GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with Mendelian Inheritance

Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology

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Example Questions

Example Question #1 : Inheritance

In a species of ant, black coloration is dominant to white. A scientist is operating under the assumption that this gene follows basic Mendelian principles; however, after crossing two heterozygotes he obtained a ratio of 2:1 of dominant to recessive offspring. Which of the following could explain this result?

Possible Answers:

The alleles display incomplete dominance

The homozygous dominant phenotype is lethal

The alleles display codominance 

The homozygous recessive phenotype is lethal

Correct answer:

The homozygous dominant phenotype is lethal

Explanation:

The only answer that properly explains a 2:1 ratio is if the homozygous dominant phenotype is lethal. In our punnett square, this would give a two-thirds chance for a heterozygous offspring and a one-third chance for a recessive offspring. We know the recessive phenotype is not lethal because homozygous recessive offspring were produced.

Parents: Aa x Aa

Offspring genotypes: AA, Aa, Aa, aa

Offspring phenotypes: lethal, dominant, dominant, recessive (only three live offspring produced)

Offspring ratio: 2 dominant to 1 recessive

There is also no evidence that codominance or incomplete dominance is present. If black and white spotted offspring or gray offspring were produced then these theories would have merit.

Example Question #1 : Help With Mendelian Inheritance

A scientist has been working with a new species of plant. He has found that there are two separate genes, which segregate according to standard Mendelian genetics, that are capable of producing the same phenotype. A single dominant allele from either gene confers red coloration of the plant's flowers. Without any dominant alleles the flowers are white. If he crosses two plants heterozygous for both traits, what will be the resulting phenotypic ratios of the offspring?

Possible Answers:

Correct answer:

Explanation:

This problem requires a standard dihybrid cross. The crossed genotypes are AaBb x AaBb. This results in a phenotypic ratio of 9 dominant for both traits, 3 dominant for a single trait, 3 dominant for the other trait, and 1 recessive for both traits. In this cross, it will result in: 9 AxBx, 3 Axbb, 3 aaBx, and 1 aabb.

Since we know that the genes are both capable of making the red coloration we actually need to add together all of the choices that contain at least a single dominant allele. Essentially, AxBxAxbb, and aaBx all show the exact same phenotype. This leaves us with a 15:1 ratio of red to white flowers. 

Example Question #2 : Help With Mendelian Inheritance

A child is curious to know what his blood type is, but he only knows his parents' blood types. If his mother had blood type A and his father had blood type AB, what are the potential blood types the child might have?

Possible Answers:

A or AB

A, B, or AB

A, B, or O

A or B

Correct answer:

A, B, or AB

Explanation:

We are not given the mother's full genotype in the question; she could reasonably carry two A alleles, or an A allele and a recessive O allele. We know that the father must carry one copy of the A allele and one copy of the B allele.

Two punnett squares can answer this question, corresponding to the two possible maternal genotypes: one crossing AA x AB and the other crossing AO x AB. From the first cross there is a 50% chance of blood type A versus 50% chance of blood type AB (half AA and half AB). The second cross shows that there is a potential chance of 50% for type A, 25% for type AB, and 25% for type B (one AA, one OA, one AB, and one OB).

Based on these possibilities, the child could have blood type A, B, or AB. The child cannot have blood type O.

Example Question #2 : Help With Mendelian Inheritance

A breeder performs a standard dihybrid cross between two plants that are heterozygous for both traits in question. How many unique genotypes could be present in the resulting offspring?

Possible Answers:

Four

Sixteen

Fifteen

Nine

Correct answer:

Nine

Explanation:

There are nine distinct genotypes present after a standard dihybrid cross. This question can easily be answered by setting up a Punnett square (AaBb x AaBb) and counting the number of unique genotypes present after doing the cross. The numbers also conveniently work out that however many offspring display the dominant phenotype is equal to the number to of genotypes present (this is true for monohybrid and trihybrid crosses as well). 

Example Question #4 : Help With Mendelian Inheritance

A scientist is performing a monohybrid homozygous cross: tall plants crossed with short plants. What fraction of the F2 generation are homozygous tall?

Possible Answers:

Correct answer:

Explanation:

A monohybrid cross between two homozygous plants would involve a parental generation that looked like this: SS (tall) x ss (short). The F1 generation would produce only heterozygous tall plants (Ss). The F2 generation would produce offspring from the following cross: Ss x Ss. A punnett square would reveal that the F2 generation would have 25% homozygous tall (SS), 50% heterozygous tall (Ss), and 25% homozygous short plants. Note that we do not need information regarding which trait is dominant in this case, and we would still get the correct answer if we took short as the dominant phenotype.

Example Question #3 : Help With Mendelian Inheritance

In apple trees, the allele for white blossoms is dominant over the allele for pink blossoms. Two trees heterozygous for this gene are crossed. What is the phenotype ratio of the offspring?

Possible Answers:

All white blossoms

All pink blossoms

3 white : 1 pink blossom

1 white : 3 pink blossom

1 white : 1 pink blossom

Correct answer:

3 white : 1 pink blossom

Explanation:

The crossing of two heterozygous parents will yield 1 homozyougs dominant offspring, 1 homozygous recessive offspring and 2 heterozygous offspring. Since white blossoms is the dominant allele, the heterozygous offspring will be white leading to a phenotypic ratio of 3 white : 1 pink blossom.

Example Question #6 : Help With Mendelian Inheritance

Assume complete dominance inheritance for the following question.

A pure-breeding red flower is mated with a pure-breeding white flower. All offspring are red in color; this is the F1 generation. Two of these offspring flowers are then mated with one another, and have F2 offspring.

Which of the following is true of the F2 offspring?

Possible Answers:

There will be more red flowers than white flowers.

All flowers will be white.

There will be a 50/50 ratio of red to white flowers.

All flowers will be red.

Correct answer:

There will be more red flowers than white flowers.

Explanation:

Since we had pure-breeding parents (also known as homozygotes for their respective colors), we can safely say the F1 offspring are heterozygotes and have a red allele and a white allele. When crossing these offspring with one another, we will expect to get a 3:1 ratio of red to white flowers. Not all flowers will be red, but 75% of the flowers will be.

Shown below is the punnett square that reflects this conclusion. (Note that "A" represents a red allele and "a" represents a white allele):

                           A                      a

 

               A       AA (red)         Aa (red)

 

 

               a     Aa (red)           aa (white)

All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

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