Exam 4 Flashcards
qualitative trait
only a few distinct phenotypes
quantitative trait
continuously variable over some measure (distribution is due to polygenic, environment, multifactorial)
polygenic
many genes involved
environment
different genotypes perform differently based on their environment
multifactorial
traits that are both polygenic and influenced by environment
3^n
number of possible genotype combinations
(n=number of loci, 3=number of alleles)
why is there a distribution, a range of phenotypes?
there are a large number of genes that influence them to dictate the phenotype, there are multiple loci segregating
in a graph that shows three distributions (AA, Aa, aa), they overlap, why? what can we assume about the person’s genotype on a particular part of the graph?
- we know the phenotype of the individual but no the genotype
- one of the genotypes is more likely than the others, but all genotypes are possible
what is the most important binary trait?
alive or dead
threshold traits
need a certain number of a particular allele before you manifest the phenotype
Nilsson-Ehle’s Wheat Kernel Color
early determination of quantitative traits, found that the extremes resembled the parents
the difference between the inheritance of genes influencing quantitative vs qualitative characteristics is the _________ ___ ____ ____ _______ ___ __________
number of loci that determine a characteristic
the proportion of F2 individuals that resemble one of the original parents can be used to estimate the number of ________ affecting a polygenic traits
genes
1/4^n
- n= the number of loci
- gives you the offspring that look like the parents
“all models are wrong but some are useful”
stuff is complex, we cannot account for everything, so we simplify it
types of distributions
- normal
- bimodel
- skewed
mean
provides information about the center of a distribution
variance
indicates the variability of a group of measurements, or how spread out the distribution is
range of variance
0 to positive infinity
standard deviation
square root of variance
what genotype would have the least amount of variance?
homogenous
covariance
how two measurements vary together
range of covariance
negative infinity to positive infinity
correlation
the strength of association between 2 measurements
range of correlation
ranges from negative 1 to positive 1
correlation does _____ equal causation
NOT
types of correlation
- positive (upward slanted line)
- negative (downward slanted line)
- strong
- weak
- none (straight line)
regression
linear relationship between 2 variables (allows predictions to be made)
y=mx+b
y=bx+a
- m/b = slope
- b/a = intercept
heritability
the proportion of the total phenotypic variation that is due to genetic differences
phenotypic variance (Vp = )
Vp = VA + VD + VI + VE + VGE
genotypic variance (VG = )
VG = VA + VD + VI
VA
- additive
- comprises the additive effects of genes on the phenotype, which can be summed to determine the overall effect on the phenotype
VD
- dominance
- alleles at a locus are not additive; rather the affect of an allele depends on the identity of the other allele at that locus (TT and Tt have the same phenotype value and tt is different)
VI
- interaction
- epistatic effects where one locus “masks” the effect of others
VE
- environment
- differences that result from environmental factors
VGE
- gene-by-environment
- effect of a gene depends on specific environment in which it is found
what is environmental impact hardest on?
plants! They cannot move
types of heritability
- broad sense of heritability
- narrow sense of heritability
broad sense heritability: equation
H^2 = VG/Vp
narrow sense of heritability: equation
h^2 = VA/Vp
H^2
broad sense heritability, represents the proportion of phenotypic variance that is due to genetic variance
h^2
narrow sense heritability, represents the proportion of phenotypic variance that is due to additive genetic variance
H^2 = 0
genetic varation does not contribute to the phenotypic variation in the trait. It is due to environmental factors.
H^2 = 1
indicates that all of the phenotypic variance results from differences in phenotype
H^2 = between 0-1
both genetic and environmental factors influence phenotypic variance
________ variance is what is able to be reproductively selected for
additive
calculating heritability
- eliminating one or more variance components
- comparing the resemblance of parents and offspring
- comparing the phenotypic variances of individuals with different degrees of relatedness
- measuring the response to selection
phenotypic variance equation
Vp = VA + VD + VI + VE + VGE
eliminating one or more variance components
- eliminate environmental variance (VE + VGE = 0)
- genetically identical individuals (VG = 0)
- raise clones or highly inbred, identically homogenous individuals in a defined environment and measure their phenotypic variance to estimate VE
VG equation
VA + VD + VI
when genetic differences are responsible for phenotypic variance, offspring should resemble their parents more than they resemble unrelated individuals, why?
offspring share 50% DNA with mom and 50% DNA with dad
why are there deviations from the expected phenotypes?
due to mendelian sampling (meiosis, recombination, independent assortment)
h^2 = b
regression of offspring mean against mean of both parents
response to selection
the extent to which a characteristic subjected to selection changes in one generation
selection differential
the difference between the mean phenotype of the selected parents and the mean phenotype of the original population
limits to the response to selection
What are two reasons why a trait may cease to respond to selection?
- no genetic variability
- biological limitations
limitations to heritability
- heritability says nothing about the degree to which genes determine a characteristic
- no meaning for a specific individual (need a group)
- specific to a given population in a given environment
- high heritability does not mean environmental factors can’t influence the expression of a characteristic
- cannot be used to draw conclusions about why populations differ in a characteristic
what does high heritability indicate?
