Download Genetics 314 – Spring, 2004

Name: ____________________________________
Genetics 314 – Spring, 2004
Exam 5 – 100 points
1. You have become intrigued with aquaculture, the ‘farming’ of fish for food. You
decide to become a fish breeder and breed designer trout for restaurants. You
discover most of the traits you are interested in are under quantitative control and
have a heritability (h2) of less than .4.
a) What is meant when a trait is under quantitative control?
If a trait is under quantitative genetic control then it is a trait that is controlled
by several to many genes with each gene having only a minor effect on
expression of the trait. Because of the number of genes there are usually no
distinct phenotypes with a quantitative trait, instead you have more of a gradient
of expression from one extreme to the other.
b) How much success will you have in breeding for improved trout with the given
heritability? Briefly explain your answer.
It is possible to make some improvement in the trait but the amount of the trait
under genetic control is relatively low with a h2 of less than .4. This means that
less than 40% of the variation observed for the trait is under genetic control.
2. You decide to breed for rapid growth in a trout population. You have a
population with an average weight gain of 50 grams/month. You want to improve
that to 75 grams/month so you select a population for breeding that has an average
weight gain of 90 grams/month.
a) If you have a heritability (h2) of .33 what would be the increase in average weight
gain/month after one generation?
genetic gain = h2(mean sel pop – mean org pop)
= .33(90 – 50)
= .33 x 40
= 13.2 grams per month or the new population mean would be 63.5
grams per month
b) Did you attain your goal? If not what could you have done differently to achieve
your target average weight gain per month?
No, the goal was 75 grams per month and the new population mean will only be
63.5 grams per month. If possible select a population with a higher mean
average weight gain per month (greater than 90 grams per month).
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Name: ____________________________________
You have decided fish farming is not for you but you want to stay in fish genetics. You
observe that there are several colors of trout possible from iridescent blue to a color best
described as ‘stealth’ because it makes the fish impossible to spot from the surface. You
find a population that these two colors are the primary colors of the fish and that the two
colors, iridescent blue (IB) and stealth (ib), are due to allelic variations at the same gene
locus with iridescent blue being the dominant allele. The following questions will relate
to this population.
3. You discover this population in an underground river and realize that the population
has been under no selection pressure due to its location and that it could be an
example of a population that meets the criteria for being in Hardy-Weinberg
equilibrium. You collect the following data on the fish population:
iridescent blue (IB_) 840
stealth (ibib)
160
a) What is meant when a population is said to be in Hardy-Weinberg equilibrium?
If a population is in Hardy-Weinberg equilibrium then the allelic and genotypic
frequencies should remain constant over generations. It also means that there are
no evolutionary forces at work, there is an unlimited population size, no mating
preference (random mating) and no differences in gamete or progeny viability.
b) If the population is in Hardy-Weinberg equilibrium, please calculate the allelic and
genotypic frequencies for your population.
q2 = 160/1000 = .160 so q would equal the square root of q2 (.16) or .4
If q = .4 then p = (1 – q) = (1 - .4) = .6
So the allelic frequencies are IB = .6 and ib = .4
For genotypic frequencies use the formula p2 + 2pq + q2
IBIB = p2 = .62 = .36
IBib = 2pq = 2 x .6 x .4 = 2 x .24 = .48
ibib = q2 = .42 = .16
4. In studying your color trait you discover the difference between iridescent blue and
stealth is a single base change mutation. Working with a sub-population with allelic
frequencies of IB = .7 and ib = .3 you determine that the forward mutation rate (IB to
ib) is 7 x 10-5 and the back mutation rate (ib to IB) is 5 x 10-6.
a) What is the change in the allelic frequency of ib after one generation?
∆q = up – vq = (7 x 10-5 x .7) – (5 x 10-6 x .3) = 4.9 x 10-5 – 1.5 x 10-6 = 4.75 x 10-5
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Name: ____________________________________
b) If mutation was the only evolutionary force at work, what would the equilibrium
frequency of ib be?
qequil = u / (u + v) = 7 x 10-5 / (7 x 10-5 + 5 x 10-6) = 7 x 10-5 / 7.5 x 10-5 = 7 / 7.5 = 0.933
So the equilibrium frequency for ib would be .933 if mutation was the only
evolutionary force acting on the population.
5. You are curious if the iridescent blue color gives a selective advantage to the trout in
the underground river. You develop molecular markers to differentiate each genotype
and tag an equal number of individuals and follow their survival and reproduction for
one generation. You collect the following data:
genotype
initial pop.
