1. In mice, black fur (B) is dominant to brown fur (b) and a short tail (T) is dominant to a long tail (t).

Two mice with the genotype BbTt have two separate litters of offspring together.

Of the data sets shown below, which data set shows the most likely outcome for the two litters?

(A)

Distractor Rationale:

This answer suggests the student may understand that it is possible to produce offspring

with any of the listed phenotypes, but does not understand how to correctly determine the

9:3:3:1 (9 black fur, short tail; 3 brown fur, short tail; 3 black fur, long tail; 1 brown fur, long

tail) expected phenotypic ratio by creating a Punnett square for a cross between two

heterozygous individuals (BbTt × BbTt). The student may assume that there is roughly an

equal probability of producing offspring with each phenotype because he or she does not

understand how the formation and combination of gametes with different allele

combinations affects the probability of producing an offspring with a particular phenotype.

(B)

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Distractor Rationale:

This answer suggests the student may understand that determining the outcome of a cross

between two heterozygous individuals (BbTt × BbTt) in a Punnett square will give an

expected phenotypic ratio of 9:3:3:1 (9 black fur, short tail; 3 brown fur, short tail; 3 black fur,

long tail; 1 brown fur, long tail) in the offspring and understands how to calculate multiples of

this ratio, but does not understand that the actual ratio of the resulting offspring usually

differs from the Punnett square’s expected ratio because the expected ratio is only a

probability, and that, due to chance during the formation and combination of gametes, the

actual ratio will likely differ from the expected ratio.

(C)

Rationale:

This answer suggests the student understands that a cross between two heterozygous

individuals (BbTt × BbTt) will produce a ratio that is close to, but not exactly, 9:3:3:1 of

phenotypes in the offspring for each litter (9 black fur, short tail; 3 brown fur, short tail; 3

black fur, long tail; 1 brown fur, long tail) because these numbers are only probabilities, and

that actual outcomes will likely differ from expected ratios due to chance.

(D)

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Distractor Rationale:

This answer suggests the student may understand that determining the outcome of a cross

between two heterozygous individuals (BbTt × BbTt) in a Punnett square will give an

expected phenotypic ratio of 9:3:3:1 (9 black fur, short tail; 3 brown fur, short tail; 3 black fur,

long tail; 1 brown fur, long tail) in the offspring, but does not understand that the actual

ratio of the resulting offspring usually differs from the Punnett square’s expected ratio

because the expected ratio is only a probability, and that, due to chance during the

formation and combination of gametes, the actual ratio will likely differ from the expected

ratio.

Aligned to: LO 3.14 CA 3.14: Apply Math to Genetics

2. Leaf color in a particular plant is controlled by the gene locus F. Plants with at least one F allele

have dark green leaves, and plants with the homozygous recessive genotype have light green

leaves. A plant with dark leaves and the FF genotype is crossed with a plant with light leaves,

and the F1 offspring are allowed to self-pollinate.

Which of these describes the possible allele combinations for the F1 and F2 generations and the

expected percentages for each?

(A) F1 ratio: 50% FF, 50% ff

F2 ratio: 100% Ff

Distractor Rationale:

This answer suggests the student may understand that alleles are passed from parents to

offspring, but does not understand that each parent contributes a single allele for each

particular gene (F or f in this case) to the offspring in the form of a gamete, and that a

parent’s full genotype is not inherited. The student may not understand how to create a

Punnett square to show how the contribution of a single allele from each parent to an

offspring can determine the allele combinations that are possible after fertilization, and how

a Punnett square can be used to determine the probability of different allele combinations

occurring.

(B) F1 ratio: 50% FF, 50% ff

F2 ratio: 25% FF, 50% Ff, 25% ff

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Distractor Rationale:

This answer suggests the student may understand that alleles are passed from parents to

offspring, but does not understand that each parent contributes a single allele (for a

particular gene) to the offspring in the form of a gamete, and that a parent’s full genotype is

not inherited. The student may not understand how to create a Punnett square to show

how the contribution of a single allele from each parent to an offspring can determine the

allele combinations that are possible after fertilization, and how a Punnett square can be

used to determine the probability of different allele combinations occurring.