only that the environmental variation to which the population is CURRENTLY exposed to is not responsible for variation in the characteristic
mendelian population
a group of interbreeding, sexually reproducing individuals that have a common set of genes
population genetics
study the variation in alleles within and between groups and the evolutionary forces responsible for shaping the patterns of genetic variation found in nature
genotype frequency = f(AA) =
number of AA individuals / N
N =
total number of individuals
genotype frequency = f(Aa) =
number of Aa individuals / N
genotype frequency = f(aa) =
number of aa individuals / N
allele frequency = p = f(A) =
2nAA + nAa / 2N
allele frequency = q = f(a) =
2naa + nAa / 2N
X-linked = p = f(XA) =
2nXAXA + nXAXa + nXAY / 2n females + n male
X-linked = q = f(Xa) =
2nXaXa + nXAXa + nXaY / 2n females + n male
the hardy-weinberg equilibrium
describes the effect of reproduction on genotypic and allelic frequencies
hardy-weinberg equilibrium: generally safe assumptions
- diploid
- sexual reproduction
- non-overlapping generations
- bi-allelic
- easy allele frequencies in males and females
hardy-weinberg equilibrium: major evolutionary forces that affect allele frequencies
- random mating
- no migration
- no mutation
- no natural selection
- no drift (large population)
hardy-weinberg equilibrium: predictions
- allele frequencies do not change
- genotype frequencies reach equilibrium after one generation with proportions
hardy-weinberg equilibrium: maximum frequency and minimum frequency
- maximum = 1
- minimum = 0
hardy-weinberg equilibrium: what does a frequency of 0 mean?
there is no genetic diversity
hardy-weinberg equilibrium is on a per locus basis, why would selection on chromosome 1 not impact a locus on chromosome 2?
independent assortment
does mutation happen frequently? why do we need mutations?
- no
- without mutation we could not evolve or adapt
implications of the hardy-weinberg equilibrium
- population cannot evolve if it meets HWE
- genotypic frequencies are determined by the allelic frequencies
- a single generation of random mating produces the equilibrium frequencies
the fact that genotypes are in HWE proportions does ______ prove that the population is free from natural selection, mutation and migration. What does it mean?
- NOT
- it means only that these forces have not acted since the last time random mating took place
positive assortative mating
tendency for like individuals to mate
negative assortative mating
tendency for unlike individuals to mate
outcrossing
preferential mating between unrelated individuals
inbreeding
preferential mating between related individuals (leads to increased homozygosity)
does inbreeding change allele frequency?
NO
alleles identical by state
originated from different chromosomes (no relation)
alleles identical by descent
two copies of the chromosome are descended from the same copy in a common ancestor
mutation
ultimate source of variation
migration: equation
- q2 = q1(m) + q2(1-m)
- m=portion of migrants
- q1=migrating
- q2=not migrating
what does migration cause
- causes the allele frequencies of two populations to become more similar
- adds genetic variation to population
genetic drift
sampling error arises when gametes unite to produce progeny
when we sample individuals, we are sampling ___________
chromosomes
the amount of genetic drift can be estimated from the _________ in allelic frequency
variance (sp^2=pq/2N)
what is the result of an allelic drift?
allelic frequencies in the different populations diverges and often become fixed for one allele or the other
causes of allelic drift
- reduced sampling size
- founder effect
- bottleneck
all genetic drift arises from ___________ _______
sampling error
founder effect
establishment of a population by a small number of individuals
bottleneck
when a population undergoes a drastic reduction in size
effects of drift
- change in allelic frequencies within a population
- reduce genetic variation within a population
- different populations diverge genetically from one another over time
what suggests populations can evolve through random chance?
genetic drift
natural selection
takes place when individuals with adaptive traits produce a greater number of offspring than do individuals not carrying such traits
the effect of natural selection on the gene pool of a population depends on the _________ values of the genotypes in the population
fitness
fitness (W)
- the relative reproductive success of a genotype
- ranges from 0 to 1
selection coefficient (s)
the relative intensity of selection against genotype
selection coefficient (s) equation
1 - W
phenotypic variance symbol
Vp
Human height is a highly polygenic trait and has a high heritability.
a) If you have (or already had) children, what would be your best estimate of their adult hight before they are even born? I’m asking how you would estimate this not an actual number.
b) What basic principle of genetics allows you to make that estimate? Don’t overthink this.
a) the mean of the parents heights
b) diploid, half of your DNA comes from mom and half from dad
We said that heritability cannot be calculated for an individual and heritability has no meaning for a specific individual. Why?
we calculate the phenotypes and genotypes based on what is shown in a larger group, it must be calculated for a population, not just an individual
forces that change allele frequency
- mutation
- migration
- selection
- drift
how does mutation change allele frequency?
ultimate source of variation, change allele frequency slow
how does migration change allele frequency?
adds new alleles, end result is that the two populations will become more similar
how does drift change allele frequency?
randomly, dictated by population size (sampling variance)
how does selection change allele frequency?
fitness, ability of a genotype to produce more or less than another genotype
migration
blending of populations with different frequencies until equilibrium is reached
what do migration, mutation, selection, and drift have in common?
time, everything is based on times, based on generations
evolutionary genetics
the study of genetic changes over time
the raw material for genetic change is _____________
mutation
molecular data advantages
- quantifiable
- can be used with all organisms
- can compare all organisms
- can be applied to a huge amount of genetic variation
biological species concept
members of a population that actually or potentially interbreed in nature
reproductive barriers
pre zygotic and postzygotic
prezygotic (definition and types)
acts before a zygote has formed (ecological, temporal, mechanical, behavioral, gametic)
postzygotic (definition and types)
acts after a zygote has formed (hybrid inviability, hybrid sterility, hybrid breakdown)
speciation
process by which new species arise
allopatric
speciation is initiated when a geographic barrier splits a population into two or more groups and prevents gene flow between those groups
sympatric
speciation arises in the absence of an geographic barrier to gene flow; in this mode of speciation, reproductive isolating mechanisms evolve within a single interbreeding population
two types of speciation
allopatric and sympatric
in sympatric speciation; reproductive isolation mechanisms arise as a ______________ of genetic differentiation
consequence
what does evolutionary genetics deal with?
populations and species, not individuals
biological species concept: members of a population that __________ or __________ interbreed in nature
actually or potentially
