MCAT Biology : Population Genetics and Hardy-Weinberg

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #21 : Population Genetics And Hardy Weinberg

The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.

The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.

Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.

 

 

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A scientist discovers another genetic disease that has similar symptoms to Prader-Willi Syndrome. He discovers that this disease is recessive, and caused not by changes to chromosome 15, but by a point mutation on chromosome 3. He calculates the recessive allele frequency in a population to be .

Assuming the that normal allele, , and recessive allele, , are the only two alleles in this population, what is the percentage of the population that has the disease?

Possible Answers:

Correct answer:

Explanation:

Based on Hardy-Weinberg principles, we can predict the phenotype and genotype frequencies of a given population based on the equation . In this equation, is the recessive allele frequency and is the recessive phenotype frequency, or frequency of homozygous recessive genotypes.

The genotype frequency of , necessary for the development of a recessive disease, is going to be the square of the recessive allele frequency, .

Example Question #22 : Population Genetics And Hardy Weinberg

A rare recessive mutation causes rabbits that are normally white to be pink. If one in a hundred rabbits is pink, what is the frequency of the pink allele?

Possible Answers:

Correct answer:

Explanation:

We know that the pink allele is recessive and that one out of every hundred is pink; thus, one out of every hundred rabbits is homozygous recessive. Using Hardy-Weinberg calculations, we should be able to calculate the allele frequency.

If the pink allele frequency is , then is ; will refer to the frequency of homozygous recessive individuals in a population.

Using this set up, we can solve for the recessive allele frequency.

Example Question #23 : Population Genetics And Hardy Weinberg

The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.

The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.

Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.

 

 

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A child is diagnosed with a genetic disease, and two years later his parents have another child. The second child also inherited the gene mutation, but does not express the disease. Which of the following is reduced in the second child?

Possible Answers:

Penetrance

Anticipation

Imprinting

Dominance

Recessivity

Correct answer:

Penetrance

Explanation:

The second child has reduced penetrance. Reduced penetrance is found when a given genotype (the presence of the mutation) fails to "penetrate" the phenotype. In other words, even though the second child has the dysfunctional allele, he does not have a clinical disease.

Penetrance refers to the level of dominance. These alleles can still be described as dominant or recessive, but penetrance describes the degree of dominance along a spectrum.

Example Question #24 : Population Genetics And Hardy Weinberg

Researchers are studying a disease that causes neurological deficits in humans. They have identified the disorder as autosomal dominant, but notice that about 20% of people with the dominant disease allele do not express any of the associated neurological impairment. What genetics term explains this phenomenon?

Possible Answers:

Degeneracy of the genetic code

Reduced penetrance

Variable expressivity

Sex-linked interitance

Codominance

Correct answer:

Reduced penetrance

Explanation:

In this example, some individuals with a certain genotype do not express the expected phenotype (symptoms of the disease) at all. The only term that properly describes this effect is reduced penetrance. Penetrance refers to the percent of individuals with a specific genotype who express the associated phenotype. In most common examples given in genetics courses, autosomal dominant diseases are 100% penetrant, meaning that all individuals with one disease allele will show symptoms to some extent. Here, however, the disease appears to show 80% penetrance.

One term often confused with penetrance is expressivity. This refers to the extent that the phenotype is expressed, and is only applicable when penetrance is 100%. If all of the individuals showed symptoms of the disease, but some showed slightly different defects than others, the disease would have variable expressivity. The other answer choices refer to unrelated genetics concepts.

Example Question #25 : Population Genetics And Hardy Weinberg

If forty percent of a Hardy-Weinberg population is phenotypic for an autosomal recessive trait, approximately what percent of the population will be heterozygous?

Possible Answers:

36%

48%

56%

16%

Correct answer:

48%

Explanation:

The phenotype for a Hardy-Weinberg population can be defined as p + q =1, where p and q represent dominant and recessive frequencies. The genotypes for the population can be defined as p2+2pq+q2=1.

In this formula, pand q2 represent the homozygous dominant and homozygous recessive genotype frequencies respectively, and 2pq represents the heterozygous genotype frequency in the population.

Since 40% of the population is phenotypic recessive, we know that 60% of the population is phenotypic dominant.

q = 0.4

p + q =1; p = 0.6

p2 and q2  are equal to 0.16 and 0.36 respectively. Solving for 2pq gives us 0.48, so approximately 48% of the population will be heterozygous.

Example Question #26 : Population Genetics And Hardy Weinberg

The Hardy-Weinberg principle is a tool used to predict the allele and disease frequencies in a given population. If we know the proportion of either the recessive or dominant allele, we can calculate the proportion of population with homozygous dominant, heterozygous, and recessive genotypes using Hardy-Weinberg principles. For example, let’s assume the frequency of the dominant allele for a particular gene is  and that for recessive allele is . Hardy-Weinberg principle states the following:

This principle also allows us to calculate the proportion of genotypes by squaring both sides of the above equation.

Where  is the proportion of homozygous dominant,  is proportion of heterozygous individuals, and  is proportion of recessive individuals. These equations can only be used for populations in Hardy-Weinberg equilibrium.

A researcher is analyzing a group of indigenous people on a remote island. He observes that a rare, autosomal recessive allele is found in 25% of the population. How many people on this island are unaffected by this disease? There are 100 people living on this island.

Possible Answers:

Correct answer:

Explanation:

We need to use the Hardy-Weinberg principle to solve this problem.

Where  is the proportion of dominant allele in a population,  is the proportion of recessive allele,  is proportion of homozygous dominant individuals,  is proportion of heterozygous individuals, and  is proportion of recessive individuals. The question tells us that  and asks us to find the amount of unaffected individuals.

