topic 7 Flashcards
why is genetic variation important
survival - some individuals will survive conditions others wont
central issue of pop genetics? 3 scientists
• Central issue in pop. Genetics is genetic variation
○ Its extent within pops.
○ How it changes over generations
○ Why it exists
• Emerged as a branch in 1920s and 30s
• ronald fisher, sewall wright, J.B.S haldane
• Played a role in newer darwinian synthesis
what is genetic variation an extention of
pop genetics is a direct extension of mendels laws of inhertiance, molecular genetics, ideas of darwin
○ Know the laws of inheritence
gene pool?
• Focus is shifted away from individual to pop
• All of the allelles of every gene in a pop makes up its gene pool
○ Only individuals that reproduce contribute to gene pool of next gen
Population geneticists study the genetic variation within
the gene pool and how it changes from one generation to
the next
define phenotype, genotype
• Phenotype: characteristic of an individual organims
Genotype: genetic constitution of an individual organism
define locus and allele
• Locus: site on a chromosome; gene that occupies a site
Allele: a particular form of a gene, usually distinguished by its effects on phenotype q
define haplotype, gene copy, and gene frequency
• Haplotype: One of the sequences of a gene or DNA segment that can be distinguished from homologous sequences
• Gene copy: number of representatives of a gene; in a 2N pop., a sample of 100 individuals represent 200 gene copies
Gene (allele) frequency: the relative proportion of a particular
define genotype freq
Genotype frequency: the relative proportion of a particular genotype in a population (a number between o and 1)
what is a pop? demes?
• A population is a group of individuals of the same
species that occupy the same region and can
interbreed with each other
• A large population is usually composed of smaller
groups called local populations
○ These are also called demes
○ Members of a local population are far likelier to breed
with each other than with members of the general
population
how are local pops separated
○ Local populations are often separated from each other
by moderate geographic barriers
Such as a local pop of large ground finch on daphne major island, the galapagos
how can a pop change? how can we predict?
A population may change in ..... • Size Geographic location Genetic composition Population geneticists have developed mathematical theories that predict how the gene pool will change in response to fluctuations in the above
describe allele freq
○ Relative commonness or rarity of an allele
○ Its proportions of all gene copies in the pop
○ Sometimes impercisely known as gene freq
○ In sexual reproducing 2N pops:
Combined into homozygous and heterozygous genotypes
define genotype freq (separate from alleles)
Genotype frequency
Proportion of population with certain genotype
how can the freq. of an allele be calculated
○ For 2N organisms, total number of alleles at a locus is total number of individuals X2
Total number of dominant alleles at a locus is 2 alleles for each homozygous dom. Individual plus 1 allele for each heterozygous inidivdual; same logic applies for recessive alleles
look at two fundamental calcs of pop genetics (allele and gene freq)
ok
or a given trait, what value are allele and genotype frequencies always
For a given trait, the allele and genotype frequencies are
always less than or equal to 1
i.e., less than or equal to 100%
alelle freq. for monomorphic gene?
For monomorphic genes
The allele frequency for the single allele will be equal or
be close to 1.0
All individuals are the same for a particular trait - only one kind
alelle freq. for polymorphic gene?
For polymorphic genes
The frequencies of all alleles should add up to 1.0
In our frog example
Frequency of G allele + frequency of g allele = 1
Frequency of G allele = 1 — frequency of g allele
= 1 — 0.2
= 0.8, or 80%
what is hardy weinberg equilibrium
• Foundation to the basis of pop genetics
• Hardy-Weinberg equation was formulated
independently by Godfrey Harold Hardy
(British mathematician) and Wilhelm
Weinberg (German physician — OB/GYN)
in 1908
A simple mathematical expression that relates allele and genotype freqs. In a pop
why is HW called an equilibrium
equation is also called an equilibrium
Under a given set of conditions
Allele and genotype frequencies do not change
over the course of many generations
why is HW a null model
like Newton's first law of motion: Every object tends to remain in a state of uniform motion (or stasis), Assuming no external force is applied to it rhe Hardy-Weinberg Equation will be satisfied, if Ill the assumptions are met...
