AP Biology Sample Student Responses and Scoring … Biology Sample Student Responses and Scoring...

2018

AP BiologySample Student Responses and Scoring Commentary

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Free Response Question 1

R Scoring Guideline

R Student Samples

R Scoring Commentary

AP® BIOLOGY 2018 SCORING GUIDELINES

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Question 1

Figure 1. Phylogenetic tree representing the evolutionary relatedness among bear populations based on mitochondrial DNA sequence comparisons

Polar bears are highly adapted for life in cold climates around the North Pole. Brown bears, black bears,

and pandas are found in warmer environments. Researchers collected complete mitochondrial DNA sequences from several populations of bears and constructed a phylogenetic tree to represent their evolutionary relatedness (Figure 1).

A researcher studying adaptation in bears sequenced the nuclear gene encoding a lysosomal trafficking protein (LYST) in polar bears, brown bears, black bears, and panda bears. There are seven inferred amino acid substitutions that are found only in polar bears. Mutations that cause similar substitutions in the human LYST protein are associated with Chediak-Higashi syndrome, an autosomal recessive condition in which pigment is absent from the hair and eyes. The researcher used the inferred amino acid sequences to build the distance matrix shown in Table 1.

TABLE 1. AMINO ACID DIFFERENCES IN THE LYST PROTEIN AMONG BEAR SPECIES

Panda Black Brown Polar

Panda −

Black 33 −

Brown 34 1 −

Polar 40 7 8 −



Question 1 (continued) (a) Use the phylogenetic tree in Figure 1 to estimate the age in hundreds of thousands of years of the most

recent common ancestor of all brown bears. Identify the population of brown bears to which polar bears are most closely related based on the mitochondrial DNA sequence comparison. Identify two populations whose positions could be switched without affecting the relationships illustrated in the phylogenetic tree.

Estimate (1 point) • First two digits of the answer must be between 30 and 35.

Identification (1 point) • European

Identification (1 point) • European/Polar OR Asian/Western (North American)

(b) Construct a cladogram on the template to represent a model of the evolutionary relatedness among the bear species based on the differences in LYST protein sequences (Table 1). Circle the position on the cladogram that represents the out-group.

Construction (1 point) • Correctly illustrated evolutionary relationship among the four species

Circling (1 point) • Correctly circled out-group based on orientation of cladogram

(c) A student claims that mitochondrial DNA sequence comparisons provide a more accurate phylogeny of

bear species than do LYST protein sequence comparisons. Provide ONE piece of reasoning to support the student’s claim.

Reasoning (1 point) • Genes show more variability (in nucleotide sequence) than proteins do (in amino acid sequences). • mtDNA genome contains multiple genes vs. one lyst gene. • The phenotype associated with the lyst gene is under strong selection.



Question 1 (continued)

(d) A researcher genetically engineers a mouse strain by deleting the mouse lyst gene and replacing it with the polar bear lyst gene. Predict the most likely difference in phenotype of the transgenic mouse strain compared to the wild-type mouse strain. Justify your prediction.

Prediction (1 point) • Mouse fur and/or eyes will not have pigment/will have reduced pigment. • Mouse (fur) will be white/lighter.

Justification (1 point) • Polar bear lyst gene/LYST protein is associated with a lack of pigment/white hair. • Mutated human lyst gene/ LYST protein is associated with a lack of pigment in hair and eyes.

(e) Describe how the mutation in the lyst gene became common in the polar bear population. If the lyst

gene were the only determinant of fur color, predict the percent of white offspring produced by a mating between a polar bear and a brown bear.

