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Lecture 5

BIO120H1 Lecture Notes - Lecture 5: Genetic Drift, Zygosity, Genetic Variation

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Kenneth Yip

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BIO120 Lecture 5: Monday, September 28, 2015
R.A. Fisher, J.B.S. Haldane, S. Wright found evolution genetics.
They brought genetics into evolutionary biology
Fundamental Measures of Genetic Diversity/Variation:
Polymorphism (P) and Heterozygosis (H)
P = proportion of genes that we sample in a population
There are more than one type of alleles
Fraction of genes that we sample, but a variable
Everybody might have the same gene, and that is not polymorphic.
We have to measure it by taking sample from a large population
H = how many of the genes are present in one individual that are
heterozygous  then average for the population
It gives us a measure of how many heterozygous genes an average
individual have
Mutation (Increase): brings in variability and caused by random
errors during replication of the DNA.
We can lose variability in a population through random genetic drift
(decrease). We lose alleles simply by chance. This is a force that
reduces genetic variability as a result of small population size. Drift is
important when populations become small.
Natural Selection (Increase or decrease depending of the
selection): takes away variability if it is bad. It is a purifying selection.
Positive selection  when a good mutation spreads because it
increases 3tness. The mutation becomes 3xed when every individual
has it.
Balancing Selection  maintains diversity
What Maintains Genetic Variation:
1. Mutation selection balance – less 3t types maintained by
repeated. Recognizes we have two forces coming in  mutation
and selection. Mutation gives variability and selection takes it
out. At any one time, we’re seeing the balance between the two
forces. The strength of selection vs the rate of mutation.
2. Selection – Heterozygote advantage  if a particularly gene
is 3tter than homozygotes, that maintains heterozygote
advantage. We will never have heterozygote going into 3xation,
because there will always be homozygotes. It will maintain
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