AP Biology
5 Agents of evolutionary changeMutation Gene Flow
Genetic Drift Selection
Non-random mating
AP Biology
Populations & gene pools Concepts
a population is a localized group of interbreeding individuals
gene pool is collection of alleles in the population remember difference between alleles & genes!
allele frequency is how common is that allele in the population how many A vs. a in whole population
AP Biology
Evolution of populations Evolution = change in allele frequencies
in a population hypothetical: what conditions would
cause allele frequencies to not change? non-evolving population
REMOVE all agents of evolutionary change
1. very large population size (no genetic drift)
2. no migration (no gene flow in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is equally fit)
AP Biology
Hardy-Weinberg equilibrium 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
AP Biology
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
AP Biology
Hardy-Weinberg theorem 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
AP BiologyWhat are the genotype frequencies?What are the genotype frequencies?
Using Hardy-Weinberg equation
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!
AP Biology
Using 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
AP Biology
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
AP Biology
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…
AP Biology
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
AP Biology
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
AP Biology
Practice Question A population called the “founder generation”, consisting of
2000 AA individuals, 2000 Aa individuals, and 6000 aa individuals is established on a remote island. Mating within this population occurs at random, the three genotypes are selectively neutral, and mutations occur at a negligible rate.
a) What are the frequencies of the alleles A and a in the founder generation?
b) Is the founder generation at Hardy-Weinberg Equilibrium?
c) What is the frequency of the A allele in the second generation (that is, the generation subsequent to the founder generation)?
d) What are the frequencies of the AA, Aa, and aa genotypes in the second generation?
AP Biology
For each of the following problems in population genetics use the Hardy-Weinberg equation. Show all of
your work and label each frequency, probability, and allele.
1. Suppose that in a breeding experiment, 7,000 AA individuals and 3,000 aa individuals mate at random. In the first generation of offspring, what would be the frequencies of the three genotypes (AA, Aa, and aa)? What would be the frequencies of the two alleles? What would be the values in the second generation?
AP Biology
2. Among African-Americans, the frequency of sickle-cell anemia (which, as you will recall is a homozygous recessive condition) is about 0.0025. What is the frequency of heterozygotes? When on African-American marries another, what is the probability that both will be heterozygotes? If both are heterozygotes, what is the probability that their first child will have sickle-cell anemia?
AP Biology
3. If q = 0.3 and there are Hardy-Weinberg proportions, what is the most common genotype and what is its frequency? What is the least frequent genotype and its frequency?
AP Biology
4. In a large, randomly mating population with no forces acting to change gene frequencies, the frequency of homozygous recessive individuals for the character extra-long eyelashes is 90 per 1000, or 0.09. What percentage of the population carries this trait but displays the dominant phenotype, short eyelashes? Would the frequency of the extra-long-lash allele increase, decrease, or remain the same if long-lashed individuals preferentially mated with each other and no one else?
Top Related