Evolution Flashcards
Gene
- segregating and heritable determinant of phenotype
- fundamental physical and functional unit of heredity which carries in from from one generation to the next
- segment of DNA with transcribed region and regulatory sequences that make transcription possible
- a DNA segment that contributes to phenotype/function
Locus
- the position on a chromosome of a gene or other chromosome marker
- can also refer to the DNA at that position
Allele
- variant of a gene
* different alleles can lead to different phenotypes
Homozygote
possesses 2 copies of the same allele
Heterozygote
• possesses 2 different alleles
Genotype
- the genetic makeup of an individual
* a description of the alleles possessed by an individual
Under random mating we expect to see
Hardy-Weinberg genotype frequencies
Hardy-Weinberg genotype frequencies
p^2
2p(1-p)
(1-p)^2
When alleles are rare they’re more commonly found in
heterozygote genotypes
Phenotype
- physical characteristics of an individual
- composed of traits
- interaction of genes and the environment
Gamete
germline cell that is able to unite with another of the opposite sex during sexual reproduction
Zygote
- the earliest developmental stage of the embryo
* produced by the fusion of 2 gametes
True or false
the terms dominant and recessive apply to genes
false
alleles
True or False
The dominant allele is the one that’s selected for
false
True or False
the dominant allele is the most common in the population
false
True or false
The dominant allele expresses its phenotype even if present in a heterozygote
true
True or false
If A is dominant over a, then individuals who are AA and Aa have the same phenotype
true
but must factor in environment
Genetic drift
- describes the process by which allele frequencies change over time due to the effect of random sampling
- takes place as a consequence of finite population size
p must add to 1
eg 2 alleles A and a
• starting allele frequency of A is p=0.6
• therefore starting allele frequency of a is p=0.4
If allele frequency gets stuck (fixed) at 1, that means that there’s only
1 allele left to sample
If we leave enough time, we can be certain that one or the other allele will become fixed, and the other will become lost
if there’s no selection, which of these events is more likely depends only on the starting allele frequency
Genetic drift is stronger in a smaller population than in a large population
BECAUSE
the effect of random sampling is greater in a small population than in a large one
One place that drift can be particularly strong is when a population undergoes
a bottleneck
Genetic drift always causes
allele frequencies to change in a random fashion over time
Selection occurs because
different individuals have different fitnesses
Fitness
the expectation of the number of descendent genes at the same stage of the life cycle in the next generation
Fitness is a property of
genotypes
• not genes
• not genotypes
Relative fitness
calculated by dividing all (absolute) fitness values by the largest values
• the fittest genotype always has a relative fitness of 1
Absolute fitness
alleles / # of individuals
Selection is a process leading to
different expectations of transmitting genes
If individuals in a population have different fitness then we say
selection is operating
• if they have the same fitness then we say that there is no selection, or that the population is evolving naturally
The fitness of different genotypes is represented by the symbol
ω
• eg fitness of AB genotype is ωAB
The strength of selection is often represented by the symbol
s
• for example if AB is not the fittest genotype then the strength of selection against heterozygotes can be thought of as the deficit from a relative fitness of 1 so that ωAB = 1 - s
the effect of high fitness is to make an individual
more likely to be the parent of offspring in the next generation
• it is still possible that a fit individual will get unlucky and have not kids
Why would we expect to see greater genetic drift on the Y chromosome compared with other parts of the genome
a smaller effective population size
As the allele frequency nears 1
the proportion in heterozygotes goes down
• allele frequency low = many in heterozygotes
Genetic drift causes allele frequencies to change over time as a result of
sampling from a finite population
• genotype frequencies are expected to remain in Hardy-Weinberg proportions every generation
The probability of identity by descent due to relatedness between parents can be measured by the parameter
f
• the chance that the 2 gene copies in a diploid individual are descended from the same copy in an earlier generation
Drift and consanguity
- both occur due to a buildup of shared incestry within a population
- drift occurs as a result as a finite population size whereas consanguinity could technically occur even in an infinitely large population
- drift results in a change in allele frequencies but genotype frequencies remain in HWE. consanguinity results in a change in genotype frequencies, but doesn’t alter allele frequency
Mutation
- the processes producing genetic variation
- the original source of all genetic variation
- permanent structure alteration in DNA
Each gene copy experiences a mutational rate
mu
• in a population of 2N genes this is a total mutation rate of 2n(mu)
• the chance of any 1 allele going to fixation is 1/(2N)
• the probability of a new mutant allele going to fixation under drift alone = mu
Identity by descent
the probability that the 2 genes in the offspring are descended from the same gene copy in an earlier generation
Selection occurs at the level of
phenotype
same phenotype = same fitness
(same value for ω)
Selection in favor of a dominant allele
eg ωAA = 1
ωAB = 1
ωBB = 0.9
• AB as fit as AA (same ω)
• hard to go completely to AA (fixation)
- A high frequency –> B rare = B mostly in heterozygotes
- AA and AB have same fitness, nothing for selection to work on in AB (little phenotypic variation)
• same reason it’s difficult to eliminate deleterious alleles from a population
Selection in favor of a recessive allele
ωAA = 1 ωAB = 0.9 ωBB = 0.9
• slow to start
• once it gets going it gets fixed rapidly
(sigmoid)
• A low = hetero, masked by B (dominant)
• lots of AB and BB, select for AA
• B not invisible (has A in AB) –> drive B out
• even when the A allele is at high frequency B allele is always visible
• from a fitness point of view selection is acting to drive out B alleles
• dominant disorders can be driven out of a population more easily than recessive disorders, and hence there are less of them around
Heterozygote advantage
ωAA = 0.9 ωAB = 1 ωBB = 0.9
• converges to an equilibrium in allele frequency (lines meet in the middle, remain in flat)
• this is true irrespective of starting allele frequency (except p=0 or p=1)
• A alleles are rare = present mostly in heterozygotes and selected for
• A alleles are common = present mostly in homozygotes = selection against A
– the equilibrium frequency is the point at which these forces balance out
Heterozygous advantage
ωAA = 1 ωAB = 0.9 ωBB = 1
- lines start middle-ish and go out to top and bottom, flat
- common alleles are drive to fixation (mostly homozygous, selected for –> fixation)
- rare alleles are out of the population (heterozygous, selected against –> lost)
• one cause of heterozygous disadvantage is formation of hybrids
Gene flow
- the processes by which individuals genes (or alleles) move from one population to another
- can be one-directional or multi-directional
- movement of individuals doesn’t necessarily imply movement of genes
- in the absence of gene flow populations tend to be come genetically differentiated from one another
- gene flow homogenizes populations and can recover lost genetic variation
