High School Biology : Understanding Codominance and Incomplete Dominance

Study concepts, example questions & explanations for High School Biology

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Example Questions

Example Question #1 : Understanding Codominance And Incomplete Dominance

A plant with red flowers is crossed with a plant with white flowers. The resulting offspring have pink flowers. What term describes the dominance of this trait?

Possible Answers:

Codominance

Recessiveness

Dominance

Incomplete dominance

Correct answer:

Incomplete dominance

Explanation:

A phenotypic "blending" of two traits is referred to as incomplete dominance, indicating that neither trait is truly dominant over the other. Instead of one color overpowering expression of the other, both colors are expressed simultaneously.

Codominance suggests that both phenotypes are dominant, but cannot be expressed at the same time. The result of codominance would be regions of dominant red expression and regions of dominant white expression, resulting in spots rather than blending.

Example Question #2 : Understanding Codominance And Incomplete Dominance

Two pure breeding plants are crossed. One plant has red flowers and the other has white flowers.

What phenotype(s) would be seen in the first generation if flower color exhibited incomplete dominance?

Possible Answers:

Half of the offspring would have red flowers, and half would have pink

All offspring would have both red and white flowers

Half of the offspring would have red flowers, and half would have white

All offspring would have pink flowers

Correct answer:

All offspring would have pink flowers

Explanation:

The genotypes of the offspring can be determined by crossing the red flowers, RR, with the white flowers, rr.

RR x rr

Offspring: all offspring are Rr.

Incomplete dominance means that neither color shows dominance in the hybrid generation. This means that instead of solid red or solid white flowers, the heterozygous plants will display a mixture of both colors. Since the entire first generation will be heterozygous for the color trait, they will all be pink (a mix of white and red).

Example Question #3 : Understanding Codominance And Incomplete Dominance

Black fur (A) is codominant with white fur (a) and brown eyes (B) are dominant to blue eyes (b) in mice. Two mice are heterozygous for both traits. If these mice are crossed, what color of fur will the offspring with genotype Aa express?

Possible Answers:

Black

Black or white, depending on each individual offspring

Gray

Black and white spotted

Correct answer:

Black and white spotted

Explanation:

Codominance means that more than one type of dominant allele for the same gene is present. If both black and white fur are dominant, then heterozygous (Aa) offspring would be spotted with black and white.

Note this is a different expression pattern from incomplete dominance, in which a blending of phenotypes occurs.

Example Question #3 : Understanding Codominance And Incomplete Dominance

Two mice are heterozygous for both fur color and eye color. If these mice were crossed and all offspring have dark brown, almost-black fur, what is the best explanation?

Possible Answers:

The alleles for black and brown fur exhibit incomplete dominance

Both parents have a recessive mutation

The alleles for black and brown fur exhibit complete dominance

Independent assortment has occurred

Correct answer:

The alleles for black and brown fur exhibit incomplete dominance

Explanation:

Incomplete dominance is when more than one type of dominant allele for the same gene is present. If black and brown alleles are incompletely dominant, they "compete" for expression, which produces offspring with a combination of the two colors.

Note that this pattern difference from codominance, in which the phenotypes will be present in separate spots of blotches.

Example Question #4 : Understanding Codominance And Incomplete Dominance

If red (R) and white (r) are codominant alleles that determine flower color, what phenotypes are possible for this gene?

Possible Answers:

Pink only

Red and white

Red, white, and red-white spotted

Red, white, and pink

Correct answer:

Red, white, and red-white spotted

Explanation:

The possible genotypes for this trait are RR, Rr, and rr. To determine the answer, we must find the phenotype that corresponds to each genotype.

We know that RR is red and rr is white, since these genotypes are homozygous.

Now you must determine if Rr is red, white, or some other phenotype. Codominance means that both phenotypes show simultaneously, so the heterozygote would be both red and white, which is a distinct third phenotype. These organisms would show spots or splotches of each color.

This gives three total phenotypes: red, white, and red-white spotted.

