BIOL 336 Chapter Notes -Genetic Drift, Blood Orange

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26 Jan 2013
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Pre-Tutorial Assignment #3
BIOL 336 Section 202
Cassandra Chan 60463080
January 25h, 2013
1. Selection acts directly on phenotypes, as it is phenotypic variation that creates differences
between survival rates and reproductive success of individuals. However, phenotypes are a
result of genotypes interacting with the environment; and genotypes are combinations of alleles.
Therefore, selection acts indirectly on genotypes and alleles.
2. An allele does not have a phenotype on a diploid organism because diploid organisms require at
least two alleles of a particular gene; even then, an expression of one particular gene does not
necessarily give rise to a phenotype. Phenotypes are usually a combination of different alleles
and their interactions with the environment. In haploid organisms, it is possible that a single
allele may be responsible for a particular phenotype if it is the only other allele in a population
of haploid organisms that shows a difference in phenotype.
3.
Run 1 Run 2 Run 3
w11 1.22 1.22 1.22
w12 1 1.22 1.11
w22 1 1 1
Colour Red Orange Yellow-Green
Fitness Effect Recessive Dominant Additive
Reasoning Since we set the population size to be infinite, we eliminate the probability
of genetic drift occurring. In Run 1, w11 is 1.22 so has an advantage over
A1A2 and A2A2 genotypes. However this phenotype is recessive, requiring
2 copies of allele A1. Therefore the number of A1 alleles is low in the
beginning and most likely found in heterozygotes. Allele A1 rapidly
increases once drift or some mechanism increases their frequency slightly,
and then selection overtakes the curve. In Run 2, allele A1 is dominant over
allele A2, so heterozygotes as well as allele A1 homozygotes dominate the
population initially as dominant alleles are exposed to selection
immediately. However, as the dominant allele increases in frequency, the
recessive allele will eventually be “hidden” in the population in
heterozygotes. Therefore, the allele A1 (the dominant allele in this case) will
never quite reach fixation in the population. In Run 3, allele A1 is an
additive allele as 2 copies of the allele bring in twice the phenotypic effect.
Additive alleles are always exposed to selection as having two copies is
always more beneficial (in this case), so allele A1 increases steadily in the
population until it reaches fixation.
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Document Summary

60463080: selection acts directly on phenotypes, as it is phenotypic variation that creates differences between survival rates and reproductive success of individuals. However, phenotypes are a result of genotypes interacting with the environment; and genotypes are combinations of alleles. Phenotypes are usually a combination of different alleles and their interactions with the environment. In haploid organisms, it is possible that a single allele may be responsible for a particular phenotype if it is the only other allele in a population of haploid organisms that shows a difference in phenotype. Since we set the population size to be infinite, we eliminate the probability of genetic drift occurring. In run 1, w11 is 1. 22 so has an advantage over. Therefore the number of a1 alleles is low in the beginning and most likely found in heterozygotes. Allele a1 rapidly increases once drift or some mechanism increases their frequency slightly, and then selection overtakes the curve.

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