Lecture 5 - Quantitative variation 3 and Mutation

Question 1

What is artificial selection?

Answer 1

The process by which humans breed plants and animals to select for certain traits

Question 2

What are the features of natural selection?

Answer 2

• reponse to selection can lead to phenotypes beyond natural variation
• there are limits to the response (no dog has six legs)

Question 3

What was Johannsen’s unsuccessful selection experiment?

Answer 3

1926

• started with an inbred line of seeds
• bred heavy seeds and light seeds
• there was no response to selection
• later repeated the experiment with a mixture of seeds which led to a response

Question 4

What does a response to selection require?

Answer 4

• genetic variability of the trait

i. e. the trait needs to be heritable

Question 5

What factors will influence the strength of selection ?

Answer 5

• the greater the heritability, the stronger the response to selection
• the strength of selection will also affect (the difference in fitness between individuals in the parental population)

Question 6

How is the response to selection limited and by what?

Answer 6

Often plataeus
-no further genetic variation 
-traits may be linked:
on the chromosome (physical linkage)
through negative pleiotropic effects

Question 7

What is pleiotropy?

Answer 7

when one gene influences multiple, seemingly unrelated phenotypic traits

Question 8

What do response to selection graphs help to visualise?

Answer 8

the different factors that determine the magnitude of the response to selection

Question 9

What is the formula for the s (selection differential)?

Answer 9

s = Ms - M

Question 10

What is the formula for the response to selection?

Answer 10

R = M’ - M

Question 11

How does the mean in the offspring generation often differ from the mean of the selected parents?

Answer 11

The mean in the offspring generation is usually smaller than the mean of the selected parents
-this means that the response is less strong than selection

Question 12

Why is the reponse to selective breeding in the offspring generation often less strong than selection?

Answer 12

• not all phenotypic variation within the parental generation is due to genetic variation i.e. h2 <1
• heritability is therefore useful for predicting the response to selection

Question 13

How can heritability help to predict the response to selection?

Answer 13

• not all phenotypic variation within the parental generation is due to genetic variation i.e. h2 <1
• through the breeders equation

Question 14

What is the breeders equation?

Answer 14

R = h^2*s

Question 15

How does a response to selection graph look if h2 = 0?

Answer 15

Use breeders equation
R=0
M’ and M are the same

Question 16

How does a response to selection graph look if h2 = 1?

Answer 16

Use breeders equation
R=s
M’ = Ms

Question 17

How was heritability and the response to selection used to predict selection on beak size in the Galapagous finches?

Answer 17

Finches hatched in 1976, before the drought
-mean bill depth - (M) = 9.0mm
Birds that survived the drought (parents)
-(Ms) = 10.2mm
Next generation
-(M’) = 9.7mm
Selection differential
Mean selected parents - population mean before drought
s = Ms - M
=10.2mm - 9mm = 1.2m
Predicted response to selection
R = h^2s
R = 0.81.2 = 0.96mm
Predicted mean bill depth next generation, M + R
= 9mm +0.96 = 9.96
Actual measurement 9.7mm
Actual response
-New mean - previous mean = 9.7 - 9 = 0.7

Question 18

What are the features of mutations?

Answer 18

• help create variation
• understanding the mutation process helps understanding of nucleotide composition of genomes and the difference in it among species (e.g. elevated TA or GC content)
• occur at very low frequencies (although highly variable)

Question 19

What are the different types of mutation?

Answer 19

Point mutations
Insertions and deletions
-gene duplication 
-copy number variation 
-repeat sequences i.e. microsatellites
Inversions
Lateral gene transfer

Question 20

Why use point mutations?

Answer 20

• they are very common
• their mutation process is very well understood
• their effect is reasonably well understood

Question 21

How much variation is there in SNPs in the human population?

Answer 21

-Human mutation rate = ~ 1 X 10^-9 muations/position/generation
-Diploid genome is ~2X3.2X10^9 nucleotides
-~6.4 mutations/diploid genome/generation
-7 billion people
-7 billion X 6.5 mutations/generation/position
= 44.8 X 10^9 mutations per generation

Large genomes and population sizes mean that there are a lot of mutations in a population

Question 22

What is the human mutation rate?

Answer 22

-Human mutation rate = ~ 1 X 10^-9 muations/position/generation

Question 23

How many nucleotides are there in a diplod human genome?

Answer 23

-Diploid genome is ~2X3.2X10^9 nucleotides

Question 24

What are the two types of effects of mutations on allele frequencies?

Answer 24

Mutations can be

• irreversible
• reversible

Question 25

What is the result of an irreversible mutation in the absence of other forces?

Answer 25

eventually leads to the loss of the initial allele

Question 26

What is the formula for the frequency of A (original allele in an irreversible mutation) after time t?

Answer 26

Pt = Po (1 - μ (mutation rate of A to a/generation))^t

Question 27

What are the results of allele frequencies with a reversible mutation?

Answer 27

A: initial allele with frequency p
Reversible mutation to a (has an initial frequency of q)
-results in an equilibrium of allele frequencies

Question 28

Can the equilibrium allele frequences of a reversible mutation be calculated?

Answer 28

Mutation rate A to a: μ
Mutation rate a to A: v
Frequency of A after t generations:
pt = pt-1(1- μ) + (1-pt-1)*v
At the equilibrium, p=pt=pt-1

P=(v/( μ+v))