A pink phenotype would only show in instances of incomplete dominance. When an organism is heterozygous for alleles that show incomplete dominance, an intermediate of blended phenotype will be seen.

Example Question #5 : Understanding Codominance And Incomplete Dominance

Yellow, blue, and red alleles all show incomplete dominance for flower color in a diploid plant species. How many phenotypes for flower color are possible in this species?

Possible Answers:

Four

Five

Six

Three

Correct answer:

Six

Explanation:

In the species the entire range of phenotypes will be expressed. Diploid organisms have two alleles of each gene, so the plant could be homozygous for any of the alleles or it could have any heterozygous combinations.

First, we can identify three homozygous phenotypes: YY is yellow, BB is blue, and RR is red.

Now we need to identify the heterozygous phenotypes. Since the alleles show incomplete dominance, these phenotypes will be blended. YB will be green, YR will be orange, and BR will be purple.

This gives a total of six possible allele combinations and six different phenotypes.

Example Question #6 : Understanding Codominance And Incomplete Dominance

Which of the following is an example of codominance?

Possible Answers:

A black dog and tan dog mate to produce a dog with black and tan spots

A black dog and tan dog mate to produce a tan dog

A black dog and white dog mate to produce a black dog

A black dog and white dog mate to produce a gray dog

A black dog and tan dog mate and produce a red dog

Correct answer:

A black dog and tan dog mate to produce a dog with black and tan spots

Explanation:

Codominance is evidenced when the phenotypes of both parents show up in the offspring. A dog that has fur that consists of colors of both parents will be an example of codominance. Only one trait can be expressed at a time, since they are both dominant phenotypes. This results in regions of one dominant allele and regions of the other, showing a spotted or mottled pattern.

Incomplete dominance occurs when neither trait is truly dominant over the other. This means that both traits can be expressed in the same regions, resulting a blending of two phenotypes. If a white and black dog produce a gray offspring, this is an example of incomplete dominance.

The answer that suggests a red offspring from a black parent and tan parent could result from one of two scenarios. The first possibility is that there are three alleles for color, with red recessive to both black and tan. Both parents carry the red allele, but do not display it, and then pass it to the offspring. Something similar happens with the O blood type. The other possibility is that red color is a new mutation.

Example Question #7 : Understanding Codominance And Incomplete Dominance

A F1 generation flower has red and white petals. One parent flower was red and the other was white. This is an example of which of the following forms of inheritance? 

Possible Answers:

Multiple alleles 

Incomplete dominance 

Polygenetic inheritance 

Co-dominance 

Correct answer:

Co-dominance 

Explanation:

In the example above, the flower has both red and white petals due to co-dominant inheritance pattern of the red and white petal alleles. If the flower’s phenotype was determined by incomplete dominance, the F1 hybrids would be in-between the parental phenotypes (i.e. if a red petal parent was crossed with a white petal parent, then the F1 generation would be pink). Polygenic inheritance occurs when two or more genes control one characteristic, such as skin color, eye color, and adult height. 

Example Question #2 : Understanding Codominance And Incomplete Dominance

A sugar flower has white petals, and is homozygous for this trait. Another sugar flower is homozygous for red petals. The white and red sugar flowers are crossed, and produce offspring with pink petals. Which of the following terms describes this type of inheritance?

Possible Answers:

Complete dominance

Epistasis

Codominance

Polygenic inheritance

Incomplete dominance

Correct answer:

Incomplete dominance

Explanation:

Incomplete dominance is described by a phenotype that is not completely dominant over another. Therefore, it will be a "blending" of colors in the case of this question, therefore the petals are pink. Codominance is when both dominant traits are expressed, therefore if white was considered dominant and red was also a dominant trait, the petals would have spots of white and red, with no pink. Polygenic inheritance is described by one characteristic influenced by multiple genes, which is not the case in this problem. Finally, epistasis involves the suppression of genes, however in this problem color is not suppressed.

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