Description (1 point) • Natural selection for the white fur phenotype

Prediction (1 point) • 0%

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AP® BIOLOGY 2018 SCORING COMMENTARY


Question 1

Overview

This question is based on recent investigations into the evolutionary history of polar bears. Students were presented with a phylogenetic tree created from mitochondrial DNA (mtDNA) data for five populations of bears. Using this phylogenetic tree, students were asked to estimate the age of the most recent common ancestor of all brown bears, to identify what population of brown bears was most closely related to polar bears, and to identify two populations of bears whose positions on the phylogenetic tree could be switched without altering the proposed relationships. Students were presented with a data table indicating the number of amino acid differences in the lysosomal trafficking protein LYST among four species of bears. Using these data, the students were asked to construct a cladogram on a provided template and circle the outgroup position on that cladogram. Students then were asked to provide one piece of reasoning to support a claim that mtDNA sequence comparisons provide a more accurate prediction of bear phylogeny. The students were then asked to predict the most likely difference in the phenotype of a mouse that was genetically altered to contain the polar bear-specific lyst allele. The students were asked to justify their prediction. Finally, students were asked to describe how the lyst allele became common in the polar bear population and to predict what percentage of the offspring resulting from a mating between a brown bear and a polar bear would be white. The key understandings and skills students were expected to demonstrate included the following:

• Basic concepts of evolution were used to evaluate and interpret data and to describe evolutionary relationships and processes.

• Knowledge of Mendelian and molecular genetics was used to predict the outcomes of a genetic alteration and a mating based on experimental design.

Sample: 1A Score: 10

The response earned 1 point in part (a) for estimating that the most recent common ancestor of all brown bears existed 325,000 years ago. The response earned 1 point in part (a) for identifying that European Brown Bears are most closely related to Polar Bears. The response earned 1 point in part (a) for identifying that Western North American Brown Bear and Asian Brown Bear positions could be switched on the phylogenetic tree. The response earned 1 point in part (b) for constructing a cladogram that correctly orders the taxa, from left to right, Panda, Polar, Black, and Brown. The response earned 1 point in part (b) for circling a correct outgroup at the leftmost terminal branch, without including the ancestral node of the cladogram. The response earned 1 point in part (c) for supporting the student’s claim by reasoning that mtDNA would include more genes that code for more proteins that could be compared in contrast to one gene coding for a single LYST protein. The response earned 1 point in part (d) for predicting that the mouse’s fur would lack pigment. The response earned 1 point in part (d) for justifying that humans having the mutant form of the LYST protein lack pigment in hair. The response earned 1 point in part (e) for describing that the lyst gene became common when white individuals had a selective advantage. The response earned 1 point in part (e) for predicting that zero percent of offspring should be white.

Sample: 1B Score: 8

The response earned 1 point in part (a) for estimating that the most recent common ancestor existed 325,000 years ago. The response earned 1 point in part (a) for identifying that European Brown Bears are most closely related to Polar Bears. The response earned 1 point in part (a) for identifying that Polar Bear and European Brown Bear positions could be switched on the phylogenetic tree. The response earned 1 point in part (b) for circling a correct outgroup at the leftmost terminal branch, without including the ancestral node of the cladogram. The response earned 1 point in part (d) for predicting that the mouse would be growing pigmentless hair. The

AP® BIOLOGY 2018 SCORING COMMENTARY


Question 1 (continued) response earned 1 point in part (d) for the justification that because the lyst gene in Polar Bears encodes for a lack of pigment in hair, the mouse would begin growing pigmentless hair. The response earned 1 point in part (e) for describing that the lyst gene became common as a result of natural selection for white Polar Bears that were able to blend into a snowy white environment. The response earned 1 point in part (e) for predicting that no offspring should be white.

Sample: 1C Score: 6

The response earned 1 point in part (a) for estimating that the most recent common ancestor existed 325,000 years ago. The response earned 1 point in part (a) for identifying that European Brown Bears are most closely related to Polar Bears. The response earned 1 point in part (a) for identifying that the position of the Polar Bear and the position of the European Brown Bear could be switched on the phylogenetic tree. The response earned 1 point in part (b) for constructing a cladogram that correctly orders the taxa, from left to right, Panda, Polar, Black, and Brown. The response earned 1 point in part (b) for circling a correct outgroup at the leftmost terminal branch, without including the ancestral node of the cladogram. The response earned 1 point in part (e) for predicting that zero percent of offspring should be white.

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