Population Genetics Flashcards
polymorphic human DNA sequence
for any gene there are many different specific versions of that gene’s sequence in the population
allele
each different version o the gene’s sequence
isoform
version of the protein that has a significantly higher or lower level of activity than the isoform made by most other people
genotype
combination of gene alleles you posses for a given gene
heterozygous genotype
you have different versions of the gene’s sequence (2 different alleles) in your 2 copies of the gene
homozygous genotype
you have same version of gene’s sequence (2 copies of the same allele) in your two copies of the gene
phenotype
your observable traits (physical traits, personality, biochemical parameters, susceptibility to specific diseases, response to specific drugs)
population
group of individuals that reproduce sexually and interbreed within that population
the population’s gene pool
all the different gene alleles that are present in the population
allele frequency
of alleles of the type
formula for X-linked allele in female-only population
f(A) = [f(AA) x 1] + [f(Aa)/2]
genotype frequency
of people w the genotype in question/total number of people (which is the same as the total # of genotypes)
X linked genes
a woman has 2 alleles per gene and a man has one (hemizygous) but both have only one genotype
Y linked genes
only males have a genotype which contains only one allele per gene (hemizygous)
changing allele frequencies and genotype frequencies
unrelated outsiders migrating into population, nonrandom mating w/i a population, gene mutations, specific gene alleles being favored by chance, natural selection, groups being forced to flee their homes by war or natural disasters
Hardy-Weinberg Equilibrium
(1) under certain circumstances frequency of all gene alleles w/i population will stabilize (2) when something changes the frequency of distribution of alleles in a population after one generation of random mating you can predict frequency of homozygous and heterozygous genotypes in population if you know frequencies of the specific alleles in questions (3) p^2 + 2pq + q^2 = 1
p
frequency of A allele
q
frequency of a allele
frequency of AA
p^2
frequency of Aa
2pq
frequency of aa
q^2
HW for 3 alleles
p^2 + q^2 + r^2 + 2pq + 2pr +2qr = 1
HW assumptions
population is large, population mates randomly w/i itself (no artificial selection), no migration into population by outsiders, no genetic mutations, no gene alleles get favored by change, no advantage/disadvantage in terms of genetic fitness in having one allele/genotype or another of a given gene
gene w frequencies not in HWE
many mean there is something interesting about that gene/allele/genotype; ex: one allele/genotype increases or decreases a person’s risk for disease or otherwise increases/decreases genetic fitness in that environment
evolution & HWE
population can’t evolve if in HWE bc its allele frequencies/genotype frequencies will never change; sexual reproduction maintains genetic diversity but can’t power evolution bc it doesn’t cause allele/genotype frequencies to change: it take mutation, migrations, selection or chance to drive evolution
1 - q =
p
degrees of freedom
of possible different genotypes - # of different alleles for that gene; use Chi square test to test whether a population is in HWE or not (gene w 2 alleles df = 1)
Chi Square Test Formula
(sigma: (O-E)^2)/expected
genetic drift
possibility that gametes that made next generation just happened to have disproportionate # the possess one particular allele of a gene; smaller pop = genetic drift is more likely (sampling error is greatest when population is small)
genetic bottleneck
small sample of population gets isolated (or survives a war) and makes up the original gene pool for the subsequent population; causes genetic drift; allele frequencies in this group will be determined by who survived the war there was no natural selection involved
founder effect
caused by genetic bottleneck; original gene pool of the population was very limited in its diversity
effects of genetic drift
inc frequency of some alleles and dec that of others; reduces genetic variability w/i population some alleles will become fixed (present at 100% freq); causes diff population to become more different from each other over time
natural selection
phenotypic traits that allow ind to thrive relative to his/her peers in that given environment
genetic fitness
ability to reproduce relative to contemporaries; symbolized as W ranges from 0 to 1.0
overdominance
heterozygotes have greater fitness than homozygotes; maintains the freq of both alleles
underdominance
heterozygotes have lower fitness than homozygotes; directional selection occurs, where one allele’s freq increases more than the other’s
selection coefficient (s)
reflects how severely a selection method works against a given genotype; s = 1 - W
formula for effect of natural selection on genotypes in next generation
(p^2W11)/wbarr
X linked gene
HW only applies for females and not males
direction selection
most favored allele will eventually get fixed at freq of 1.0; if heterozygous genotype has lowest fitness
recessive disease allleles
removed very slowly from population
negative eugenics
actively reducing freq of “bad” alleles (ex: sterilized handicapped people)
postive eugenics
actively encouraging propagation of “good” alleles (ex: selective mating, sperm and egg banks)
selective mating
you select your mates on the basis of some characteristics you think are important
negative eugenics & ethical standards
sterilize unaffected parents w 3/4 chance of unaffected child, unaffected relatives
positive assortative mating
mating w people who have characteristics similar to yours; nonrandom mating
negative assortative mating
mating w people who have characteristics different from yours; nonrandom mating
assortative mating
choosing people based on their phenotypes; only affects allele frequencies of genes that influence those traits and genes that are linked to those genes
inbreeding
positive assortative mating for relatedness; affects all genes’ alleles; increases proportion of homozygotes; - increase in freq of genetic disorders bc recessive mutations come out more
inbreeding coefficient (F)
proportion of ind’s homozygous genotypes in which 2 alleles are derived from common ancestor; range from 0 to 1.0; reduced by 2Fpq increases by Fpq
gene mutations
eventually rate of forward and reverse mutations equalizes; u = forward rate v = reverse rate equi = q = u/(u+v)
effects of migration
freq changes by m(q1-q2)
