Biology 1001A Lecture Notes - Lecture 6: Allele Frequency, Genotype Frequency, Null Hypothesis

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emeraldcattle11

7 May 2018

School

Western University

Department

Biology

Course

Biology 1001A

Professor

Beth Mac Dougall- Shackleton

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18.2: Population Genetics

Populations are made up of individuals of the same species each with its own genotype. In humans (and

all diploid orgaiss, ad idiidual’s geotype icludes to alleles at each gee locus.

The su of all alleles at all gee loci i all idiiduals is called the populatio’s gene pool.

How do scientists describe the structure of the gene pool?  Scientists first identify the genotypes in a

representative sample and calculate genotype frequencies (percentages of individuals possessing each

genotype - 0 < gf < 1). They can then calculate allele frequencies (relative abundance of different alleles)

from the genotype frequencies (0 < af <1 ). For a locus with two alleles, scientists use the symbol p to

identify the frequency of one allele and q to identify the frequency of another allele.

Ex. Table 18.1

Once we have described the genotype and allele frequencies of a population (i.e. the gene pool of a

population), the next question is, is the populatio eolig i.e. are the allele frequencies

chagig?.

When describing experiments, scientists often use controls to see if the experimental treatment had an

effect on the group, but in studies using observational data, there is no suitable control so they use null

models (the null hypotheses states that a particular factor has no effect on a group).

We use a null hypothesis known as the Hardy-Weinberg principle to evaluate whether allele frequencies

of the population for a particular gene locus will never change (change in allele frequencies = evolution)

The Hardy-Weinberg principle does this by stating the conditions under which a population of diploid

organisms achieves genetic equilibrium (the point at which allele frequencies or genotype frequencies

change in succeeding generations).

The conditions under which genetic equilibrium is possible:

1. No mutations are occurring (no mutations)

2. The population is closed to migration from other populations (no immigration/emigration)

3. The population is infinite in size (no genetic drift)

4. All genotypes in the population survive and reproduce equally well (no natural selection)

5. Individuals in the population mate randomly wrt to the trait being considered (no non-random

mating)

If all the conditions are met the allele frequencies of the population for an identified gene locus will

never change and the genotype frequencies will stop changing after one generation. The opposite of

the conditions outlined in the Hardy-Weinberg principle are possible causes of evolution.

Note: If conditions are met, population is not evolving in terms of the identified gene locus, but if

conditions are not met, population could either be evolving or not evolving (the converse of HW

principle is not necessarily true).

Ex. Applications of the Hardy-Weinberg Principle (p.415-416) - Hardy Weinberg Principle States that both

allele frequencies and genotype frequencies remain constant from generation to generation over time if

the population is in genetic equilibrium for a gene locus. This exercise shows that although allele

frequencies did not change between the parent generation and its offspring, the genotype frequencies

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Document Summary

Populations are made up of individuals of the same species each with its own genotype. In humans (and all diploid orga(cid:374)is(cid:373)s(cid:895), a(cid:374)d i(cid:374)di(cid:448)idual"s ge(cid:374)otype i(cid:374)cludes t(cid:449)o alleles at each ge(cid:374)e locus. The su(cid:373) of all alleles at all ge(cid:374)e loci i(cid:374) all i(cid:374)di(cid:448)iduals is called the populatio(cid:374)"s gene pool. Scientists first identify the genotypes in a representative sample and calculate genotype frequencies (percentages of individuals possessing each genotype - 0 < gf < 1). They can then calculate allele frequencies (relative abundance of different alleles) from the genotype frequencies (0 < af <1 ). For a locus with two alleles, scientists use the symbol p to identify the frequency of one allele and q to identify the frequency of another allele. Once we have described the genotype and allele frequencies of a population (i. e. the gene pool of a population), the next question is, (cid:862)is the populatio(cid:374) e(cid:448)ol(cid:448)i(cid:374)g(cid:863) (cid:894)i. e. (cid:862)are the allele frequencies cha(cid:374)gi(cid:374)g? (cid:863)(cid:895).

Related Questions

Consider a single locus with three alleles: A₁, A₂, and A₃.

1) Write down all of the diploid genotypes possible at this locus.

2) Derive the equation for the frequency of allele A₁ as a function of the genotype frequencies.

Consider the following two datasets:

At the M locus:

In a study of 1000 adults, the following genotype numbers were found:

M₁M₁ - 213 individuals

M₁M₂ - 489 individuals

M₂M₂ - 298 individuals

At the S locus:

In a study of 4555 adults, the following genotype numbers were found:

S₁S₁ - 3564 individuals

S₁S₂ - 979 individuals

S₂S₂ - 12 individuals

Assignment: For both loci

Calculate the frequency of the alleles.

