GRE Subject Test: Biology : Genetics, DNA, and Molecular Biology

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

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Example Question #1 : Genetics, Dna, And Molecular Biology

Colorblindness is a recessive sex-linked disease that is caused by a defective gene on the X-chromosome. If a mother who is a carrier for the trait mates with a normal sighted male, what percentage of their daughters will have the disease?

Possible Answers:

Correct answer:

Explanation:

Since colorblindness is a recessive disease, all copies of the X-chromosome must have the diseased allele in order for the person to be colorblind. Daughters have two copies of the X-chromosome: one from the mother and the other from the father. Males only have one copy of the X-chromosome (from the mother) and a Y-chromosome from the father.

Since we know that the father has normal vision, he does NOT carry the colorblind allele. Since the daughters for this couple can only potentially receive one colorblind allele (from the mother), all of their daughters will have normal vision. This means that there is a zero percent chance for colorblindness in their daughters.

The cross would look like this, taking Xb as the colorblind allele:

Parents: XXb x XY

Offspring: XX or XXb (normal daughters), XY (normal son), YXb (colorblind son)

The chance of a colorblind daughter will be zero, but the chance of a colorblind son will be 50%.

Example Question #1 : Genetics, Dna, And Molecular Biology

Hemophilia A is a blood clotting disorder that is usually inherited as an X-linked recessive trait. If a woman who does not have hemophilia A has a daughter with a man who does not have hemophilia A, what is the probability that the daughter will have hemophilia A if the maternal grandfather did have hemophilia A?

Possible Answers:

Correct answer:

Explanation:

X-linked recessive inheritance dictates that expression of themutant phenotype will only occur if the individual is homozygous for the mutation on the X-chromosomes. Therefore, a female must have inherited two mutant X-chromosomes to have hemophilia A, while a male only requires one mutant X-chromosome to have the disorder. By virtue of the father not having hemophilia A, we know the daughter is inheriting at least one wild-type X-chromosome, and therefore there is zero chance she will be homozygous and have hemophilia A.

Example Question #2 : Understanding Modes Of Inheritance

Two parents are heterozygous for an allele that determines hair color for this species. These parents have offspring with the following genotypic ratios:

50% Bb, 25% BB, 25% bb 

The B allele produces black hair, and the b allele produces white hair. However, the phenotypic expression of this gene's traits do not follow Mendelian patterns. If allele B exhibits incomplete dominance, which of the following is true for the offspring?

Possible Answers:

There is not enough information provided to determine the offspring phenotypes

All three genotypes produce black hair

All three genotypes (BB, bb, Bb) produce a different hair color phenotype

The homozygotes will have black hair, and the heterozygotes will have gray hair

All three genotypes produce an intermediate phenotype

Correct answer:

All three genotypes (BB, bb, Bb) produce a different hair color phenotype

Explanation:

Incomplete dominance indicates that there is no dominant allele. In these cases, the phenotype associated with inheriting one copy of each allele (the heterozygotes, Bb) is often a blending of the phenotypes associated with homozygosity of each allele. As such, a genotype of BB will result in black hair, bb will produce white hair, and Bb will result in grey hair.

The incorrect answers are too limited in scope to be cases of incomplete dominance. The correct answer identifies that there will be three unique phenotypes. 

Example Question #4 : Genetics, Dna, And Molecular Biology

Which of the following statements about autosomal-dominant disorders is false?

Possible Answers:

There is often a great deal of variability in the severity of the phenotype associated with an autosomal dominant disorder

An individual with an autosomal dominant mutation has a 50% chance of passing it to their offspring

An individual's risk of inheriting an autosomal dominant disorder is independent from whether or not their sibling has the disorder

If an individual does not have the disorder, they can still pass on the mutant gene if one of their parents has the disorder

Male and female individuals have the same chance of being affected

Correct answer:

If an individual does not have the disorder, they can still pass on the mutant gene if one of their parents has the disorder

Explanation:

Because the disorder is autosomal dominant, the statement "If an individual does not have the disorder, they can still pass on the mutant gene if one of their parents has the disorder" must be false.

If the indivdual in question does not have the disorder, that means they did not inherit ANY copies of the mutant gene, and therefore cannot pass it on.

Example Question #2 : Genetics, Dna, And Molecular Biology

Which of the following inheritance patterns only requires a single copy of a mutant allele for presentation of a specific gene disorder?

Possible Answers:

Autosomal dominant inheritance 

Complex inheritance

Mendelian inheritance 

Autosomal recessive inheritance 

X-linked recessive inheritance

Correct answer:

Autosomal dominant inheritance 

Explanation:

For autosomal dominant disorders, the individual only needs to inherent a single copy of a mutated allele to then show symptoms of that disorder. If it were recessive, both alleles would have to be mutant. X-linked recessive is incompletely correct for males since they only have one X-chromosome, and incorrect for females since 2 copies of the X-chromosome are needed, and thus 2 copies of the allele. Complex inheritance describes situations beyond a single gene, and Mendelian inheritance is not a specific method of inheritance. Note that Y-linked disorders are passed from father to son, and since males only have one copy of the Y-chromosome, if there is a genetic mutation on the Y-chromosome, the individual will be affected.

