AP Biology : Understand epistasis

Study concepts, example questions & explanations for AP Biology

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Example Questions

Example Question #251 : Evolution And Genetics

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 black fur

bbee results in brown 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 #1 : 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

epistatic, hypostatic

dominant, recessive

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 #1 : Understand Epistasis

Which of the following is true?

Possible Answers:

Epistasis describes the interaction of two alleles

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

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 #1 : 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 #1 : 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 #1 : 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 yellow snapdragon

bbAa results in yellow snapdragon

BbAA results in red 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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