GRE Subject Test: Biology : Understanding Modes of Inheritance

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

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Example Questions

Example Question #1 : Understanding Modes Of Inheritance

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 : Understanding Modes Of Inheritance

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 #3 : 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:

All three genotypes produce black hair

All three genotypes produce an intermediate phenotype

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

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

There is not enough information provided to determine the offspring phenotypes

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 : Understanding Modes Of Inheritance

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

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

An individual's risk of inheriting an autosomal dominant disorder is independent from whether or not their sibling 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 #5 : Understanding Modes Of Inheritance

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 

Mendelian inheritance 

Autosomal recessive inheritance 

X-linked recessive inheritance

Complex 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.

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