Download Population Genetics (Hardy

Population Genetics & Hardy-Weinberg
EQ: How do you determine if a population is evolving or not?
What are the components to the Hardy-Weinberg Principle? If
and component of H-W is violated, what does this infer?
 Population Genetics – the study of how populations change
genetically over time.
 Gene pool: The combined genotypes of all the members of a
population.
 Allele frequency: how many specific alleles occur out of the
total alleles for a gene in a gene pool.
 A population is evolving when allele frequencies change over
time.
Ex. Trait: Hair Texture
Phenotypes: Straight, Wavy, Curly (Incomplete Dominance)
Genotypes: HSHS, HSHC, HCHC
 Generation #1
There are 2 straight haired people, 2 wavy haired people, and 2
curly haired people:
Allele frequency
HS = 3/6 = 50%
HC = 3/6 = 50%
(3 comes from = 3 HS alleles and 3 HC alleles; 6 comes from there are 6 total alleles in
this gene pool)
 Genotypic and phenotypic frequencies can also be analyzed:
Let’s look at phenotypic frequency …
Phenotypic ratio: [there are 3 total genotypes (bottom number), each expressing
a phenotype; out of the 3 genotypes there is 1 straight, 1 wavy, and 1 curly (top
numbers)]
Straight = 1/3
Wavy = 1/3
Curly = 1/3
 If the phenotypic ratio changes from one generation to the
next, it DOES NOT indicate the population is evolving.
 See Generation #2
1 straight, 4 wavy, 1 curly
HSHS,
HSHC, HSHC, HSHC, HSHC,
(straight)
(wavy)
Allele frequency
HS = 6/12 = 50%
HCHC
(curly)
HC = 6/12 = 50%
Phenotypic frequencies
Straight = 1/6
Wavy = 4/6 = 2/3
Curly = 1/6
**Population at Hardy-Weinberg equilibrium (or) …is at Genetic
Equilibrium (or)…is not evolving. [All mean the same thing]
 The allele frequencies must change over time to show a
population that is evolving. [Different combinations of alleles can create
different phenotypes even if the allele frequency doesn’t change. Since the allele
frequency didn’t change from generation #1 to generation #2 this population didn’t
evolve; so population is said to be at Hardy-Weinberg equilibrium.]
 See Generation #3
4 straight, 1 wavy, 0 curly
HSHS, HSHS, HSHS, HSHS,
(Straight)
Allele frequency
HS = 9/10
HSHC
(wavy)
HC 1/10
Phenotypic frequencies
Straight = 4/5
Wavy = 1/5
Curly = 0/5 = 0
So,….since this population’s allele frequency changed from generation #2 to generation
#3, the population has evolved.
 The Hardy-Weinberg Law = Frequencies of alleles in a given
population will remain constant unless acted on by an outside
factor.
 The Hardy-Weinberg Principle suggests reasons why a
population would not evolve.
1. Large population – therefore no genetic drift.
 Genetic drift: random loss of alleles by chance
- Alleles frequency is more constant in large populations
- drift happens in small populations and have drastic
effects
2. No gene flow – no migration
- No immigration (migrating into)
- No emigration (migrating out of [exit])
3. No mutations – no introduction of new alleles
4. Random mating – no sexual selection; no selection of a mate
due to specific traits.
5. No natural selection – If so, then individual carrying alleles
that equip them with genes that express favorable traits will
alter the allele frequencies.