AP Biology : Understanding Co-dominance and Partial Dominance

Study concepts, example questions & explanations for AP Biology

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Example Questions

Example Question #1 : Understanding Co Dominance And Partial 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:

A

All are possible

B

O

AB

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 #2 : Understanding Co Dominance And Partial 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:

Distinct red and white spots

White

Red

Pink

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 #3 : Understanding Co Dominance And Partial 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:

codominance

Mendelian dominance

nondominance

complete dominance

incomplete dominance

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 #1 : Understanding Co Dominance And Partial Dominance

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

Possible Answers:

Somatic

Pleiotropic

Polygenic

Epistatic

Polysomatic

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 #253 : 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:

Incomplete dominance

Codominance

Green is the dominant allele

Yellow is the dominant allele

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 #5 : Understanding Co Dominance And Partial 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:

Incomplete dominance

Red is the dominant allele

Codominance

Blue is the dominant allele

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.

Example Question #2 : Understanding Co Dominance And Partial Dominance

A white homozygous recessive plant is crossed with a homozygous dominant red plant. The plants produced from this cross are all pink. What kind of inheritance does this plant's color follow?

Possible Answers:

Incomplete dominance

Sex-linked dominance

Complete dominance

Codominance

Multiple alleles

Correct answer:

Incomplete dominance

Explanation:

In incomplete dominance, heterozygotes express an intermediate phenotype. Since neither parent expresses the pink phenotype, we know that plant color follows incomplete dominance since neither red nor white is fully expressed as would be the case with complete dominance; rather they are both incompletely expressed.

Example Question #3 : Understanding Co Dominance And Partial Dominance

Which occurs when a heterozygous genotype produces blended traits?

Possible Answers:

Monosomy

Pure genotype

Incomplete dominance

Codominance

Trisomy

Correct answer:

Incomplete dominance

Explanation:

Incomplete dominance involves expression of an intermediate phenotype. The heterozygotes express a phenotype that is a blend of both the dominant and recessive phenotypes. One common example is a flower with white petals and a flower with red petals sexually reproduce to create flowers with pink petals. 

Example Question #4 : Understanding Co Dominance And Partial Dominance

Which of these is an example of polygenic inheritance?

Possible Answers:

Color blindness

Skin color

Duchenne muscular dystrophy 

Sickle cell anemia

Freckles

Correct answer:

Skin color

Explanation:

Polygenetic inheritance is where multiple genes affect a single trait. Human skin color depends on three sets of alleles: Aa, Bb, and Cc. A cross between two parents with any combination of these three alleles determines skin color; there is no single skin color gene.

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