Download - Option D: Evolution D4: The Hardy- Weinberg Principle.

Option D:Evolution

D4: The Hardy- Weinberg Principle

• Darwin’s could not explain how inherited variations are maintained in populations - not “trait blending”

• A few years after Darwin’s “Origin of Species”, Gregor Mendel proposed his hypothesis of inheritance: Parents pass on discrete heritable units (genes) that retain their identities in offspring

D 4.1D 4.1 ExplainExplain how the Hardy-Weinberg equation how the Hardy-Weinberg equation is derived.is derived.

• Frequencies of alleles & genotypes in a population’s gene pool remain constant from generation to generation unless acted upon by agents other than sexual recombination (gene shuffling in meiosis)

• Equilibrium = allele and genotype frequencies remain constant


• Hypothetical, non-evolving population▫preserves allele frequencies

• Serves as a model (null hypothesis)▫natural populations rarely in H-W

equilibrium▫useful model to measure if forces are acting

on a population measuring evolutionary change

W. Weinbergphysician

G.H. Hardymathematician


Hardy-Weinberg Theorem:

Hardy-Weinberg theorem•Counting Alleles

▫assume 2 alleles = B, b▫frequency of dominant allele (B) = p ▫frequency of recessive allele (b) = q

frequencies must add to 1 (100%), so: p + q = 1

bbBbBB


• Counting Individuals▫ frequency of homozygous dominant: p x p = p2 ▫ frequency of homozygous recessive: q x q = q2 ▫ frequency of heterozygotes: (p x q) + (q x p) =

2pq frequencies of all individuals must add to 1 (100%), so:

p2 + 2pq + q2 = 1

bbBbBB

Hardy-Weinberg theorem


•Alleles: p + q = 1

•Individuals: p2 + 2pq + q2 = 1

bbBbBB

BB

B b

Bb bb

Hardy-Weinberg theorem


What are the genotype frequencies?What are the genotype frequencies?

q2 (bb): 16/100 = .16

q (b): √.16 = 0.40.4

p (B): 1 - 0.4 = 0.60.6

q2 (bb): 16/100 = .16

q (b): √.16 = 0.40.4

p (B): 1 - 0.4 = 0.60.6

population: 100 cats84 black, 16 whiteHow many of each genotype?

population: 100 cats84 black, 16 whiteHow many of each genotype?

bbBbBB

p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16

Must assume population is in H-W equilibrium!Must assume population is in H-W equilibrium!

D 4.2D 4.2 CalculateCalculate allele, genotype and phenotype allele, genotype and phenotype frequencies for two alleles of a gene, using the frequencies for two alleles of a gene, using the Hardy-Weinberg equation.Hardy-Weinberg equation.

bbBbBB

p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16

Assuming H-W equilibriumAssuming H-W equilibrium

Sampled data Sampled data bbBbBB

p2=.74p2=.74 2pq=.102pq=.10 q2=.16q2=.16

How do you explain the data? How do you explain the data?

p2=.20p2=.20 2pq=.642pq=.64 q2=.16q2=.16

How do you explain the data? How do you explain the data?

Null hypothesis Null hypothesis



•Using the calculated gene frequency to predict the EXPECTED genotypic frequencies in the NEXT generation

OR •to verify that the PRESENT population is

in genetic equilibrium


BB 0.18AB 0.25

AB 0.25AA 0.32

B 0.43

A 0.57

B 0.43A 0.57

Assuming all the individuals mate randomly

SPERMS

EGGSp*p= p2 p*q

p*q q*q= q2


• Close enough for us to assume genetic equilibrium

Genotypes Expected frequencies

Observed frequencies

AA p2 = 0.32 233 747 = 0.31

AB 2pq =0.50 385 747 = 0.52

BB q2 =0.18 129 747 = 0.17

Application of H-W principle•Sickle cell anemia

▫inherit a mutation in gene coding for hemoglobin oxygen-carrying blood protein recessive allele = HsHs

normal allele = Hb

▫low oxygen levels causes RBC to sickle breakdown of RBC clogging small blood vessels damage to organs

▫often lethal

Sickle cell frequency•High frequency of heterozygotes

▫1 in 5 in Central Africans = HbHs

▫unusual for allele with severe detrimental effects in homozygotes 1 in 100 = HsHs

usually die before reproductive age

Why is the Hs allele maintained at such high levels in African populations?Why is the Hs allele maintained at such high levels in African populations?

Suggests some selective advantage of being heterozygous…Suggests some selective advantage of being heterozygous…

Malaria

Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells

Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells

1

2

3

Heterozygote Advantage• In tropical Africa, where malaria is common:

▫homozygous dominant (normal) die or reduced reproduction from malaria: HbHb

▫homozygous recessive die or reduced reproduction from sickle cell anemia: HsHs

▫heterozygote carriers are relatively free of both: HbHs

survive & reproduce more, more common in population

Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.

Hypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite. Frequency of sickle cell allele &

distribution of malaria

Hardy-Weinberg Theorem describes a non-evolving population.

1. Extremely large population size (no genetic drift). 2. No gene flow (isolation from other populations).3. No mutations.4. Random mating (no sexual selection).5. No natural selection.

D 4.3D 4.3 StateState the assumptions made when the the assumptions made when the Hardy-Weinberg equation is used.Hardy-Weinberg equation is used.

• If any of the Hardy-Weinberg conditions are not met microevolution occurs

•Microevolution = generation to generation change in a population’s allele frequencies

D 4.3D 4.3 StateState the assumptions made when the the assumptions made when the Hardy-Weinberg equation is used.Hardy-Weinberg equation is used.

Mutation Gene Flow

Genetic Drift Selection

Non-random mating

1. Mutation & Variation •Mutation creates variation

▫new mutations are constantly appearing•Mutation changes DNA sequence

▫changes amino acid sequence?▫changes protein?

changes structure? changes function?

▫changes in protein may change phenotype & therefore change fitness

2. Gene Flow•Movement of individuals &

alleles in & out of populations▫seed & pollen distribution by

wind & insect▫migration of animals

sub-populations may have different allele frequencies

causes genetic mixing across regions

reduce differences between populations

Human evolution today• Gene flow in human

populations is increasing today▫transferring alleles

between populations

Are we moving towards a blended world?Are we moving towards a blended world?

3. Non-random mating•Sexual selection

Warbler

finch

Tree

finc

hes

Ground finches

4. Genetic drift•Effect of chance events

▫founder effect small group splinters off & starts a new

colony▫bottleneck

some factor (disaster) reduces population to small number & then population recovers & expands again

Founder effect•When a new population is started

by only a few individuals▫some rare alleles may be at high

frequency; others may be missing

▫skew the gene pool of new population human populations that

started from small group of colonists

example: colonization of New World

Bottleneck effect•When large population is drastically

reduced by a disaster▫famine, natural disaster, loss of habitat…▫loss of variation by chance event

alleles lost from gene pool not due to fitness

narrows the gene pool

Cheetahs •All cheetahs share a small number of alleles

▫less than 1% diversity▫as if all cheetahs are

identical twins•2 bottlenecks

▫10,000 years ago Ice Age

▫last 100 years poaching & loss of habitat

Conservation issues•Bottlenecking is an important

concept in conservation biology of endangered species▫loss of alleles from gene pool▫reduces variation▫reduces adaptability

Breeding programs must consciously outcrossBreeding programs must consciously outcross

Peregrine Falcon

Golden Lion Tamarin

5. Natural selection•Differential survival & reproduction

due to changing environmental conditions

climate change food source availability predators, parasites, diseases toxins

▫combinations of alleles that provide “fitness” increase in the population adaptive evolutionary change