MCAT Biology : Population Genetics and Hardy-Weinberg

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #1 : Population Genetics And Hardy Weinberg

Which of the following is not a necessary condition of Hardy-Weinberg equilibrium?

Possible Answers:

Migration of individuals

Large population

Random mating

No mutations

No natural selection

Correct answer:

Migration of individuals

Explanation:

There are five conditions for Hardy-Weinberg equilibrium.

  1. No natural selection
  2. Large population
  3. Random mating
  4. No mutations
  5. No migration

Example Question #1 : Population Genetics And Hardy Weinberg

Which statement best describes the Hardy-Weinberg principle?

Possible Answers:

Recessive alleles eventually disappear in large populations.

When there is a large population, the mechanism of inheritance does not change allele frequencies.

Dominant alleles become more prevelant in large populations.

Expected frequencies of alleles are impossible to predict mathematically.

When a population is large, the allele frequency will change over time.

Correct answer:

When there is a large population, the mechanism of inheritance does not change allele frequencies.

Explanation:

Hardy-Weinberg principle mathematically describes how inheritance does not change allele frequency in large populations. This helps explain why dominant and recessive alleles are both found in populations. A change in the predicted genotypes of a population may indicate evolotuion at work.

Example Question #2 : Population Genetics And Hardy Weinberg

All of the following are important aspects of Hardy-Weinberg equilibrium EXCEPT __________.

Possible Answers:

mating patterns

migration

population size

temperature

mutations

Correct answer:

temperature

Explanation:

Hardy-Weinberg equilibrium is acheived when the gene frequencies in a population do not change over time. This means the population is not evolving. The five conditions for this are large population size, no mutations, random mating, no net migration, and equally successful reproduction potential for all genes in the population. Temperature is a directly important aspect of this.

Example Question #3 : Population Genetics And Hardy Weinberg

Which of the following conditions are required for a population to be in Hardy-Weinberg equilibrium?

Possible Answers:

No gene flow between populations

All of these are conditions of Hardy-Weinberg equilibrium

Large population

Random mating

No mutations

Correct answer:

All of these are conditions of Hardy-Weinberg equilibrium

Explanation:

The correct answer is "all of these." The choices listed are all required assumptions for a population to achieve Hardy-Weinberg equilibrium. "No natural selection" is another condition that is not listed here, bringing the total to 5 conditions.

Example Question #4 : Population Genetics And Hardy Weinberg

Cryptosporidium is a genus of gastrointestinal parasite that infects the intestinal epithelium of mammals. Cryptosporidium is water-borne, and is an apicomplexan parasite. This phylum also includes Plasmodium, Babesia, and Toxoplasma. 

Apicomplexans are unique due to their apicoplast, an apical organelle that helps penetrate mammalian epithelium. In the case of cryptosporidium, there is an interaction between the surface proteins of mammalian epithelial tissue and those of the apical portion of the cryptosporidium infective stage, or oocyst. A scientist is conducting an experiment to test the hypothesis that the oocyst secretes a peptide compound that neutralizes intestinal defense cells. These defense cells are resident in the intestinal epithelium, and defend the tissue by phagocytizing the oocysts. 

She sets up the following experiment:

As the neutralizing compound was believed to be secreted by the oocyst, the scientist collected oocysts onto growth media. The oocysts were grown among intestinal epithelial cells, and then the media was collected. The media was then added to another plate where Toxoplasma gondii was growing with intestinal epithelial cells. A second plate of Toxoplasma gondii was grown with the same type of intestinal epithelium, but no oocyst-sourced media was added.

You are conducting a study of an isolated tribe in New Guinea, and you find that there is widespread resistance to cryptosporidium infection. You determine that the gene for resistance is inherited in a recessive fashion. The incidence of resistance in a normal population is 1/900. In New Guinea, it is 1/25. What are the carrier frequencies in the normal population and in New Guinea, respectively? Assume that the populations are in Hardy-Weinberg equilibrium.

Possible Answers:

58/300, 8/25

58/900, 8/25

58/900, 1/625

58/300, 1/25

2/900, 2/5

Correct answer:

58/900, 8/25

Explanation:

The Hardy-Weinberg equilibrium expression says that p2+2pq+q2 = 1.

We know that the incidence of q2 (getting two recessive alleles, and thus being resistant) is 1/900 in a general population, and 1/25 in New Guinea. The recessive allele frequency, q, will be 1/30 and 1/5, respectively.

The carrier frequency is 2pq, where p = 1-q.

Using this information, we can find the respective carrier frequencies.

General population:

New Guinea:

Example Question #5 : Population Genetics And Hardy Weinberg

Cryptosporidium is a genus of gastrointestinal parasite that infects the intestinal epithelium of mammals. Cryptosporidium is water-borne, and is an apicomplexan parasite. This phylum also includes Plasmodium, Babesia, and Toxoplasma. 

Apicomplexans are unique due to their apicoplast, an apical organelle that helps penetrate mammalian epithelium. In the case of cryptosporidium, there is an interaction between the surface proteins of mammalian epithelial tissue and those of the apical portion of the cryptosporidium infective stage, or oocyst. A scientist is conducting an experiment to test the hypothesis that the oocyst secretes a peptide compound that neutralizes intestinal defense cells. These defense cells are resident in the intestinal epithelium, and defend the tissue by phagocytizing the oocysts. 

