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Evol Gen > Evolution > Flashcards

Evolution Flashcards

1
Q

Gene

A
  • 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
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2
Q

Locus

A
  • the position on a chromosome of a gene or other chromosome marker
  • can also refer to the DNA at that position
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3
Q

Allele

A
  • variant of a gene

* different alleles can lead to different phenotypes

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4
Q

Homozygote

A

possesses 2 copies of the same allele

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5
Q

Heterozygote

A

• possesses 2 different alleles

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6
Q

Genotype

A
  • the genetic makeup of an individual

* a description of the alleles possessed by an individual

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7
Q

Under random mating we expect to see

A

Hardy-Weinberg genotype frequencies

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8
Q

Hardy-Weinberg genotype frequencies

A

p^2
2p(1-p)
(1-p)^2

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9
Q

When alleles are rare they’re more commonly found in

A

heterozygote genotypes

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10
Q

Phenotype

A
  • physical characteristics of an individual
  • composed of traits
  • interaction of genes and the environment
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11
Q

Gamete

A

germline cell that is able to unite with another of the opposite sex during sexual reproduction

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12
Q

Zygote

A
  • the earliest developmental stage of the embryo

* produced by the fusion of 2 gametes

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13
Q

True or false

the terms dominant and recessive apply to genes

A

false

alleles

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14
Q

True or False

The dominant allele is the one that’s selected for

A

false

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15
Q

True or False

the dominant allele is the most common in the population

A

false

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16
Q

True or false

The dominant allele expresses its phenotype even if present in a heterozygote

A

true

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17
Q

True or false

If A is dominant over a, then individuals who are AA and Aa have the same phenotype

A

true

but must factor in environment

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18
Q

Genetic drift

A
  • 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
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19
Q

p must add to 1

A

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

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20
Q

If allele frequency gets stuck (fixed) at 1, that means that there’s only

A

1 allele left to sample

21
Q

If we leave enough time, we can be certain that one or the other allele will become fixed, and the other will become lost

A

if there’s no selection, which of these events is more likely depends only on the starting allele frequency

22
Q

Genetic drift is stronger in a smaller population than in a large population
BECAUSE

A

the effect of random sampling is greater in a small population than in a large one

23
Q

One place that drift can be particularly strong is when a population undergoes

A

a bottleneck

24
Q

Genetic drift always causes

A

allele frequencies to change in a random fashion over time

25
Q

Selection occurs because

A

different individuals have different fitnesses

26
Q

Fitness

A

the expectation of the number of descendent genes at the same stage of the life cycle in the next generation

27
Q

Fitness is a property of

A

genotypes
• not genes
• not genotypes

28
Q

Relative fitness

A

calculated by dividing all (absolute) fitness values by the largest values
• the fittest genotype always has a relative fitness of 1

29
Q

Absolute fitness

A

alleles / # of individuals

30
Q

Selection is a process leading to

A

different expectations of transmitting genes

31
Q

If individuals in a population have different fitness then we say

A

selection is operating

• if they have the same fitness then we say that there is no selection, or that the population is evolving naturally

32
Q

The fitness of different genotypes is represented by the symbol

A

ω

• eg fitness of AB genotype is ωAB

33
Q

The strength of selection is often represented by the symbol

A

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

34
Q

the effect of high fitness is to make an individual

A

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

35
Q

Why would we expect to see greater genetic drift on the Y chromosome compared with other parts of the genome

A

a smaller effective population size

36
Q

As the allele frequency nears 1

A

the proportion in heterozygotes goes down

• allele frequency low = many in heterozygotes

37
Q

Genetic drift causes allele frequencies to change over time as a result of

A

sampling from a finite population

• genotype frequencies are expected to remain in Hardy-Weinberg proportions every generation

38
Q

The probability of identity by descent due to relatedness between parents can be measured by the parameter

A

f

• the chance that the 2 gene copies in a diploid individual are descended from the same copy in an earlier generation

39
Q

Drift and consanguity

A
  • 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
40
Q

Mutation

A
  • the processes producing genetic variation
  • the original source of all genetic variation
  • permanent structure alteration in DNA
41
Q

Each gene copy experiences a mutational rate

A

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

42
Q

Identity by descent

A

the probability that the 2 genes in the offspring are descended from the same gene copy in an earlier generation

43
Q

Selection occurs at the level of

A

phenotype
same phenotype = same fitness
(same value for ω)

44
Q

Selection in favor of a dominant allele

A

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

45
Q

Selection in favor of a recessive allele

A 
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

46
Q

Heterozygote advantage

A 
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

47
Q

Heterozygous advantage

A 
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

48
Q

Gene flow

A
  • 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

Evol Gen (2 decks)

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