AP Biology : Understand incomplete dominance

Study concepts, example questions & explanations for AP Biology

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Example Questions

Example Question #81 : Inheritance

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

Possible Answers:

Polysomatic

Pleiotropic

Somatic

Epistatic

Polygenic

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 #1 : 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:

Incomplete dominance

Red is the dominant allele

Blue is the dominant allele

Codominance

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 #91 : Inheritance

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

Codominance

Sex-linked dominance

Multiple alleles

Complete dominance

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 #92 : Inheritance

Which occurs when a heterozygous genotype produces blended traits?

Possible Answers:

Monosomy

Trisomy

Pure genotype

Codominance

Incomplete dominance

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 #93 : Inheritance

Which of these is an example of polygenic inheritance?

Possible Answers:

Color blindness

Freckles

Duchenne muscular dystrophy 

Sickle cell anemia

Skin color

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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