Crosses between flowering plants
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Crosses between flowering plants
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[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Peony flowers range in color from white to red and can also include yellow. While there are 25-40 different species of peonies, it can be difficult to trace an individual peony to a specific species simply based on the flower phenotype. Therefore, in order to understand the genetic and physical characteristics of the peony flower, plants of known genotype and phenotype were crossed. The resulting peonies in the first filial (F1) generation were then repeatedly crossed with each other until a group of 522 offspring in the second filial (F2) generation were grown. The phenotypic distribution of petal color for the F2 generation is summarized in Table 1.
Table 1 Distribution of F2 generation peony petal color phenotype
Peonies share similarities with camellias, a genus of flowering plants belonging to the Theaceae family and primarily found in eastern and southern Asia. The camellia genus comprises 100-250 described species, exhibiting a color palette akin to that of peonies. To gain insights into the genetics of camellias, the peony study was replicated, replacing peonies with camellias. The F2 generation phenotypes resulting from the camellia flower crosses are outlined in Table 2.
Table 2 Distribution of F2 generation camellia petal color phenotype
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[question] => Which of the two alleles coding for peony flower petal color is the dominant allele?
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[answer] => 2
[description] => Reason for Correct Answer:
The heterozygous flowers, which have one red allele and one white allele, have a pink phenotype.
Since the heterozygous flowers produce a mixed phenotype, it suggests an incomplete dominance inheritance.
Neither allele is dominant over the other in the peony flower.
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[answers] => Array
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[each_answer] => A. The allele coding for white flower petals is the dominant allele as the hybrid color more closely resembles the white color than it does the red color.
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[each_answer] => B. Neither allele is dominant over the other, hence the appearance of a hybrid color in heterozygous flowers.
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[each_answer] => C. The allele coding for red flower petals is the dominant allele as the hybrid color more closely resembles the red color than it does the white color.
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[each_answer] => D. This information cannot be determined from the data given as the passage only presents information regarding flower phenotype, without giving any information about flower genotype.
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[quiz_unique_key] => 1403770772
[question] => What phenotypic dominance pattern for petal color is observed in the camellia population?
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[answer] => 3
[description] => Reason for Correct Answer:
In this experiment, the first crosses are of all red petal or all white petal camellias.
One resulting phenotype from these crosses are petals that are both red and white.
The camellias exhibit an co-dominance pattern, as the two alleles are dominantly expressed together.
This is different from the incomplete dominance observed with peonies, where there is a blend, or intermediate phenotype observed, instead of two discrete colors being observed.
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[answers] => Array
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[each_answer] => A. Hybrid inheritance
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[each_answer] => B. Incomplete dominance
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[each_answer] => C. Co-dominance
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[each_answer] => D. Traditional dominance
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[quiz_unique_key] => 1403770772
[question] => Based on the information in the passage and in Table 1, what would you expect the distribution of the peony F1 generation petal color phenotype to look like?
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[answer] => 3
[description] => Reason for Correct Answer:
The F2 generation is created by crossing members of the F1 generation.
In this case, the F2 generation shows this breakdown of phenotypes: a quarter has red flowers (RR), a quarter has white flowers (WW), and half has pink (RW).
This is consistent with a heterozygous F1 generation:
If individuals from the F1 generation are heterozygous, then they will all be pink, since heterozygous peonies exhibit incomplete dominance, with a pink color resulting from the blend between the red and white phenotypes.
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[0] => Array
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[each_answer] => A. Half of the peonies in the F1 generation have white petals, while the other half of the peonies have red petals.
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[each_answer] => B. One quarter of the peonies in the F1 generation have red petals, one quarter have white petals, and one half have pink petals.
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[each_answer] => C. All of the peonies in the F1 generation have pink petals.
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[each_answer] => D. This information cannot be determined from the data provided.
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[question] => A red flower and a pink flower from the peony F2 generation are crossed to create an F3 generation. Based on the information in this passage, what can you expect regarding the distribution of petal color phenotype in this generation?
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[answer] => 2
[description] => Reason for Correct Answer:
Draw a Punnett square for the cross between a red flower and a pink flower.
A red flower is homozygous for the red allele (RR) and a pink flower is heterozygous (RW).
The punnet square would therefore look like this:
Accordingly, 50% of the new generation flowers will have red petals (RR), and 50% will have pink petals (RW). None of the flowers resulting from this cross will have white flowers (WW).
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[each_answer] => A. The flowers would be a new shade of dark pink color.
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[each_answer] => B. No white flowers would be seen in the F3 generation.
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[each_answer] => C. The majority of the flowers in the F3 generation would have red petals.
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[each_answer] => D. The F3 generation would be very similar, if not identical, to the F1 generation.
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[quiz_unique_key] => 1403770772
[question] => Would any members of the P generation of camellias be expected to exhibit a mixture of red and white flowers?
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[answer] => 4
[description] => Reason for Correct Answer:
The data provided in table 2 is for the F2 generation. The F2 generation results from a cross between the F1 generation members, which result from a cross between the P generation.
As with the peonies, it looks like the F1 generation for the camelias was heterozygous, since again 25% of the F2 generation is red, 25% is white, and half are mixed.
This means that the P generation must consist of individuals with genotypes that would result in the production of only RW offspring.
This is consistent with a cross between RR and WW plants:
Accordingly, half of the plants of the P generation should be red (RW) and half should be white (WW).
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[each_answer] => A. Yes, all of the camellia P generation flowers have a mixture of red and white petals.
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[each_answer] => B. Yes, half of the camellia P generation flowers have a mixture of red and white petals.
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[each_answer] => C. No, all of the camellia P generation flowers have pink petals.
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[each_answer] => D. No, half of the camellia P generation flowers have red petals, and the other half have white petals.
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[quiz_unique_key] => 1403770772
[question] => What is the frequency of the allele coding for red petals in the peony population?
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[answer] => 3
[description] => Reason for Correct Answer:
You can use the numbers shown in Table 1 as well as the known genotypes for red (RW), pink (RW), and white (WW) to determine the frequency of the red alleles in this pool of genes. The table shows there are 128, 137, and 257 plants that are white, red, and pink, respectively.
This means that the plants have a total of these numbers of alleles:
128 white plants (WW) = 2 x 128 W alleles = 256 W alleles
137 red plants (RR) = 2 x 137 R alleles = 274 R alleles
257 pink plants (RW) = 257 R alleles + 257 W alleles
Therefore, there is a total of 513 W alleles and 531 R alleles, and 1,044 alleles in all.
This means that just over 50%, or about 51%, of the alleles in the population are R alleles.
Note that we didn’t have to go through the math here. Because the pink flowers have an equal number of R and W alleles, we could have just compared the number of red and white flowers to understand the difference between these allele frequencies. Because the table shows that there are slightly more red flowers, we know that there are slightly more R alleles in the population.
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[each_answer] => A. 33%
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[each_answer] => B. 47%
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[each_answer] => C. 51%
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[each_answer] => D. 66%
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