1. Due today – Phylogenetic Trees & Variation & Gene Pools
AP Bio Learning Target – Illustrate gene pool dynamics & microevolution
Due today – Phylogenetic Trees & Variation & Gene Pools
Chapter 21 Reading Guide due Monday

2. There are 5 agents of evolutionary change
Mutation
Gene Flow
Non-random mating
Genetic Drift
Selection

3. Populations & gene pools
a population is a localized group of interbreeding individuals
gene pool is a collection of alleles in a particular population (remember difference between alleles & genes!)
allele frequency is how common that allele is in the population (how many of A or a in whole population)

4. Evolution of populations
Evolution is a change in allele frequencies in a population
What conditions would cause allele frequencies not to change?
In a non-evolving population you would need to remove all agents of evolutionary change
- very large population size (no genetic drift)
- no migration (no gene flow in or out)
- no mutation (no genetic change)
- random mating (no sexual selection)
- no natural selection (everyone is equally fit)

5. Hardy-Weinberg equilibrium
In a non-evolving population allele frequencies are preserved (they are said to be in Hardy-Weinberg equilibrium), but:
natural populations are rarely in Hardy-Weinberg equilibrium
However it provides a useful model to measure if evolutionary forces are acting on a population
G.H. Hardy (the English mathematician) and W. Weinberg (the German physician) independently worked out the mathematical basis of population genetics in Their formula predicts the expected genotype frequencies using the allele frequencies in a diploid Mendelian population. They were concerned with questions like "what happens to the frequencies of alleles in a population over time?" and "would you expect to see alleles disappear or become more frequent over time?"
G.H. Hardy
mathematician
W. Weinberg
physician

6. Hardy-Weinberg theory
Counting Alleles
assume 2 alleles = B, b
frequency of dominant allele (B) = p
frequency of recessive allele (b) = q
frequencies must add up to 1 (100%), so:
p + q = 1 BB Bb bb

7. Hardy-Weinberg theory
Counting Genotypes
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 BB Bb bb

8. H-W formulas Alleles: p + q = 1 Genotypes: p2 + 2pq + q2 = 1 B b BB Bb

9. Using Hardy-Weinberg equation
population: 100 cats
84 black, 16 white
How many of each genotype?
First calculate frequency of b from the known number of bb genotypes. BB Bb bb

10. Using Hardy-Weinberg equation
population: 100 cats
84 black, 16 white
How many of each allele?
q2 (bb)= 16/100 = .16
q (b): √.16 = 0.4
Now work out p
p (B)= = 0.6 BB Bb bb

11. Using Hardy-Weinberg equation
population: 100 cats
84 black, 16 white
How many of each genotype?
p2 + 2pq + q2 = 1
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): = 0.6
Now work out genotype frequencies
p2=0.36
2pq=0.48
q2=0.16 BB Bb bb

12. How to Solve H-W Problems
(p + q)2 = p2 + 2pq + q2 = 1 B b B b
Frequency of allele types
p = Frequency of allele B
q = Frequency of allele b
Frequency of allele combinations
p2 = Frequency of BB (homozygous dominant)
2pq = Frequency of Bb (heterozygous)
q2 = Frequency of bb (homozygous recessive) BB Bb bb
Frequency of allele combination BB in the population = p2
Frequency of allele combination bb in the population = q2
Frequency of allele combination Bb in the population (add these together to get 2pq)

13. How to Solve H-W Problems
Remember to use proportions in your calculations, not percentages!
Examine question to determine what information is given. In most cases this is the frequency of the homozygous recessive phenotype q2 or the allele q
Take the square root of q2 to find q or multiply q to find q2
Find p by subtracting q from 1 (p = 1 – q)
Find p2 by multiplying it by itself (p2 = p x p)
Find 2pq by multiplying p x q x 2
Check that your calculations are correct by adding values for p2 + q2 + 2pq (the sum should be 1)

14. A population of mice has a gene consisting of 90% B alleles (black fur) and 10% b alleles (gray fur). Determine the proportion of offspring that will be black and the proportion that will be gray.
Recessive allele q = 0.1
Dominant allele p =
Recessive phenotype q2 =
Homozygous dominant p2 =
Heterozygous 2pq =

15. A population of mice has a gene consisting of 90% B alleles (black fur) and 10% b alleles (gray fur). Determine the proportion of offspring that will be black and the proportion that will be gray.
Recessive allele q = 0.1 Given
Dominant allele p = – q = p – 0.1 = 0.9
Recessive phenotype q2 = 0.01 q x q = q x 0.1 = 0.01
Homozygous dominant p2 = 0.81 p x p = p x 0.9 = 0.81
Heterozygous 2pq = x p x q = 2pq
2 x 0.9 x 0.1 = 0.18
Check q2 + p2 + 2pq = = 1

16. 1% of the population will be gray
A population of mice has a gene consisting of 90% B alleles (black fur) and 10% b alleles (gray fur). Determine the proportion of offspring that will be black and the proportion that will be gray.
Recessive allele q = 0.1
Dominant allele p = 0.9
Recessive phenotype q2 = 0.01
Homozygous dominant p2 = 0.81
Heterozygous 2pq = 0.18
1% of the population will be gray
99% of the population will be black

