Pop Gen 3/4 - Diploid selection/mutation Flashcards
True breeding
Parents produce offspring with genotypes identical to their own (discounting mutations)
ALSO DESCRIBED AS parental traits passed to progeny without substantial variation
Examples:
Only occurs in diploids that reproduce asexually
Haploid models
Plant crop inbred in the past therefore homozygous at all loci
Modification of W values for diploids
must represent all allele combinations:
WAA, WAa, Waa
fraction of allele combinations in diploids represented by
xAA, xAa, xaa
to calculate fraction of each genotype
is this final?
WAAXAA. WAaXAa. WaaXaa
No, not final!!! these wouldnt sum to 1
how to determine frequency of single allele combination after selection
WAAXAA OR WAaXAa OR WaaXaa DIVIDED BY sum of (WAAXAA + WAaXAa + WaaXaa)
therefore:
xAA[t + 1] = WAAXAA[t] / Wbar [t]
how does sexual reproduction impact selection?
change due to selection is the same, but meiosis breaks apart and resorts diploid gametes
Two processes modeled to predict sexual reproduction’s modification of selection
- Meiosis segregates alleles to create haploid gametes
to determine the likelihood of either allele A or allele a gamete:
sum the frequency of each allele combination (ex XAA) multiplied by the chance it yields the allele in question
ex. frequency of A gamete: p = XAA + 1/2 XAa
- Gamete union brings alleles back together in diploids
where p = freq of A gamete calculated before, and q = freq of a gamete calculated before
multiply freq of that specific gamete by the frequency of the gamete it would have to combine with to gain a specific comby
ex. AA diploid: XAA = p^2
Aa diploid: XAa = 2pq
aa diploid: xaa = q^2
Freq. of 3 genotypes in diploid sexuals without selection
When freq of A gamete = p[t]
Freq of a gamete = q[t]
AA diploid: XAA[t] = p[t]^2
Aa diploid: XAa[t] = 2p[t]*q[t]
aa diploid: Xaa[t] = q[t]^2
NO MUTATION, DRIFT, SELECTION
Assumptions in diploid sexuals without selection
- random combinations of gametes from gamete pool
- No differences in fitness between genotypes
- A very large population (no chance effects changing genotype frequencies at any step)
- No mutation or migration altering variants in populations
If populations are not in Hardy-weinberg proportions indicates that…
one of the assumptions is wrong
Fundamental equation of evolution with selection in a diploid model (freq of A gamete)
p[t + 1] = (WAAp[t]^2 + WAap[t]q[t])/ (WAAp[t]^2 + WAa^2p[t]*q[t] + Waaq[t]^2)
Key feature of random mating and selection
Allele frequencies CHANGE!
Diploids are at Hardy-Weinberg proportions at birth, they may not be after selection
Equilibrium
a point of a system that when started at that point, the system no longer changes. Equilibria are denoted with a caret (hat) on top. Can be stabel OR unstable
Equilibrium equation
phat = (WAa - Waa) / (2WAa - WAA - Waa)
Mean fitness always…
INCREASES
change in W bar >/= 0
Is there genetic variation at a polymorphic equilibrium, alleles A and a are equally fit on average
NO
Mutation
An alteration to the DNA of an organism
Ultimate source of novel genetic variation
Types of mutation
point mutations (silent, nonsense, missence mutation)
structural mutations (deletion, duplication, inversion)
Genomic mutations (ex duplication of entire genome)
Silent mutations
change in nucleotide seq but still codes for same amino acid
Nonsense mutation
introduces a stop codon
Missense mutation
changes amino acid sequence of a protein
Do mammals or bacteria have higher mutation rates?
Mammals, we’re worse at replicating out DNA
Rate of point mutations in eukaryotes
10^(-8) - 10^(-10) per basepair per generation. OR btwn 10^(-5) - 10^(-7) per generation
Mutation rates vary between…
Organisms
Genes
Alleles involved - mutations disturbing wild type function (forward mutations) often occur at higher rates than mutations restoring wild type function (back mutations)
Mutation Load
Reduction in fitness due to mutations represented by funky u
S
SELECTION COEFFICIENT
if = 1, Lethal mutation
if = 0, equal fitness to wildtype
indicates severity of mutations, higher S = lower frequency at equilibrium but greater negative effect if carried
h
dominance coefficient
q hat
Frequency of deletarious ALLELE
could determine given frequency of homozygotes, or heterozygotes based on 2pq, q^2 or p^2