IBIB
IBib
ibib
500
500
500
surviving pop.
total progeny
300
250
400
30,000
10,000
8,000
a) What are the absolute fitness values for each genotype?
genotype
IBIB
IBib
ibib
surviving pop.
ave. no. progeny
absolute fitness
300/500 = .6
250/500 = .5
400/500 = .8
30,000/300 = 100
10,000/250 = 40
8,000/400 = 20
.6 x 100 = 60
.5 x 40 = 20
.8 x 20 = 16
b) What are the relative fitness values for each genotype?
genotype
IBIB
IBib
ibib
absolute fitness/ optimum absolute fitness = relative fitness
60 / 60 = 1
20 / 60 = .33
16 / 60 = .27
c) What are the selection coefficients for each genotype?
genotype
IBIB
IBib
ibib
1 – relative fitness = selection coefficient
1–1=0
1 - .33 = .67
1 - .27 = .73
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Name: ____________________________________
d) How will the genotypic and/or allelic frequencies change due to selection?
The number of homozygous dominant (IBIB) or blue individuals will increase while
the number of heterozygous and homozygous recessive (ibib) individuals will
decrease. This will result in the allelic frequency of IB increasing and the allelic
frequency of ib decreasing.
6. What are the three types of selection and how do they differ?
The three types of selection are directional, stabilizing and destabilizing and they
differ by which genotype is being favored. In directional selection one of the
homozygous genotypes is being selected. In stabilizing the heterozygous genotype is
being favored over the two homozygous genotypes. In destabilizing both
homozygous genotypes are favored and the heterozygous genotype is being selected
against.
7. During exploratory drilling for natural gas in an adjoining wilderness area an above
ground river is accidentally connected with the underground river and trout from the
above ground river migrate into your population before the error is discovered. If
your population of 3000 fish had allelic frequencies of IB = .8 and ib = .2 and 600
above ground fish with allelic frequencies of IB = .9 and ib = .1 migrated into the
underground river;
a) What is the change in allelic frequencies in your population due to migration?
∆p = m(pmig – porg)
migration rate (m) = no. of migrants / total population (no. original pop + migrants)
m = 600 / (3000 + 600) = 600 / 3600 = .1667
∆p = .1667(.9 - .8) = .1667 x .1 = .01667
p = p + ∆p = .8 + .01667 = .81667
q = 1 – p = 1 - .81667 = .1833
b) What two factors control the level of impact of migration?
The two factors that control the level of impact of migration on the allelic
frequencies of a population are the migration rate and the difference in the allelic
frequencies of the two populations.
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Name: ____________________________________
8. A mining enterprise in the wilderness area discovers the underground river upstream
from your population and diverts the water for placer mining. Alarmed by the sudden
drop in water you rush to save as many fish as possible to preserve the population but
you only have enough buckets to save 100 fish. You have three possible subpopulations you can save:
population
1
2
3
no. males
30
40
45
no. females
IB
ib
70
60
55
.7
.6
.9
.3
.4
.1
a) What is the effective population size of each sub-population?
Use the following formulat to calculate effective population size
Ne = (4 x Nm x Nf)/(Nm + Nf)
pop 1 = (4 x 30 x 70) / (30 + 70) = 8,400/100 = 84
pop 2 = (4 x 40 x 60) / (40 + 60) = 9,600/100 = 96
pop 3 = (4 x 45 x 55) / (45 + 55) = 9,900/100 = 99
b) If you wanted to preserve genetic variability in your population which subpopulation would you want to save and why?
To preserve genetic variability you would want the largest population with the
smallest difference in allelic frequencies. Population 2 would be the best population
with an effective population size of 96 and an allelic difference of .2. Population 3
has a slightly larger effective population size but it has the largest difference in
allelic frequencies (.8) of the 3 populations.
9. You are concerned about inbreeding having a detrimental effect on your new
population.
a) What is the potential problem with inbreeding?
Inbreeding allows for the expression of deleterious recessive alleles causing a
reduction in the fitness of a population. This can also lead to a loss of genetic
variability as alleles in the population become fixed or are lost.
b) What factors affect the impact inbreeding will have on allelic frequencies in a
population?
Key factors affecting the impact of inbreeding are effective population size, ratio of
males to females and allelic frequencies on the population.
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Name: ____________________________________
10. The IB allele appears favorable in underground situations but could be considered a
genetic load on the population in above ground rivers. What is meant by genetic load
and is it necessarily bad for the trout population?
A genetic load indicates that a specific allele or genotype causes a reduction in the
overall fitness of a population because it is not considered the optimum genotype for
the population.
This does reduce the fitness of the population but it also maintains genetic diversity
in the population allowing the population to change if the environment (selection
pressure) changes.
11. What three things would need to occur for your iridescent blue trout to become a new
species? Briefly explain your answer.
1. Something needs to cause a change in allelic frequencies such as mutation,
selection or genetic drift.
2. Sub-populations must be geographically isolated so they are in different
environments (different selection pressures) and can not inter-mate.
3. Enough time must be allowed to occur to allow any allelic changes to become
fixed in the different sub-populations.
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