EB8

Question 1

what usually happens when there are two alelles for a gene in a population
what can happen

Answer 1

usually one allele becomes fixed.
ocassionally selection changes favouring one or another allele depending on the genetic and environmental background of the allele and/or the allele frequency. = balancing selection

Question 2

what are the three processes leading to balancing selection with their examples.

Answer 2

1. Heterozygote advantage: CF, SCA, Soay Sheep
2. Negative frequency dependent selection: snails and parasites, elderflower orchids, prey switching
3. Positive frequency dependence: driving on the wrong side of the road, Mullarian mimicry butterflies

Question 3

what is favoured if it is advantageous to have both alleles present in a genome

Answer 3

a gene duplication will be favoured with each gene hosting both alleles = this tends to remove heterozygote adv.

Question 4

when does heterozygote adv. occur

Answer 4

when the hetero is more fit than either of the homos preventing extcintion of one allele - heterozygote adv. is unlikely to be long lasting in evo time.

Question 5

how does heterozygote adv. contain an element of frequency dependence

Answer 5

when mutation arises leading to favourable hetero, freq of mutation increases, when it becomes common it will occur in a homo state and be less favourable (freq dep)

Question 6

What is CF caused by and what is the frequency of disease causing allele

Answer 6

loss of function of CFTR (deletion F508)
strong selection against the disease; little survival to reproductive age
freq: 2% in europeans (q=0.02)

Question 7

what does modelling CF tell us

Answer 7

new mutations cannot explain the maintenace of CF at high freq of 0.02

Question 8

how can CF be modelled to tell us that frequency is not due to mutation selection balance

Answer 8

Recessive and nearly lethal so assume h~0, s~1
Fitness of homozygote = q’ q’ =√(u⁄s) , u = 0.02^2 = 0.0004
Actual mutation rate is lower = 6.7*10-7 therefore mutation can’t explain frequency.

Question 9

what are the two beta globin alleles and what are their frequencies and patterns

Answer 9

A and S beta globin alleles
S frequency = 15% in some parts of Africa yet homo is virtually lethal.
*prevalance of malaria correlates with HbS allele frequency

Question 10

what did field data for malaria and SCA allele tell us

Answer 10

over 12000 ind. measured in Nigeria (malarai zone) freq A and S measured. showed not in HWE.
The inverse of the respective selection coefficent is equal to the ratio of allele frequency q/p = s/t
s= SC for AA, t= SC for SS

Question 11

what could we expect to happen to S allele if malaria was eliminated

Answer 11

begin to be selected against in population

Question 12

what do males of Soay sheep of St. kilda compete for and what is correlated with success

Answer 12

males compete for access to oestrous females

horn size postively correlated to male reprodutcive success

Question 13

what gene is responsible for the genetic variation in horn morphology in Soay sheep

Answer 13

RXFP2 relaxin like receptor
two alleles
Ho+ for large horns
Hop = small horns

Question 14

what did the long term study on 1700 sheep over 21 years show

Answer 14

Ho+/+ had highest reproductive success and lowest survival success vv for Hop/p.
heterozygote only marginally lower for both.
male fitness highest for hetero (Ho/p) and marginally higher for Ho+/+ than Hop/p

Question 15

What is negative frequency dependence

Answer 15

fitness is a function of frequency

selection favours rare alleles (fitness is decreasing function of frequency of allele)

Question 16

how does negative frequency dependence allow two alleles to exist in a population

Answer 16

As the frequency of one type increases its fitness declines

Question 17

which freshwater snails and parasites are invovled in neg freq dep

Answer 17

freshwater snail P. antipodarum and trematode parasites Microphallus spp.

Question 18

describe the negative frequency dependence of snails and parasites

Answer 18

parasites are best at infecting snails of the same lake (adaptation to local genotype) and best at infecting common clones of snails
*asexual snails, rare clones escape infection from locally adaptive parasites favouring rare genotypes.
clones become more common and are more likely to be infected

Question 19

what are flowers show neg freq dep

Answer 19

Elderflower orchids: D. sambucina

Question 20

how do elderflower orchids show negative frequency dependence

Answer 20

orchids pacakge pollen which is picked up by bumblebees visiting in search of nectar.
orchids may cheat bumbles by producing no nectar in some flowers leading to risk of bumbles learning to avoid empty flowers.

Question 21

what was the experiment showing negative frequency dependence in elderflower orchids

Answer 21

10 plots of 50 flowers with varying Y and P flowers
** genetic polymorphism for flower colour; yellow and purple, yellow is cheat with no nectar.
when Y is rare more fit, more pollena nd bees and more fruit
when Y is common less fit than P, as bees presumably learned to avoid

Question 22

what is prey switching behaviour

Answer 22

rare morphs of prey are fitter as predators concentrate on more frequent morphs`

Question 23

what is positive frequency dependence

Answer 23

selection favours common alele which drives fixation and removes variation

Question 24

give an example of positive frequency dependence

Answer 24

H. erato and H. mepomene butterflies differ widely in warning patterns across geographic locations, (both being distastful) but within one area they share the same patterns. this is because deviations from this pattern are likley to be eaten as they would not be recognised as being distasteful.

Question 25

what is a common polymorphism in British Grove snails, what may be driving this polymorphism and how can it be studied

Answer 25

rich shell polymorphism
predation by birds seems to be main selective force driving polymorphism
often targeted by song thrush bird which snailss have been targeted can be determined by fragments of shell left behind

Question 26

where can grove snails be found

Answer 26

snail colonies cna be found in woodland, hedgerow and grassland

Question 27

what are the three types of selection acting on the Grove snails

Answer 27

1. thrush hunt by sight, so those least adapted to habitat will be preferntially chosen
2. certain heterozygote physiological advantage
3. negative frequency dependent selection as birds take the most common morph, as they are visual predators they target the morph wich has given a good result previously (despite availibility of other morphs)

Question 28

How are the snail polymorphism genes arranged

Answer 28

alleles controlling polymorphism form a super gene with near absolute linkage, which saves the population from a high proportion of undesirable recombinants