Week 3 Genetic Basis of Evolution

Question 1

Define a Gene

Answer 1

a DNA segment that contributes to phenotype/function

Question 2

Define a Locus (pl. Loci)

Answer 2

– The position on a chromosome of a gene or other chromosome marker

– Can also refer to the DNA at that position

– The use of locus is sometimes restricted to mean regions of DNA that are expressed

Question 3

What procedure can be used to find specific DNA sequences in the genome?

Answer 3

FISH = flourescence in situ hybridisation

MYCN is an oncogene in Neuroblastoma, the FISH procedure can find the locus of this oncogene

Question 4

Define an Allele

Answer 4

Variant of a gene

Different alleles can lead to different phenotype

Question 5

How many copies of each gene does a diploid carry?

Answer 5

2 copies of each gene

Question 6

Alleles present in a homozygote and heterozygote?

Answer 6

• *homozygote:** possesses two copies of the same allele
• *heterozygote:** possesses two different alleles

Question 7

What is the allele frequency (proportion) calculation?

Answer 7

(2xHomozygote + Heterozygote) / (total)

Question 8

Define a Genotype

Answer 8

The genetic makeup of an individual (one or more loci)

Question 9

Genotype Frequency Calculation

Question 10

When alleles are rare, are they more commonly found in homozygote or heterozygote genotypes?

Answer 10

Heterozygote genotypes

Question 11

Define a Phenotype

Answer 11

The physical/behavioural (etc.) characteristics of an individual

Interaction of genes and environment. Genetic component of the phenotype is heritable, environmentally acquired component of phenotype is not.

Question 12

Define a Gamete

Answer 12

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

– Produced by meiosis
– Contains half the chromosomes of the parents

Question 13

Define a Zygote

Answer 13

The earliest developmental stage of the embryo

– Produced by the fusion of two gametes

Question 14

Does the terms ‘dominant’ and ‘recessive’ apply to genes or alleles?

Answer 14

Alleles

Question 15

What is the Hardy-Weinberg Equilibrium?

Answer 15

Developed as a null model, in order to understand how changes in gene frequencies occur. That is, we need to understand the conditions under which NO evolution will occur

In 1908, GH Hardy and W Weinberg independently published a theorem showing how allele frequencies behave given a particular set of assumptions.

Question 16

Assumptions of Hardy-Weinberg Equilibrium:

Answer 16

• The organism is diploid

• Reproduction is sexual

• Generations are non-overlapping

• Mating is random

• Populations are infinite

• There is no mutation

• There is no migration

• There is no natural selection/drift

Question 17

HWE: Given the adult population, what is the value of f(p) and f(q)?

Answer 17

f(p) = 137/200 = 0.685

f(q) = 63/200 = 0.315

Question 18

HWE: Gene and genotype frequencies must sum up to what?

What are the equations for the theorem?

p + q = 1
p2 + 2pq + q2 = 1

Answer 18

1

Question 19

HWE: If the assumptions of HWE are met, frequecies change or do not change?

Answer 19

do not change

Question 20

HWE: When observed genotype frequencies are equal to the expected genotype frequencies under the HWT, we say that the population is in ….

Answer 20

HWE

Question 21

HWE: A population will go to HWE in only …. generation of random mating

Answer 21

in only one generation of random mating

Question 22

Define Evolution

Answer 22

A change in allele frequencies over time

Question 23

What are the two factors which are major contributors to genetic differentiation?

Answer 23

Genetic drift & Selection

Question 24

Define Genetic Drift

(what is it a consequence of & where does it take place)

Answer 24

process by which allele frequencies change over time due to the effects of random sampling

• Takes place as a consequence of finite population size
• Genetic drift takes place in all populations, and any selection must occur against this background of drift

Question 25

How does genetic drift work?

Answer 25

• the change is due to “sampling error” in selecting the alleles for the next generation from the gene pool of the current generation.
• By pure chance we might sample a particular allele more or less often than expected, causing the allele frequencies to change from one generation to the next

Question 26

Graph of a particular allele frequency as it changes over time:

what happens?

Answer 26

• allele frequency eventually gets to p=1

Question 27

Graph of a particular allele frequency as it changes over time: why does the allele frequncy get stuck at p=1?

Answer 27

Fixation - stuck at p=1 because there is only 1 allele left to sample

The allele has become fixed in the population

The other possibility is that the allele gets lost, in which case the other allele must have become fixed (assuming two alleles)

Question 28

Graph of genetic drift with the process of evolution:

what happens?

Answer 28

• Equal chance of drifting up or down
• Overtime it is certain that one or the other allele will become fixed and the other will become lost.
• The likelyhood of these events depends on the starting allele frequency
• No selection in this model

Question 29

Is genetic drift stronger in a smaller or larger population?

Answer 29

Genetic drift is stronger in a small population

The effect of random sampling is greater

Question 30

Genetic Drift: what is the bottleneck effect?

Answer 30

an extreme example of genetic drift that happens when the size of a population is severely reduced

Events like natural disasters can decimate a population, killing most indviduals and leaving behind a small, random assortment of survivors

Question 31

Why does selection occur?

Answer 31

Because different individuals have different fitness

Question 32

Define Fitness

Answer 32

The reproductive success of a genotype, usually measured as the number of offspring produced by an individual that survive to reproductive age relative to the average for the population.

Question 33

How do you measure Absolute Fitness?

Answer 33

calculate the change in abundance of a genotype from one generation to the next (assuming infinite population size, no mutation, and non-overlapping generations)

the most fit genotypes will increase in abundance compared to less fit genotypes.

Question 34

How do you measure Relative Fitness?

Answer 34

calculated by dividing all fitness values by the largest value

Relative fitness is always measured with respect to the “best” or “most fit” genotype in our sample

meaning the fittest genotype always has a relative fitness of 1

Question 35

What is selection, in regards to fitness occuring or not?

Answer 35

a process (not really a ‘force’) leading to different expectations of transmitting genes to the next generation

• different fitness amoung a popultion means selection is occuring
• same fitness means no selection, or that the population is evolving neutrally (evolution by genetic drift)

Question 36

The fitness of different genotypes is often represented by what symbol?

Answer 36

w (omega)

Example: fitness of AB genotype = w_AB

Question 37

The strength of selection is often represented by what symbol?

Answer 37

s

AB not the fittest genotype - strength of selection against heterozygotes can be thought of as the deficit from a relative fitness of 1, so that

w_AB =1–s

Question 38

Selection and Drift: The effect of high fitness is to make an individual …… to be the parent of offspring in the next generation

But..

Answer 38

The effect of high fitness is to make an individual more likely to be the parent of offspring in the next generation

But.. It is still possible that a fit individual will get unlucky and end up having no offspring

Question 39

Selection and Drift: what is occuring in this graph?

A model in which A is dominant and has high fitness

Answer 39

Allele frequencies still drift around as before, but now there is a systematic change in an upward direction.

There is still one case in which, despite the high fitness of individuals with the A allele, the A allele gets lost due to pure chance

Question 40

What is stronger, selection or drift?

Answer 40

Selection