She sets up the following experiment:

As the neutralizing compound was believed to be secreted by the oocyst, the scientist collected oocysts onto growth media. The oocysts were grown among intestinal epithelial cells, and then the media was collected. The media was then added to another plate where Toxoplasma gondii was growing with intestinal epithelial cells. A second plate of Toxoplasma gondii was grown with the same type of intestinal epithelium, but no oocyst-sourced media was added.

You are conducting a study of an isolated tribe in New Guinea, and you find that there is widespread resistance to cryptosporidium infection. Upon historical investigation, you find that the population you were studying all derived from a single group of four people that landed on the island 2000 years ago. Which phenomenon is most likely responsible for the observations of cryptosporidum resistance?

Possible Answers:

Balanced heterozygosity

Penetrance

Founder effect

Genetic bottleneck

Pleiotropy

Correct answer:

Founder effect

Explanation:

The founder effect is the abnormal abundance of an allele in a population derived from a small initial population. If, by chance, the initial population had an abnormal abundance of a certain allele, this abnormality will generally persist for future generations.

Example Question #2 : Population Genetics And Hardy Weinberg

Cryptosporidium is a genus of gastrointestinal parasite that infects the intestinal epithelium of mammals. Cryptosporidium is water-borne, and is an apicomplexan parasite. This phylum also includes Plasmodium, Babesia, and Toxoplasma. 

Apicomplexans are unique due to their apicoplast, an apical organelle that helps penetrate mammalian epithelium. In the case of cryptosporidium, there is an interaction between the surface proteins of mammalian epithelial tissue and those of the apical portion of the cryptosporidium infective stage, or oocyst. A scientist is conducting an experiment to test the hypothesis that the oocyst secretes a peptide compound that neutralizes intestinal defense cells. These defense cells are resident in the intestinal epithelium, and defend the tissue by phagocytizing the oocysts. 

She sets up the following experiment:

As the neutralizing compound was believed to be secreted by the oocyst, the scientist collected oocysts onto growth media. The oocysts were grown among intestinal epithelial cells, and then the media was collected. The media was then added to another plate where Toxoplasma gondii was growing with intestinal epithelial cells. A second plate of Toxoplasma gondii was grown with the same type of intestinal epithelium, but no oocyst-sourced media was added.

You are conducting a study of an isolated tribe in New Guinea, and you find that there is widespread resistance to cryptosporidium infection. You determine that the population is in Hardy-Weinberg equilibrium. Which of the following is true of this population?

I. There is random mating

II. There is no immigration or emigration

III. There is a constant rate of mutation

Possible Answers:

I, II, and III

I and II

I and III

I only

III only

Correct answer:

I and II

Explanation:

Hardy-Weinberg equilibrium states that there is random mating, no immigration/emigration, and that there are no mutations.

Example Question #7 : Population Genetics And Hardy Weinberg

Which of these populations could meet the criteria required for Hardy-Weinberg equilibrium?

Possible Answers:

A group of about 40 swans displays random mating and does not migrate to areas with other swan populations.

A population of over 50,000 elephants routinely travels from the western region of Africa to a more central region to deal with seasonal food supply fluctuations.

A very large group of people has a near-zero mutation rate, no migration, and a tendency to marry their first cousins.

A population of about 100,000 wild cats mate randomly and stay in the same area. Their mutation rate is negligible and their environment contains no factors that select for specific traits.

Finches randomly mate on a small island in the Caribbean. These birds eat hard-shelled seeds, so finches with short, strong beaks experience directional selection.

Correct answer:

A population of about 100,000 wild cats mate randomly and stay in the same area. Their mutation rate is negligible and their environment contains no factors that select for specific traits.

Explanation:

To meet Hardy-Weinberg criteria, a population must be very large (preferably infinite) and exhibit no mutation, no net migration, no natural selection, and no non-random mating. Of the choices, all break one of these criteria except the large population of wildcats.

Example Question #2 : Population Genetics And Hardy Weinberg

A species of birds off the coast of Africa follows Hardy-Weinberg population principles in determining beak color. The dominant phenotype is represented by a black beak, while the recessive phenotype is represented by a grey beak.

If half of the population carries the recessive allele, what percentage of the birds have black beaks? (Assume complete dominance)

Possible Answers:

25%

50%

75%

65%

Correct answer:

75%

Explanation:

If 50% of the population carries the recessive allele, then 50% carry the dominant allele. To determine the genotype breakdown we use the equation p2 + 2pq + q2, where prepresents homozygous dominant genotype, 2pq represents heterozygous genotype, and q2 represents homozygous recessive genotype. The question tells us the value of allele frequency for the recessive allele, giving us the value of q in this equation. Since p + q = 1, and q is 0.50, p must also be 0.50.

p2 = 0.25

2pq = 0.50

q2 = 0.25

Setting p and q both to 0.50 gives us 25% homozygous dominant, 50% heterozygous, and 25% homozygous recessive; therefore, 75% of the population will display the dominant phenotype (black beak), while 25% will display the recessive phenotype (grey beak). Remember that both homozygous dominant and heterozygous genotypes will display the dominant phenotype.

Example Question #9 : Population Genetics And Hardy Weinberg

 

Which of the following populations cannot be in Hardy-Weinberg equilibrium?

Possible Answers:

A population with no gene flow

A population with no selection 

A randomly mating population

A small population

A population with no mutation

Correct answer:

A small population

Explanation:

By definition, the Hardy-Weinberg principle states that genotype and allele frequencies will remain constant throughout generations. In order for equilibrium to occur, there must be a large, randomly mating population with no selection, genetic drift, migration, or mutation. A small population cannot be in Hardy-Weinberg equilibrium.

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