Since the disease is recessive, both homozygous dominant and heterozygous individuals will be unaffected by this disease. Thus we need to find :

This means that about 94 people will be unaffected (out of 100 people).

Example Question #21 : Population Genetics And Hardy Weinberg

The Hardy-Weinberg principle is a tool used to predict the allele and disease frequencies in a given population. If we know the proportion of either the recessive or dominant allele, we can calculate the proportion of population with homozygous dominant, heterozygous, and recessive genotypes using Hardy-Weinberg principles. For example, let’s assume the frequency of the dominant allele for a particular gene is  and that for recessive allele is . Hardy-Weinberg principle states the following:

This principle also allows us to calculate the proportion of genotypes by squaring both sides of the above equation.

Where  is the proportion of homozygous dominant,  is proportion of heterozygous individuals, and  is proportion of recessive individuals. These equations can only be used for populations in Hardy-Weinberg equilibrium.

Which of the following is not an assumption of Hardy-Weinberg principle?

Possible Answers:

Small population size

Diploid organisms

Random mating

Only sexual reproduction occurs in the population

Correct answer:

Small population size

Explanation:

Hardy-Weinberg principle is a tool used to predict the frequency of alleles in a given population; however, it is only valid if the following assumptions are met.

1). no migration or transfer of individuals

2). large population

3). few or no mutations

4). random mating

5). no natural selection

If a population meets these five assumptions, then a it is said to be in Hardy-Weinberg equilibrium. Note that this rarely, if ever, occurs in nature.

Example Question #28 : Population Genetics And Hardy Weinberg

The Hardy-Weinberg principle is a tool used to predict the allele and disease frequencies in a given population. If we know the proportion of either the recessive or dominant allele, we can calculate the proportion of population with homozygous dominant, heterozygous, and recessive genotypes using Hardy-Weinberg principles. For example, let’s assume the frequency of the dominant allele for a particular gene is  and that for recessive allele is . Hardy-Weinberg principle states the following:

This principle also allows us to calculate the proportion of genotypes by squaring both sides of the above equation.

Where  is the proportion of homozygous dominant,  is proportion of heterozygous individuals, and  is proportion of recessive individuals. These equations can only be used for populations in Hardy-Weinberg equilibrium.

Which of the following is true regarding allele and disease frequency?

Possible Answers:

Allele frequency is always higher

Allele and disease frequencies depend on the population size

Allele and disease frequencies depend on the type of allele

Disease frequency is always higher

Correct answer:

Allele and disease frequencies depend on the type of allele

Explanation:

Allele frequency signifies how commonly a particular allele is found in a population whereas disease frequency signifies the prevalence of the disease. Allele frequency is usually designated as  or  for dominant and recessive alleles, respectively. Disease states are determined by the genotype. The disease frequency of the three possible genotypes homozygous dominant, heterozygous, and recessive are designated as , , and , respectively.

To determine which frequency would be higher, we need to first determine whether the disease is recessive or dominant. If the disease is recessive, then the allele frequency will be higher (because the disease frequency will be  whereas the allele frequency will be ) . If the disease is dominant, then the disease frequency will be higher (because disease frequency would be  whereas the allele frequency would just be ).

Example Question #29 : Population Genetics And Hardy Weinberg

The Hardy-Weinberg principle is a tool used to predict the allele and disease frequencies in a given population. If we know the proportion of either the recessive or dominant allele, we can calculate the proportion of population with homozygous dominant, heterozygous, and recessive genotypes using Hardy-Weinberg principles. For example, let’s assume the frequency of the dominant allele for a particular gene is  and that for recessive allele is . Hardy-Weinberg principle states the following:

This principle also allows us to calculate the proportion of genotypes by squaring both sides of the above equation.

Where  is the proportion of homozygous dominant,  is proportion of heterozygous individuals, and  is proportion of recessive individuals. These equations can only be used for populations in Hardy-Weinberg equilibrium.

In a patient, it is observed that a gene on an autosomal chromosome is silenced whereas its homologous counterpart is active. The activated gene codes for a protein that causes a particular type of disease. It is found that 2 percent of the population have this activated gene. What is the incidence of this rare disease in the population?

Possible Answers:

Correct answer:

Explanation:

We need to use Hardy-Weinberg principle for this question.

Where  is the proportion of dominant allele for a disease in a population,  is the proportion of recessive allele,  is the proportion of homozygous dominant individuals,  is the proportion of heterozygous individuals, and  is the proportion of recessive individuals.

The question states that 2 percent of the population have this allele. The question also states that having the disease allele in just one chromosome causes the disease (heterozygous individuals); therefore, the disease must be dominant. So,

The proportion of affected individuals is:

This means that 4 percent of the population will have this disease. 

Example Question #30 : Population Genetics And Hardy Weinberg

The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.

The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.

Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.

 

 

Untitled

A scientist discovers another genetic disease that has similar symptoms to Prader-Willi Syndrome. He discovers that this disease is recessive, and caused not by changes to chromosome 15, but by a point mutation on chromosome 3. He calculates that  of the population is healthy. Assuming the normal allele, , and recessive allele, , are the only two alleles in this population, what is the allele frequency of 

Possible Answers:

Correct answer:

Explanation:

The Hardy-Weinberg equation for this situation is as follows, where  is the dominant allele frequency and  is the recessive allele frequency.

is the frequency of homozygous individuals and is the frequency of heterozygous individuals. We are given the frequency of healthy individuals in the population, , which will be equal to the sum of homozygous dominant and heterozygous individuals.

We want to solve for , so we plug this sum into the equation.

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