Population Genetics 2/3 Flashcards
diploid (2)
- when an organism has two copies of each gene
- the vast majority of multi-cellular plants and animals are diploid during most of their life cycle
what is an important characteristic of haploid models that differ from diploid models (2)
- haploid models “breed true” whether they produce sexually or asexually (A-bearing parent -> A-bearing offspring)
- only diploids that reproduce asexually breed true (Aa-bearing parents -> Aa-bearing offspring); with sexual reproduction, diploid models do not breed true and will produce a variety of offspring (Aa-bearing parents -> AA/Aa/aa-bearing offspring)
what are the characteristics of diploid selection in asexuals (4)
- stages
- stage for natural selection
- frequencies tracked
- fitness values tracked
- two stages: diploid and asexual reproduction
- natural selection acts during the diploid stage
- track 3 frequencies: xAA + xAa + xaa = 1
- track 3 fitness values: W(AA), W(Aa), W(aa)
mean fitness for diploid selection in asexuals (3)
- symbol
- formula
- change over time
- symbol: Wbar[t]
- W(AA)xAA[t] + W(Aa)xAa[t] + W(aa)*xaa[t]
- in asexual populations, the mean fitness Wbar, increases over time (or stays the same)
how does the frequency of alleles change in diploid selection of asexuals if there is a heterozygous advantage for the allele?
- the population will experience a dramatic change in allele frequencies where Aa will increase greatly and the other alleles will decrease
how does the frequency of alleles change in diploid selection of sexuals if there is a heterozygous advantage for the allele?
- the change due to selection is the same as in asexuals, but meiosis breaks apart and reassorts the diploid genotypes
- Key Point: sex (segregation) can undo genetic associations built by selection
what are the two processes we need to model for diploid sexuals? (2)
- meiosis segregating alleles to create haploid gametes
- gamete union bringing alleles back together in diploids
meiosis segregates alleles to create haploid gametes model
- AA, Aa and aa frequencies are divided into the total frequencies of either A or a alleles
gamete union brings alleles back together in diploids
- A and a allele frequencies in both egg and sperm meet to create AA, Aa, and aa frequencies
probability tree diagrams (3)
- help calculate chance of different events
- all options from one node sum to one
- multiply all probabilities along a path from start to finish to calculate probability of any one path
frequency of A gamete in diploid life cycle after meiosis
p = xAA + (1/2)*xAa
frequency of a gamete in diploid life cycle after meiosis
q = (1/2)*xAa + xaa
what is the frequency of the AA diploid after gamete union assuming random mating among gametes
xAA = p^2
what is the frequency of the Aa diploid after gamete union assuming random mating among gametes
2pq
what is the frequency of the aa diploid after gamete union assuming random mating among gametes
q^2
Hardy-Weinberg proportions (2)
p^2 + 2pq + q^2
- occurs during random rating and no selection: allele frequencies do not change after each generation and diploids are immediately in Hardy-Weinberg proportions
what are the assumptions are the Hardy-Weinberg equation (4)
- random combination of gametes from the gamete pool
- no differences in fitness among genotypes
- a very large populations (no chance effects changing genotype frequencies at any step)
- no mutations or migration altering in population
how do we apply the Hardy-Weinberg proportions?
- if populations of adults are not at Hardy-Weinberg proportions, it indicates that one of our assumptions is violated (there may be selection or non-random mating, etc)
diploid sexuals with selection
- with random mating and selection, allele frequencies do change
- while diploids are at Hardy-Weinberg proportions at birth, they may not be after selection
what happens to the allele A frequency if W(AA) > W(Aa) > W(aa) during long term natural selection (2)
p[t] -> 1
- “directional selection” favouring A
what happens to the allele A frequency if W(AA) < W(Aa) < W(aa) during long term natural selection (2)
p[t] -> 0
- “directional selection” favouring a
what happens to the allele A frequency if W(AA) < W(Aa) > W(aa) during long term natural selection (3)
p[t] -> phat
- “heterozygote advantage” or “overdominance” where population fixes on certain intermediate frequency value; polymorphism maintained
- stable equilibrium
what happens to the allele A frequency if W(AA) > W(Aa) < W(aa) during long term natural selection (3)
p[t] -> 0 or 1; depending on starting condition (below, above or at phat)
- “heterozygote disadvantage” or “underdominance”
- unstable equilibrium
equilibrium (3)
- a point of a system that when started at that point, the system no longer changes
- denoted with a caret on top (hat)
- may be stable (points nearby approach the equilibrium) or unstable (points nearby are repelled away from the equilibrium)
when considering the spread of a new beneficial allele (A) in a population of wildtype alleles (a) for a sexual diploid population, how do we measure fitness?
- we typically measure fitness relative to the wildtype
W(aa) = 1
W(Aa) = 1 + h*s
W(AA) = 1 + s
selection coefficient (2)
- symbol: s
- measures the fitness of one homozygote (AA) relative to the other (aa)
dominance coefficient (2)
- symbol: h
- measures how dominant A is with respect to fitness
for diploid organisms, do we use the absolute or relative values of fitness to determine allele frequencies?
- relative fitness values
how do different values of dominance coefficient (5)
- A is recessive to a when h = 0
- A is partially recessive when 0 < h < 0.5
- A is additive with a when h = 0.5
- A is partially dominant when 0.5 < h < 1
- A is dominant to a when h = 1
what occurs when h=large
- allele A is more dominant
- the frequency of allele A rises faster at first, but slows down when it becomes more frequent
how does the value of s affect the allele frequency change
- if s is 10x smaller, it takes 10x longer to observe the same amount of frequency change
notes on dominance (3)
- dominance is NOT a characteristic of an allele, but reflects the interaction between two alleles
- allele A might be dominant with respect to a, but recessive with respect to another allele a’
- dominance depends on the phenotype being measured
sickle-cell anemia (4)
- definition
- cause
- human disease affecting the shape and flexibility of RBCs; mutant form of hemoglobin (S) tends to crystallize and form chains, causing distortions in the RBCs
- caused by mutation in the sixth amino acid of the chain of hemoglobin
- where malaria is common, adults are more often heterozygous than predicted by the HW proportions
- due to the heterozygote advantage of sickle-cell anemia, the frequency of the A allele will settle at a certain phat
sickle-cell anemia and diploid allele combinations (3)
- homozygous SS experience the greatest degree of sickling and tend to suffer severe anemic attack
- heterozygous AS also suffer from sickling of RBCs, but to a lesser degree
- homozygous AA do not experience sickle-cell anemia
malaria and diploid allele combinations (3)
- heterozygous AS are less likely to die from malaria as RBCs infected with parasites causing malaria tend to sickle and be destroyed
- homozygous SS do not die from malaria due to the sickling of the cells
- homozygous AA are likely to die from malaria
mean fitness in diploid sexuals
- mean fitness always increases (or stays the same)
- deltaWbar > 0
