Chp 17: selection &
evolution
“Genetic drift & Hardyweinberg”
1
Recap definitions
GENOTYPE: Allele possessed by an organism
ALLELE: Different varieties of the same gene
HOMOZYGOUS: having 2 identical alleles of a gene (same genotype)
HETEROZYGOUS: Having 2 different alleles of a gene (different genotype)
GENE POOL: All the alleles of all genes within a population at one time
POPULATION: All the individuals of one species in one area at one time
ALLELE FREQUENCY: The proportion of an allele within the gene pool
2
Genetic drift
what
: A change in allele frq that occurs by chance, bcs
only some of the organisms of each generation
reproduce
5
How to know ?
✗ Small no. Of individuals are seperated from the
rest of a large population
✗ The separated population only have a a small
sample of ori population
✗ Thus, allele frq of separated & ori are different
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What happens if genetic drift continues?
✗ Allele frw changes more
✗ Evolution occur
7
Types of genetic drift
✗ Founder Effect
: small population alter
allele frequencies further
& evolution occur in a
different direction from
original large population,
forming an isolated small
population.
✗ Bottle Neck Effect
: population sharply
decreases in size which
leads to change in allele
frequency
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9
3
HARDY-WEINBERG PRINCIPLE
what
✗ Allele & genotype frequency that remain
constant from generation to generation in the
absence of other evolutionary influences
( NO SELECTION PRESSURE )
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HARDY-WEINBERG EQUATIONS
p – frq of dominant allele
p+q=1
q – frq of recessive allele
p2 + 2pq + q2 = 1
2pq - frq of
heterozygous genotype
p2 - frq of homozygous
dominant genotype
q2 - frq of homozygous
recessive genotype
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Principles does not apply when...
✗ Small population
✗ New mutation
✗ Natural/artificial/sexual
✗ Migration of individuals
selection
✗ Selective pressure against
1 of the genotypes
✗ Migration
✗ Overlapping generations
✗ Non-diploid organism
✗ Asexual reproduction
✗ Non-random
mating/inbreeding
13
4
exercises
✗ Cystic Fibrosis (CF) is caused by
a recessive allele.
✗ 0.02% of the UK population
suffer from CF. What proportion
of the UK are carriers?
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