assumtions of HW? (5)
- NO mutations. By altering alleles or (in large-scale
changes) deleting or duplicating entire genes, muta-
tions modify the gene pool. - Random mating. If individuals mate preferentially
within a subset of the population, such as their close
relatives (inbreeding), random mixing of gametes
does not occur, and genotype frequencies change. - No natural selection. Differences in the survival
and reproductive success of individuals carrying dif-
ferent genotypes can alter allele frequencies. - Extremely large population size. The smaller the
population, the more likely it is that allele frequencies
will fluctuate by chance from One generation to the
next (genetic drift). - No gene flow. By moving alleles into or out of popu-
lations, gene flow can alter allele frequencies.
pq for allele freqs?
By convention, if there are 2 alleles at a locus,
and q are used to represent their frequencies
○ P is dom and q is recessive
• Frequency of all alleles in a population will add
up to 1
○ Therefore, p + q = 1
based on HW, when is a pop evolving
Hardy-Weinberg principle describes a
• population that is not evolving
If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving
practice punnett squares. freq of gametes is equal to what in HW eq?
The frequency of gametes carrying a particular allele is equal to the
allele frequency for a population in Hardy-Weinberg equilibrium.
Multiplying the allele frequencies gives the proportion of each allele
combination in the population.
are the conditions for HW often met?
Hardy-Weinberg theorem describes a
hypothetical population
In real populations, allele and genotype
frequencies do change over time
ie can be met at certain points but not forever
why is HW the fundamental null model? what does it demonstrate for mendelian segregation
The Hardy-Weinberg Equilibrium characterizes the
distributions of genotype frequencies in populations that
are not evolving and is thus the fundamental null model
for population genetics.
It demonstrates that Mendelian loci segregating for
multiple alleles in diploid populations will retain
predictable levels of genetic variation in the absence of
forces that change allele frequencies.
evolutionary implications for HW?
• Population heterozygosity(the frequency of
heterozygotes) is highest when p = q = 0.5
Rare alleles are found primarily in heterozygotes,
as they must be, given that q^2 is much smaller than 2pq when q is near zero, and p^2 is much
smaller than 2pq when p is near zero
Look at hw blood type example. when is HW at eq when comparing allelles
• When comparing, if the expected is within 95% confidence, it is at equilibrium
More than 5% diff = not
how can pops. evolve and be at HW eq at the same time?
• Natural populations can evolve at some loci,
while being in Hardy-Weinberg equilibrium at
other loci
In the blood group ex., that pop is at equilibrium for blood group - only for that particular characteristic, doesn’t mean the pop is not evolving
describe PKU ex
• Autosomal recessive metabolic genetic disorder
○ Est. 1 in 12,000 newborns in N. Am.; ~300 cases every year
○ USA: 1 in ~10,000
• Mutation causes production of nonfunctional hepatic
enzyme – phenylalanine hydroxylase (PAH)
○ Phenylalanine (PHE) accumulates & released as phenylketone in
urine
• Untreated PKU may lead to Mental retardation, seizures, brain damage, & other medical symptoms
• How treated?
○ Reducing phenylalanine intake through low PHE diet
○ Providing necessary alternative to natural protein through medical formula
○ Periodic monitoring of blood PHE levels
how is HW applied to pku?
• We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that:
○ PKU gene mutation rate is low
○ Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele
○ Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions - pku suppressed if they follow diet
○ Population is large
○ Migration has no effect as many other populations have
similar allele frequencies
two important implications of HW
○ Genotype frequencies reach H-W values after one generation of random mating
Allele frequencies also remain unchanged from generation to generation
3 steps to find if a pop is in HW? do examples
Step 1: Calculate the allele frequencies
Step 2: Calculate expected numbers of each
genotype (i.e. figure out how many
homozygotes and heterozygotes you would
expect)
Step 3: Compare your expected and
observed data