Calculate expected number of each genotype if the population were in Hardy-Weinberg equilibrium.

Identify which locus most clearly violates at least one of the H-W assumptions

Q5. A sunflower population in Hardy-Weinberg equilibrium has the following allele frequencies for petal color: 0.75 dominant, 0.25 recessive. If the assumptions of Hardy-Weinberg equilibrium continue to hold, then after 200 generations:

The allele frequencies will be: 0.75 dominant and 0.25 recessive.

The allele frequencies will be: 0.50 dominant and 0.50 recessive.

The allele frequencies will be: 1.0 dominant and 0 recessive. T

here is no way to predict allele frequencies, given the information provided.

Q6. Mead and his colleagues proposed a hypothesis for the apparent heterozygote advantage for the PrP locus. What did they suggest as the basis for the high presence of both alleles?

History of cannibalism.

Protection from malaria.

Protection against plague.

They did not propose a heterozygotic advantage for the PrP locus.

Q7. A frog population has two alleles for skin color at the following relative frequencies: Allele SC1= 0.8, Allele SC2 = 0.2. Allele SC2 is:

Dominant

Recessive

Co-dominant

Impossible to determine dominance from the information given.

Q8. A bird population has two alleles for feather color at the following relative frequencies: Allele FC1 = 0.5, Allele FC2 = 0.5. Allele FC1 is:

Dominant

Recessive

Co-dominant

Impossible to determine dominance from the information given.

Q9. Use the following information for both questions 9 and 10: You encounter a few populations of millipedes with variation in skin color. Genotype can be inferred from coloration: CC millipedes are black, Cc millipedes are brown, and cc millipedes are yellow. [You can launch a calculator byclicking here.] If one millipede population has 60 black individuals, 20 brown individuals, and 20 yellow individuals, what is the relative frequency of allele C in that population?

0.5

0.6

0.7

0.8

Impossible to calculate from the information given.

Q10. If another millipede population's allele frequencies are 0.5 for C and 0.5 for c, what is the expected frequency of the cc genotype in the population, if the population is in Hardy-Weinberg equilibrium?

0.125

0.25

0.5

1.0

Impossible to calculate from the information given.

viridianlocust882

Q1. What are alleles?Different forms of the same gene.Different genes.The different characteristics of an organism.Genes found only in humans.Q2. If you add up the frequency of homozygotes + the frequency of heterozygotes for one gene (i.e. at one locus) in a population, what number do you get?Q3. People who are heterozygous for the sickle-cell allele:Are susceptible to malaria but not sickle-cell anemia.Are susceptible to sickle-cell anemia but not malaria.Are not susceptible to either sickle-cell anemia or malaria.Are susceptible to both sickle-cecell anemia and malaria.Q4. In the case study in this lab, which genotype is represented by "2pq" in the Hardy-Weinberg equation?Homozygous HbAHomozygous HbSHeterozygous HbA/HbSNo specific genotypeQ5. Suppose that, in an isolated village, the proportion of individuals that have sickle-cell anemia is 0.1 (or 1 of every 10 people). What proportion of the population should be sickle-cell carriers, according to Hardy-Weinberg expectations?Q6. When you simulated the elimination of malaria at the beginning of the lab, once the allele frequencies stabilized, what was the frequency of the HbA allele?Q7. When you investigated regional differences, what was the frequency (over many generations) of the HbS allele in the village in the "slightly wet" part of Africa?Q8. What is the number of deaths due to sickle-cell anemia in a region without mosquitoes expected to be?Very lowVery highModerateRelated to the frequency of malaria allelesQ9. As long as there are multiple alleles of a gene in a population, why will the frequencies of the alleles always change over time?Because natural selection changes all allele frequencies in populations over time.Because genetic drift causes random fluctuations of allele frequencies in populations.Because diseases will eliminate some of the alleles, generating fluctuations in their frequencies.Because allele frequencies fluctuate before the alleles eventually become fixed.Q10. Imagine a huge population with a gene that has ten functionally equivalent, neutral alleles. A small but genetically representative group of individuals from the big population is transported to an island, forming a small population. Which of the statements below best describes what will likely happen over time, and why?Because natural selection doesn't act on neutral alleles, the frequencies in the two populations should remain the same over time.Genetic drift will cause one allele to first become fixed in the island population, and later to become fixed in the original, large population.Genetic drift will result in the allele frequencies of the two populations diverging over time, with alleles being lost sooner in the island population.The allele frequencies of the two populations will change similarly due to a combination of genetic drift natural selection.

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Biology 1001A Lecture Notes - Lecture 6: Allele Frequency, Genotype Frequency, Null Hypothesis

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