17. A population of 134 lizards has 81 individuals with green skin and a gg genotype. The remaining 53 individuals have yellow skin and therefore have either the GG or Gg genotype. What proportion of the population are homozygous dominant? Recessive allele q = Dominant allele p = Recessive phenotype q2 = /134 = .60 Homozygous dominant p2 = Heterozygous 2pq =

18. A population of 134 lizards has 81 individuals with green skin and a gg genotype. The remaining 53 individuals have yellow skin and therefore have either the GG or Gg genotype. What proportion of the population are homozygous dominant? Recessive allele q = 0.77 √0.60 = 0.77 Dominant allele p = = 0.23 Recessive phenotype q2 = /134 = 0.60 Homozygous dominant p2 = x 0.23 = 0.05 Heterozygous 2pq = x 0.77 x 0.23 = = 1

19. Hardy Weinberg problems
In humans, the ability to taste the chemical phenylthiocarbamide (PTC) is inherited as a simple dominant characteristic. You find that 360 out of 1000 college students could not taste the chemical. What is the frequency of the allele for tasting PTC? What percentage of students in this population are heterogynous?

20. Hardy Weinberg problems
While working with pea plants you find that 24 plants out of 400 exhibit the recessive dwarf trait. What is the frequency of the tall gene? What percentage of the plants have the recessive allele?

21. Hardy Weinberg problems
Albinism is recessive to normal pigmentation in humans. The frequency of the albino allele was 10% in a population. Determine the proportion of people that you would expect to be albino.

22. Gene Pool Dynamics & MicroevolutionAa AA AA Aa Aa AA AA AA Aa AA Aa AA AA

23. Gene Pool Dynamics & Microevolutionaa Aa AA AA Aa aa
A’A aa Aa aa AA AA AA Aa AA Aa AA aa AA

24. Gene Pool Dynamics & Microevolution
One aspect of gene flow is immigration & emigration – alleles may be gained from or lost to other gene pools aa Aa AA AA Aa aa aa Aa
A’A aa AA AA AA Aa AA Aa AA aa AA
Spontaneous mutations can alter allele frequencies and create new alleles. Important to evolution – original source of variation providing new material for natural selection
What is the other source of variation in a population?

25. Gene Pool Dynamics & Microevolution
One aspect of gene flow is immigration & emigration – alleles may be gained from or lost to other gene pools aa Aa AA AA Aa aa aa Aa
A’A aa AA AA Aa AA Aa AA aa AA
Selection pressure against certain allele combinations may reduce reproductive success or cause death. Natural selection accumulates and maintains favorable genotypes, reduces genetic diversity within gene pools, and increases differences between populations

26. Gene Pool Dynamics & Microevolution
Deme 1 aa Aa AA AA Aa aa aa Aa
A’A aa AA AA AA Aa AA Aa AA aa AA
Deme describes a local population that is genetically isolated from other populations. Usually have clearly definable genetic or other character that sets them apart.

27. Gene Pool Dynamics & Microevolution
Geographical barriers isolate the gene pool and prevent regular gene flow between populations
Gene flow between populations can be the source of new genetic variation aa Aa AA aa Aa aa aa Aa aa AA Aa AA Aa AA aa aa AA

28. Gene Pool Dynamics & Microevolutionaa Aa AA aa Aa aa aa Aa aa AA Aa AA Aa AA aa aa AA
Mate choice (non-random mating): Individuals may not select mates randomly, seeking particular phenotypes, increasing the frequency of these “favored” alleles in the population.

29. Gene Pool Dynamics & Microevolutionaa Aa AA aa Aa aa aa Aa aa AA Aa
Genetic drift: Chance events can cause the allele frequency of small populations to change randomly from generation to generation. Can play a significant role in the microevolution of small populations. The founder effect (small population colonizes new area) and the bottleneck effect (population size dramatically reduced by catastrophic event) AA Aa AA aa AA

30. Founder effect

31. Bottleneck effect

32. Bottleneck effect Cheetahs ~ 20,000 Very little genetic diversity
Nearly went extinct at end of last ice age
Lack of variation creates problems – sperm abnormalities, decreased fecundity, high cub mortality, sensitivity to disease

33. Application of H-W principle
Sickle cell anemia
Caused by inheriting a mutation in the gene coding for haemoglobin
oxygen-carrying blood protein
recessive allele = HsHs
normal allele = Hb
low oxygen level causes RBC to sickle
clogging small blood vessels
depriving tissues of oxygen
damage to organs
The condition is often lethal

34. 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
Sickle Cell:
In tropical Africa, where malaria is common, the sickle-cell allele is both an advantage & disadvantage. Reduces infection by malaria parasite.
Cystic fibrosis:
Cystic fibrosis carriers are thought to be more resistant to cholera:
1:25, or 4% of Caucasians are carriers Cc
Why is the Hs allele maintained at such high levels in African populations?
Suggests some selective advantage of being heterozygous…

35. Malaria
Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells 1 2 3

36. 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
Frequency of sickle cell allele & distribution of malaria

37. Any Questions??