AP Biology : Understand epistasis

Study concepts, example questions & explanations for AP Biology

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Example Questions

Example Question #1 : Understand Epistasis

Epistasis controls the fur color of labradors, with the B locus controlling color (Black is dominant to brown), and E locus determining expression of B locus. This is a case of recessive masking epistasis. Which of the following is true?

Possible Answers:

bbee results in brown fur

bbEe results in black fur

BBee results in black fur

BbEe results in black fur

Correct answer:

BbEe results in black fur

Explanation:

Epistasis describes the interaction of genes, where the epistatic locus masks the effects of a gene at another locus. In this example, locus E is epistatic. As this is stated to be recessive masking epistasis, when the E locus is homozygous recessive (ee), this locus will mask the effect of the B locus (color). Thus, any combination of B/b with ee will result in yellow fur. When the E locus is heterozygous (Ee) or homozygous dominant (EE), the effect of the B locus will not be masked. Thus, BbEe will result in black fur (as black B is dominant to brown b), and the E locus does not mask the B locus here, as the E locus is heterozygous in this individual (Ee).

Example Question #2 : Understand Epistasis

When two genes interact, and one gene masks the expression of a second gene, the first gene is __ to the second or ___ gene

Possible Answers:

hypostatic, epistatic

dominant, recessive

epistatic, hypostatic

recessive, dominant

Correct answer:

epistatic, hypostatic

Explanation:

Epistasis describes the interaction of genes, where the epistatic gene masks the effects of another gene, called the hypostatic gene.

Example Question #3 : Understand Epistasis

Which of the following is true?

Possible Answers:

The epistatic locus can suppress the effect of the hypostatic locus

The hypostatic locus masks the effects of the epistatic locus

Epistasis describes the interaction of two genes producing a new phenotype

Epistasis describes the interaction of two alleles

Correct answer:

The epistatic locus can suppress the effect of the hypostatic locus

Explanation:

Epistasis describes the interaction of genes, where the gene at the epistatic locus masks the effects of another gene at the hypostatic locus.

Example Question #4 : Understand Epistasis

In a case of recessive epistasis, A is epistatic to B, and B is dominant to b, which genotype will give the dominant phenotype?

Possible Answers:

aabb

aaBB

AaBb

AAbb

Correct answer:

AaBb

Explanation:

As this is recessive epistasis, only the genotype aa will mask gene B (Aa will not mask B). BB or Bb will give the dominant phenotype as B is dominant to b. Thus, AaBb will yield the dominant phenotype.

Example Question #5 : Understand Epistasis

Epistasis controls the color of squash, with the B locus controlling color (yellow is dominant to green), and E locus determining expression of B locus. This is a case of dominant epistasis. Which of the following is true?

Possible Answers:

bbEe results in yellow squash

bbee results in yellow squash

Bbee results in yellow squash

bbEe results in green squash

Correct answer:

Bbee results in yellow squash

Explanation:

Epistasis describes the interaction of genes, where the epistatic locus masks the effects of a gene at another locus. In this example, locus E is epistatic. As this is stated to be dominant epistasis, when the E locus is either Ee or EE this locus will mask the effect of the B locus (color). Thus, any combination of B/b with Ee or EE will result in white squash. When the E locus is homozygous recessive (ee), the effect of the B locus will not be masked. Thus, BbEe will result in yellow squash as ee will not mask the color, and B (yellow) is dominant.

Example Question #6 : Understand Epistasis

Snapdragons’ color is controlled by recessive epistasis, with the B locus controlling color and the A locus determining expression of B locus. B (red) is dominant to b (yellow) Which of the following is false?

Possible Answers:

BbAA results in red snapdragon

bbAa results in yellow snapdragon

bbAA results in yellow snapdragon

bbaa results in yellow snapdragon

Correct answer:

bbaa results in yellow snapdragon

Explanation:

Epistasis describes the interaction of genes, where the epistatic locus masks the effects of a gene at another locus. In this example, locus A is epistatic. As this is stated to be recessive epistasis, when the A locus is aa this locus will mask the effect of the B locus (color). Thus, any combination of B/b with aa will result in white snapdragon. Thus, bbaa will result in white snapdragon, as the epistatic locus will mask the effect of the B locus.

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