linkage with mutations contributing to disease


founder effect

new territory populated by small founder population
eventually everyone is related (inbreeding inevitable) and homozygosity increases
Lead to amplification of rare alleles - genotype of founder population amplified proportionally as population grows


genetic drift

effect of statistical variation (in small populations, normal statistical variation can have large impact)
can lead to disappearance or multiplication of rare alleles, without selection for or against it - chance


genome-wide association studies (GWAS)

millions of SNPs analyzed to compare prevalence of certain SNPs in patients to their prevalence in control cases
Odds ratio calculated for each SNP that describes strength of association of SNP with disease


polymorphic allele

allelic variants, present in more than 1% of population


highly polymorphic genes

determine individual characteristics


Nonpolymorphic genes

code for proteins with essential cellular functions, mutations in these are incompatible with life


population bottleneck

large part of population wiped out by catastrophic event & has to recover from small founder population of survivors
population bottleneck leads to amplification of rare alleles in founder's genotype that provide a selective advantage


Ever human can be expected to carry

3 deleterious recessive mutations


How many SNPs are there between unrelated humans?

6 million (about 0.1% of genome)


How much of world's polymorphisms can be found within any given population?



How any polymorphisms set apart races?



Hardy-Weinberg equations

calculate allele frequencies in gene pools and genotype frequencies in ideal populations - allele frequencies do not change over time in ideal populations


Ideal populations have following characteristics:

large population size, equal fitness of offspring, random mating, no influx of new alleles by migration or mutations


Probability of being heterozygous (Aa) using Hardy-Weinberg equation (carrier frequency)

or 2*sq.root of prevalence


Probability for being homozygous for an allele using Hardy-Weinber equation

f(a)^2 or f(A)^2


allele frequency of a gene pool

f(A) = 1-f(a)
if recessive disease is 1/10,000, f(a)=1/100
f(A) = 0.99


X-linked recessive allele frequency

fraction of males who are affected



will reduce number of detrimental mutant alleles in population (as selection favors against mutant allele)
frequency of mutant alleles will stabilize at low level (negative selection equals occurrence of new mutations)


positive selection of heterozygotes (heterozygote advantage)

responsible for high mutant allele frequencies
Some heterozygote alleles are favored - disease preventing


heterozygote advantage in CFTR

In homozygous state causes Cystic fibrosis
Heterozygote protects against Typhoid fever
have higher resistance to S. typhimurium bacterium which binds to nonmutated CFTR protein to enter human - can not enter bloodstream as well
people heterozygote for CFTR lived during Typhoid epidemics and passed on the mutant allele
Populations with repeated typhoid exposure have high CFTR mutant allele frequencies and a high incidence of cystic fibrosis


heterozygote advantage in Hemoglobin B

homozygous state = sickle cell anemia or B-thalassemia
heterozygous = protects against malaria


heterozygote advantage in HFE

homozygous state = Hemochromatosis
heterozygous protects against plague


Assortative mating

selection of partners based on specific genetic trait
disturbs equilibrium of alleles


consanguineous matings

mating of genetically similar individuals increases degree of homozygosity in population
does not change allele frequencies in gene pool - only change is an increase in homozygote frequency above expected level = increase in recessive diseases


Ellis van Creveld syndrome

EVC gene recessive mutation
affects genetically isolated populations - propagation of rare allele in Old Order Amish of Lancaster County (carrier frequency 12.3% vs. general population 0.8%)
skeletal dysplasia, shortening of forearms and lower legs, postaxial polydactyly and heard defects


Diseases affecting genetically isolated populations

Ellis van Creveld Syndrome - Old Order Amish of Lancaster County
Tay-Sachs Disease - Ashkenazi Jews
Tyrosinemia - French Canadians (Quebec)


linkage disequilibrium

original combination of alleles is found more frequently than predicted
find alleles that contribute to certain disease - find markers that are more often associated with disease trait than predicted by allele frequency alone
Genetically young populations (LD observed when disease introduced to population recently) & small populations (only few genes with little heterogeneity contributing to disease) most useful in these studies