All GRE Subject Test: Biology Resources
Example Questions
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?
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?
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 : Genetics, Dna, And Molecular Biology
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?
All three genotypes produce an intermediate phenotype
The homozygotes will have black hair, and the heterozygotes will have gray hair
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
All three genotypes (BB, bb, Bb) produce a different hair color phenotype
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 #1 : Understanding Modes Of Inheritance
Which of the following statements about autosomal-dominant disorders is false?
An individual's risk of inheriting an autosomal dominant disorder is independent from whether or not their sibling has the disorder
There is often a great deal of variability in the severity of the phenotype associated with an autosomal dominant 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
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
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 : 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?
X-linked recessive inheritance
Autosomal recessive inheritance
Autosomal dominant inheritance
Complex inheritance
Mendelian inheritance
Autosomal dominant inheritance
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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