AP Biology : Inheritance

Study concepts, example questions & explanations for AP Biology

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Example Questions

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 #2 : 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 red 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.

Example Question #1 : Understand Co Dominance

A mother with type A blood and a father with type B blood have a child. What blood type is impossible for that child to have?

Possible Answers:

AB

O

A

All are possible

B

Correct answer:

All are possible

Explanation:

The mother's possible genotypes for blood are AO and AA, while the father's are BO and BB; therefore, the child could have any blood type because we could receive an O allele from either parent.

The full possibilities are:

A from mother, O from father - blood type A

A from mother, B from father - blood type AB

O from mother, B from father - blood type B

O from mother, O from father - blood type O

Example Question #1 : Understand Co Dominance

In a specific type of flower the genes coding for color display codominance. What would you expect the phenotype to be for a cross between a flower homozygous for white coloration and a flower homozygous for red coloration?

Possible Answers:

Pink

Red

Distinct red and white spots

White

Correct answer:

Distinct red and white spots

Explanation:

Codominance is a phenomenon in which the phenotypes associated with both alleles will be expressed in their entirety. This expression pattern results in mottled expression, creating distinct red and white spots for the flower. This is different than incomplete dominance, in which the two phenotypes appear to blend together.

Example Question #1 : Understand Co Dominance

In humans, blood type is determined by three alleles: A, B, and O. Both the A and B alleles are dominant to the O allele. When both A and B are present, however, a combined phenotype of AB results. In AB individuals, both A and B antigens are fully expressed on the cell surface. This is an example of __________.

Possible Answers:

incomplete dominance

Mendelian dominance

nondominance

complete dominance

codominance

Correct answer:

codominance

Explanation:

Since the A and B alleles both seem to exert a form of dominance, this is clearly not our common example of a complete dominance scenario. We can conclude that blood type is determined by either incomplete dominance or codominance.

In incomplete dominance, both alleles exert influence to a lesser degree resulting in a "blended" phenotype. In blood type, both alleles exert their full influence together. Instead of yielding a blended phenotype, this situation results in a phenotype that is functionally equivalent to having both A and B blood types at once. The A and B alleles are codominant.

Example Question #301 : Evolution And Genetics

Scientists are trying to figure out the dominance hierarchy for a newly discovered plant. They have found that when a true-breeding yellow plant is crossed to a true-breeding green plant, the resulting offspring have distinct spots of yellow and green. What is the most likely explanation for this result?

Possible Answers:

Yellow is the dominant allele

Incomplete dominance

Green is the dominant allele

Codominance

Correct answer:

Codominance

Explanation:

In the resulting offspring, both phenotypes are displayed equally. This is a classic example of codominance. If an intermediate phenotype was observed, incomplete dominance would be the correct answer.

In codominance, both alleles are considered dominant. This means that both alleles will be fully expressed in different regions, resulting in spots. In incomplete dominance neither allele is fully dominant, so both can be expressed simultaneously in a given area. The result is a blending of both alleles.

Example Question #1 : Understand Co Dominance

The shorthorn cattle coat color exhibits codominance. If a homozygous red individual and homozygous white individual produce an offspring, what will the resulting coat color be?

Possible Answers:

White

Roan

Red

Pink

Correct answer:

Roan

Explanation:

In cases of codominance, the offspring have both alleles expressed at the same time. Thus, the coat color will be roa, which contains both white and red hair.

Example Question #5 : Understand Co Dominance

Blood type exhibits codominance.  and  are dominant alleles, and i is recessive.  results in blood type results in blood type , and i results in blood type . If an individual with genotype  produces offspring with an individual with blood type  what will be the blood type of the resulting offspring?

Possible Answers:

A

A

AB

O

Correct answer:

AB

Explanation:

In cases of codominance, the offspring have both alleles expressed at the same time. As both  and  are dominant alleles, the resulting offspring will have blood type .

Example Question #81 : Inheritance

Which of the following describes a single genetic locus that controls more than one trait?

Possible Answers:

Epistatic

Somatic

Polygenic

Polysomatic

Pleiotropic

Correct answer:

Pleiotropic

Explanation:

The ability of a gene to affect an organism is multiple ways is called pleiotropy. During post-transcriptional modification, introns are removed from the mRNA sequence and exons are spliced together to create the desired protein product. By splicing the gene in different ways, different proteins can be produced, which will affect different traits.

Consider the sentence: The man ran on the track, but fell.

By splicing different portions of the sentence, it can take on different meanings: The man ran. The man on the track fell. The man fell. The man ran, but fell.

Where pleiotropic genes affect more than one trait, polygenic traits are affected by multiple genes. Epistatic genes are regulated by the activation of other genes.

Example Question #2 : Understand Incomplete Dominance

Scientists are trying to figure out the dominance hierarchy for a newly discovered plant. They have found that when a true-breeding red plant is crossed to a true-breeding blue plant, the resulting offspring are purple. What is the most likely explanation for this result?

Possible Answers:

Codominance

Blue is the dominant allele

Red is the dominant allele

Incomplete dominance

Correct answer:

Incomplete dominance

Explanation:

An intermediate phenotype is observed in the offspring. This is a classic example of incomplete dominance. Neither allele is dominant over the other, allowing both phenotypes to be expressed simultaneously. A plant expressing both blue and red will appear purple.

Codominance refers to a dominance pattern in which both alleles are dominant, and cannot be expressed simultaneously. Certain regions will express one dominant allele, while other regions will express the other allele. The result is a mottled or spotted appearance.

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