Evolution

Question 1

Gene

Answer 1

• 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

Question 2

Locus

Answer 2

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

Question 3

Allele

Answer 3

• variant of a gene

* different alleles can lead to different phenotypes

Question 4

Homozygote

Answer 4

possesses 2 copies of the same allele

Question 5

Heterozygote

Answer 5

• possesses 2 different alleles

Question 6

Genotype

Answer 6

• the genetic makeup of an individual

* a description of the alleles possessed by an individual

Question 7

Under random mating we expect to see

Answer 7

Hardy-Weinberg genotype frequencies

Question 8

Hardy-Weinberg genotype frequencies

Answer 8

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

Question 9

When alleles are rare they’re more commonly found in

Answer 9

heterozygote genotypes

Question 10

Phenotype

Answer 10

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

Question 11

Gamete

Answer 11

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

Question 12

Zygote

Answer 12

• the earliest developmental stage of the embryo

* produced by the fusion of 2 gametes

Question 13

True or false

the terms dominant and recessive apply to genes

Answer 13

false

alleles

Question 14

True or False

The dominant allele is the one that’s selected for

Answer 14

false

Question 15

True or False

the dominant allele is the most common in the population

Answer 15

false

Question 16

True or false

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

Answer 16

true

Question 17

True or false

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

Answer 17

true

but must factor in environment

Question 18

Genetic drift

Answer 18

• 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

Question 19

p must add to 1

Answer 19

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

Question 20

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

Answer 20

1 allele left to sample

Question 21

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

Answer 21

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

Question 22

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

Answer 22

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

Question 23

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

Answer 23

a bottleneck

Question 24

Genetic drift always causes

Answer 24

allele frequencies to change in a random fashion over time

Question 25

Selection occurs because

Answer 25

different individuals have different fitnesses

Question 26

Fitness

Answer 26

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

Question 27

Fitness is a property of

Answer 27

genotypes • not genes • not genotypes

Question 28

Relative fitness

Answer 28

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

Question 29

Absolute fitness

Answer 29

alleles / # of individuals

Question 30

Selection is a process leading to

Answer 30

different expectations of transmitting genes

Question 31

If individuals in a population have different fitness then we say

Answer 31

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

Question 32

The fitness of different genotypes is represented by the symbol

Answer 32

ω | • eg fitness of AB genotype is ωAB

Question 33

The strength of selection is often represented by the symbol

Answer 33

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

Question 34

the effect of high fitness is to make an individual

Answer 34

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

Question 35

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

Answer 35

a smaller effective population size

Question 36

As the allele frequency nears 1

Answer 36

the proportion in heterozygotes goes down | • allele frequency low = many in heterozygotes

Question 37

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

Answer 37

sampling from a finite population | • genotype frequencies are expected to remain in Hardy-Weinberg proportions every generation

Question 38

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

Answer 38

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

Question 39

Drift and consanguity

Answer 39

* 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

Question 40

Mutation

Answer 40

* the processes producing genetic variation * the original source of all genetic variation * permanent structure alteration in DNA

Question 41

Each gene copy experiences a mutational rate

Answer 41

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

Question 42

Identity by descent

Answer 42

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

Question 43

Selection occurs at the level of

Answer 43

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

Question 44

Selection in favor of a dominant allele

Answer 44

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

Question 45

Selection in favor of a recessive allele

Answer 45

``` 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

Question 46

Heterozygote advantage

Answer 46

``` 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

Question 47

Heterozygous advantage

Answer 47

``` 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

Question 48

Gene flow

Answer 48

* 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