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Example Questions
Example Question #1 : 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?
All offspring would have pink flowers
Half of the offspring would have red flowers, and half would have white
Half of the offspring would have red flowers, and half would have pink
All offspring would have both red and white flowers
All offspring would have pink flowers
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 #1 : 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?
Black
Gray
Black or white, depending on each individual offspring
Black and white spotted
Black and white spotted
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 #2 : 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?
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
The alleles for black and brown fur exhibit incomplete dominance
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 #1 : 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?
Red and white
Red, white, and pink
Pink only
Red, white, and red-white spotted
Red, white, and red-white spotted
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 #1 : 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?
Three
Six
Four
Five
Six
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 #1 : Understanding Codominance And Incomplete Dominance
Which of the following is an example of codominance?
A black dog and tan dog mate to produce a tan dog
A black dog and white dog mate to produce a gray dog
A black dog and tan dog mate to produce a dog with black and tan spots
A black dog and white dog mate to produce a black dog
A black dog and tan dog mate and produce a red dog
A black dog and tan dog mate to produce a dog with black and tan spots
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 #4 : 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?
Incomplete dominance
Co-dominance
Polygenetic inheritance
Multiple alleles
Co-dominance
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?
Codominance
Complete dominance
Epistasis
Incomplete dominance
Polygenic inheritance
Incomplete dominance
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.
Example Question #1 : Hardy Weinberg Equilibrium
Which of the following is NOT a requirement for Hardy-Weinberg equilibrium?
Mating must be random in nature
Organisms entering or leaving the population must not change the gene pool frequencies
The population must be at carrying capacity
The population must have a large number of organisms
The population must be at carrying capacity
Hardy-Weinberg equilibrium has a set of conditions that must be met in order for the population to have unchanging gene pool frequencies. There must be random mating, no mutation, no migration, no natural selection, and a large sample size.
It is not necessary for the population to be at carrying capacity. The population can grow or shrink while maintaining the gene pool.
Example Question #2 : Hardy Weinberg Equilibrium
A species of bird has an allele for black feathers and an allele for blue feathers. Which of the following scenarios would violate the necessary conditions for Hardy-Weinberg equilibrium in a population of this species?
Birds with black feathers prefer to mate with birds that have similarly colored feathers
The population is isolated on an island, and new birds are unable to fly to the island
There is a large population of birds on an island
Predators do not discriminate between members of the species with different colored feathers
Birds with black feathers prefer to mate with birds that have similarly colored feathers
If a population is in Hardy-Weinberg equilibrium, there is no evolution taking place in the population. One of the violations of Hardy-Weinberg equilibrium is selective mating. If birds prefer to mate with others that are similarly colored, then Hardy-Weinberg equilibrium is violated and the gene pool in the population is changing.
Hardy-Weinberg requires no migration, random mating, large population size, no natural selection, and no mutation.