Population Genetics... the Hardy-Weinberg Equilibrium
1908 G.H. Hardy, English mathematician & G. Weinberg, German physician
Law of Genetic Equilibrium...
mathematically describes the gene pool (i.e., all the alleles present)
defines... the ideal case for a NON-evolving population.
for an ideal case... a number of criteria must be met...
- must be an infinitely large populations (large sample sizes)
- should exhibit random mating
- absence of forces of evolution, which can change allele frequencies,
such as... no migration (in/out)
each allele is equally viable (no lethals)
HW law states -->
original percentage of a genotypes alleles remains CONSTANT
HW Equilibrium... is defined algebraically, by the binomial expansion
any gene with 2 allelic forms... A and a
let frequency of one allele (A) = p
& frequency of other allele (a) = q
then by definition, p + q = 1
HW equation... (p + q)2 = p2 + 2 pq + q2 = 1
GG Gg gg
|In population of canes... Orange is dominant (GG) to Green (gg)|
of 1000 canettes, we observe that...
40 are Green (gg) & thus 960 are Orange (GG or Gg)
q2 = freq homozygous recessive (green gg) = 0.04
40/1000 = [0.04] x 1000 = 40
q = freq of recessive allele sq.root of 0.04 = 0.20
p = freq dominant allele [G] = 1 - q = 1 - 0.2 = 0.80
2pq = freq of heterozygote [Gg]
= 2 (0.2) (0.8) = [0.32 x 1000] = 320
p2 = freq of homozygous dominant
= (0.8)2 = [0.64 x 1000] = 640
(an example) cf
you may skip this point on. -
agents of evolution...
But..... Allele frequencies do change over time via....
Mutation... 1/10,000, random, non-directional
Gene Flow... migration of breeders... in/out
Genetic Drift... random loss of alleles - due to failed matings (23.4)
in very small populations, it's a statistical anomaly
can lead to fixation or deletion of alleles
Bottleneck Effect... natural disasters leave survivors
which are not representative of whole population (23.5)
Non-Random Mating... in-breeding lessens heterozygosity...
Founder's Principle... little dispersal - new allele predominates - small human tribes
Selection... better fit individuals are better reproducers
Artificial - animal husbandry selects best - mustards- (22.6)
Natural - acts on individuals of populations
factors that may affect selection...
ABIOTIC - non-living factors
temp, humidity, presence heavy metals, Chernobyl
BIOTIC - living factors
predators, parasites, population density, growth rates
Types of Selection (23.11)
STABILIZING - limits extremes of population
one optimum phenotype - ex: human birth weight
DIRECTIONAL- one best phenotype, not the mean
gradual replacement one by another
DIVERSIFYING (disruptive)- increases the fitness of extremes
no optimum phenotype (2 or more) - patchy environments
ex: sexual dimorphism
How do you win the game of evolution:
the mother of all bumper stickers......
the one who leaves behind the best fit genes, wins...
Cystic Fibrosis (cc)
is a recessive GENETIC DISEASE of childhood that is characterized by
respiratory & digestive problems and is usually fatal.
The average life span of its victims is only about 24 years.
CF symptoms include heavy production of thick mucus in respiratory tracts, which increases
susceptibility to respiratory infections; 90 percent of all patients die of chronic lung disease.
Secretions that block pancreatic ducts cause important digestive enzymes to fail to reach the
small intestine. Treatment is directed toward relief of symptoms, and no cure is yet known.
apparently is caused by the inability of
chloride ions to cross the specialized
of salivary, mucus, and sweat glands and the pancreas. The ductal systems become clogged with
CF is a recessive condition from a mutation to a single gene, which produces a
dysfunctional chloride channel protein.
Gene lies on Chromosome 7
CFTR gene: cystic fibrosis transmembrane conductance regulator protein
mutation: ΔF508 ; deletion of 3 nucelotides at positions 507-508 in the CFTR protein
T ISOLEUCINE 506
T ISOLEUCINE 507
C ---> ISOLEUCINE 507
T PHENYLALANINE 508 --> NO 508
G GLYCINE 509
T VALINE 510
The disease is a commonly inherited disease among
in U.S. population 1/2,500 whites have the disease cc (0.0004 %)
How many carriers Cc are there is U.S. population?
cc is 1/2500 = 0.0004%
thus freq of "c" is sq.root = 0.02%
if freq of
c = 0.02 then freq of
C = 1 - 0.02 =
then freq of 2(Cc) = 2 (.98) (.02) = 0.0392
or 4% of American whites are asymptomatic
carriers of the allele
4 out of 100 whites are carriers (Cc)
1 out of 25 in U.S. Caucasian population is a carrier.
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