Example Question #3 : Genetics, Dna, And Molecular Biology

A scientist is working with a breed of dog and has noticed that two traits, ear length and color, behave in normal dominant-recessive hierarchies. Long ears (A) are dominant to short ears (a) and black coloration (B) is dominant to yellow coloration (b). If he breeds a long eared, black dog (AaBb) with a short eared yellow dog (aabb), what would be the resulting phenotypic ratios of the offspring?

Possible Answers:

15 long ears, black : 1 short ears, yellow

9 long ears, black : 3 long ears, yellow : 3 short ears, black : 1 short ears, yellow

1 long ears, black : 1 long ears, yellow : 1 short ears, black : 1 short ears, yellow

9 long ears, black : 3 long ears, yellow : 4 short ears, black

Correct answer:

1 long ears, black : 1 long ears, yellow : 1 short ears, black : 1 short ears, yellow

Explanation:

This question can be solved by making a punnett square. The genotypes are given: AaBb x aabb.

The potential gametes the AaBb dog can produce are AB, Ab, aB, and ab. The aabb dog can only produce one gamete: ab.

Putting these gametes in our punnett square we can see that we end up with the following potential offspring: AaBb, Aabb, aaBb, and aabb.

Each of these possible offspring carries a different phenotype. AaBb will carry both dominant traits and be black with long ears. Aabb will be yellow with long earsaaBb will be black with short ears. Finally, aabb will be yellow with short ears. Each of these gametes is produced in the same ratio, making these phenotypes exist in a 1:1:1:1 probability.

Example Question #1 : Inheritance

Which of the following choices represents information contained in a punnett square?

I. Potential genotype ratios of offspring

II. Possible gametes produced by parent generation

III. Allele frequencies of the population

Possible Answers:

I and II

II and III

I only

I, II, and III

Correct answer:

I and II

Explanation:

Punnett squares give information about the potential genotype ratios of offspring possible from the cross of two members of the parental generation. The letters represent alleles of various genes, but do not give any information about the allele frequencies. To get information about the allele frequencies, more information about the size and make-up of the population would be needed. The actual cross is between potential gametes produced by the parental generation. Each square shows the potential offspring from these potential gametes.

Example Question #4 : Genetics, Dna, And Molecular Biology

Peas in pea plants can be either yellow or green, with yellow being the dominant color. The peas can also be smooth or wrinkled, with smooth being the dominant shape. Suppose that a pea plant that is heterozygous for both traits is self crossed.

What proportion of the next generation will have smooth, green peas?

Possible Answers:

Correct answer:

Explanation:

The shortcut for this problem involves the standard phenotypic ratios for a dihybrid cross. Nine offspring will show both dominant traits. Three will show one dominant trait and the other recessive trait. Three will show the inverse phenotypes, with the opposite dominant trait and recessive trait combination. One offspring will show both recessive traits. Based on these ratios, we can see that three of the sixteen offspring will show the dominant smooth trait and the recessive green phenotype.

We can also solve by using a dihybrid punnett square. The cross described will be AaBb x AaBb, in which the A alleles signify color and the B alleles signify shape.

Consider the color of the peas. In order to have green peas, two recessive alleles must combine in the next generation. According to a punnett square where both sides are heterozygous for the trait, there is only a one in four chance of this taking place. Since smooth is the dominant shape for the peas, a punnett square where each side is heterozygous shows a three in four chance that pea plants will have this shape. By multiplying these two probabilities, we determine that three out of sixteen pea plants will have smooth, green peas.

Example Question #1 : Prokaryotic Genetics

Which answer choice correctly pairs the mode of bacterial genetic transfer with its definition?

Possible Answers:

Transformation—a bacterial cell directly transmits part of its F plasmid to another bacterium via a bridgelike structure called a pilus

 Transduction—a bacteriophage picks up genetic material from one bacterium and then infects another, transferring the material to the second cell

Transformation—a bacteriophage picks up genetic material from one bacterium and then infects another, transferring the material to the second cell

Transduction—a bacterium picks up a piece of genetic material from its external environment

Conjugation—a bacterium picks up a piece of genetic material from its external environment

Correct answer:

 Transduction—a bacteriophage picks up genetic material from one bacterium and then infects another, transferring the material to the second cell

Explanation:

The three main modes of genetic transfer for prokaryotes are transformation, transduction, and conjugation. Transformation occurs when a bacterium picks up a piece of genetic material from its external environment and incorporates it into its own genome. Transduction is genetic transfer using a bacteriophage as a vector. Conjugation is direct gene transfer via sex pili.

Example Question #1 : Genetics, Dna, And Molecular Biology

Frederick Griffith's experiments showed that formerly harmless IIR bacteria had become deadly in mice. This change took place when the harmless IIR bacteria were exposed to the remains of heat killed IIIS bacteria. Based on this experiment, what was the process that converted the harmless bacteria to a deadly strain?

Possible Answers:

Binary fission

Mutation

Conjugation

Transduction

Transformation

Correct answer:

Transformation

Explanation:

The IIR bacteria became deadly when exposed to the remains of the IIIS bacteria. This means that the IIR bacteria managed to receive genetic material from the environment and incorporate it into their genome. This is an example of transformation, a process that results in genetic recombination. In this case, the recombination made the formerly harmless bacteria deadly in mice.

Transduction is the process by which new genetic information is introduced to a bacterium via a vector, such as a bacteriophage. Conjugation is the transfer of genetic material between bacteria via a sex pilus. Binary fission is not a means of recombination; rather, the parent cell divides to produce two identical copies of itself.

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