cycle 6 Flashcards
population genetics
involve many individuals, no controlled crosses, don’t show Mendelian ratios
no selection in a population
frequency of all genotypes stay constant, allele frequencies also stay the same
selection against the dominant phenotype
frequency of genotypes that include dominant allele go to 0, recessive goes to 100
what happens when allele frequencies change?
evolution occurs
microevolution
change in allele frequencies that occurs from one generation to the next within a population
macroevolution
speciation, the evolution of a new species
hardy-weinberg principle
if a population experiences no selection, mutation, immigration/emigration, genetic drift, and is randomly mating then it is in HWE
HWE allele vs genotype frequencies
observed allele frequencies:
-dominant (p), recessive (q)
expected genotype frequencies:
-homozygous dominant (p^2), heterozygous (2pq), homozygous recessive (q^2)
P + Q = 1
how to tell if a population is in HWE by frequencies?
a population is in HWE if expected genotype frequencies match observed genotype frequencies
why determine if a population is in HWE?
used to predict genotype frequencies, helps to indicate if a population might be evolving or if individuals are not mating randomly
absolute fitness (W)
a measurable quantity, sometimes a proxy for # of offspring
relative fitness (w)
absolute fitness divided by the absolute fitness of the most successful genotype (w=W/Wmax)
-most successful genotype has w=1
selection against the dominant phenotype relative fitness
WBB = WBR < WRR
selection against the recessive phenotype relative fitness
WBB = WBR > WRR
heterozygote advantage relative fitness
WBB < WBR > WRR
heterozygote disadvantage relative fitness
WWW > WWS < WSS
allele frequencies in selection against the dominant phenotype
frequency of dominant allele goes to 0, frequency of recessive allele goes to 100% (fixation)
allele frequencies in selection against the recessive phenotype
frequency of the recessive allele never goes to 0 (because of heterozygotes)
- because selection acts on phenotypes not genotypes
- impossible to eliminate recessive alleles
heterozygote advantage and malaria
heterozygous for disease, mild anaemia but also resistance to malaria
heterozygote advantage
maintains genetic variation, balancing selection, rare alleles increase in frequency and common alleles decrease (for 2 allele system, each allele will reach 0.5 frequency)
heterozygote disadvantage
results in less genetic variation, common alleles have the advantage and rare alleles decrease in frequency (can disappear)
heterozygote advantage and disadvantage
occurs if heterozygotes have a different phenotype than homozygotes (incomplete dominance/codominance)
directional selection
one of the extremes are favoured
stabilizing selection
mean is favoured
disruptive selection
both extremes are favoured
balancing selection
each phenotype is favoured
gene flow
any movement of individuals (or genetic material) from one population to another (alleles added/lost from populations)
genetic drift
by chance (not everyone will reproduce), change in allele frequencies due to the effect of chance, anyone can mate with anyone and pass one any one of their alleles; e.g. survival during a severe storm- randomly lost alleles, but rare alleles are more likely to be completely lost
bottleneck and founder effect
change in allele frequencies due to random sampling of a very small number of individuals; catastrophic reduction in population (bottleneck) or small number of founders (founder)
-leads to reduced genetic variation
consequences of genetic drift
reduces genetic variation, beneficial mutations are rare and have a higher probability of being lost, can lead to an increased frequency of often recessive deleterious alleles (especially in bottleneck/founder)
consequences of bottleneck and founder effects
small populations lead to inbreeding (increased homozygosity), increase in phenotypes of recessive deleterious traits
non-random mating
individuals select mates based on phenotype, does not cause changes in allele frequencies (assortative mating, inbreeding, disassortative mating, inbreeding avoidance)
-leads to a departure from HWE, but not evolution because allele frequencies stay the same
assortative mating
leads to an increased frequency of homozygotes- assortative mating among relatives leads to inbreeding (homozygosity across the genome)
consequences of inbreeding
increase in homozygosity exposes phenotype of deleterious recessive alleles, increases the prevalence of these harmful phenotypes (inbreeding depression- low lifespan)
disassortative mating
leads to increased heterozygosity- mating between unrelated individuals is inbreeding avoidance
-can lead to heterozygote advantage in organisms