(C) F1 ratio: 100% Ff

F2 ratio: 25% FF, 50% Ff, 25% ff

Rationale:

This answer suggests the student understands, through creating a Punnett square for each

generation, that crossing a homozygous dominant plant (FF) with a homozygous recessive

plant (ff) will produce all heterozygous (Ff) offspring in the F1 generation because only one

allele combination is possible (the F allele from one parent plant combines with the f allele

from the other parent plant), and that the F1 parent alleles then combine during fertilization

to produce F2 offspring with an expected frequency of FF: 25%, Ff: 50%, and ff: 25% in the

F2 generation.

(D) F1 ratio: 100% Ff

F2 ratio: 50% FF, 50% ff

Distractor Rationale:

This answer suggests the student may understand that crossing a homozygous dominant

plant (FF) with a homozygous recessive plant (ff) will produce all heterozygous (Ff) offspring

in the F1 generation, but does not understand how to illustrate, through creating a Punnett

square, that both homozygotes and heterozygotes are produced in the F2 generation

because alleles will segregate and recombine during meiosis and fertilization to produce

three different allele combinations (FF: 25%, Ff: 50%, ff: 25%) in the offspring of the F2generation.

Aligned to: LO 3.14 CA 3.14: Apply Math to Genetics

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3. In cattle, roan coat color (mixed red and white hairs) occurs in heterozygous offspring of a cross

between red homozygous and white homozygous parents. The table below describes the

phenotypes of the calves (offspring) of a bull (male) and a cow (female) that have been mated

several times.

Which cross describes the most likely phenotypes of the bull and the cow?

(A) red × roan

Distractor Rationale:

This answer suggests the student may understand that red and roan parents would produce

red and roan calves, but does not understand that a white calf could not result from

fertilization of the alleles from the parents because the red parent does not have, and

therefore cannot contribute, the white hair allele necessary to produce white offspring. The

student may not realize that a Punnett square can be used in this case to determine the

phenotypes of the parents by first filling in the resulting allele combinations of the offspring

(based on their genotypes and/or phenotypes) and then determining the parent alleles

necessary to produce the given offspring.

(B) red × white

Distractor Rationale:

This answer suggests the student may understand that red and white homozygous parents

would produce roan calves, but does not understand that during fertilization, the offspring

receives one allele from each parent, so red and white calves could not be produced

because every combination of the parents’ alleles during fertilization would result in a

heterozygous (roan) calf. The student may not realize that a Punnett square can be used in

this case to determine the phenotypes of the parents by first filling in the resulting allele

combinations of the offspring (based on their genotypes and/or phenotypes) and then

determining the parent alleles necessary to produce the given offspring.

(C) roan × white

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Distractor Rationale:

This answer suggests the student may understand that roan and white parents would

produce roan and white offspring, but does not understand that a red calf could not be

produced because the white parent does not have, and therefore cannot contribute, the red

hair allele necessary to produce red offspring. The student may not realize that a Punnett

square can be used in this case to determine the phenotypes of the parents by first filling in

the resulting allele combinations of the offspring (based on their genotypes and/or

phenotypes) and then determining the parent alleles necessary to produce the given

offspring.

(D) roan × roan

Rationale:

This answer suggests the student understands that a Punnett square can be created from

the offspring data to show that crossing two heterozygous individuals would yield a 1 red:2

roan:1 white ratio in the offspring, so a roan × roan cross is the only possible combination

that would produce the phenotypic ratio shown in the offspring.

Aligned to: LO 3.14 CA 3.14: Apply Math to Genetics

4. A couple has three boys and plans to have two more children. Assuming there are no external

factors that might influence the gender of future offspring, what is the probability that the two

additional children would also be boys?

(A) 1/32

Distractor Rationale:

This answer suggests the student may understand that the probability of having a male

offspring is 1/2 and calculates that the probability of having five males and no females would

be (1/2)5 = 1/32, but does not understand that for every instance (birth), the probability is

still 1/2 male and 1/2 female because past outcomes do not affect future probabilities.

(B) 1/8

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Distractor Rationale:

This answer suggests the student may understand that the probability of having a male

offspring is 1/2 and calculates that the probability of three males already having been born

with no females would be (1/2)3 = 1/8, but does not understand that for every instance

(birth), the probability is still 1/2 male and 1/2 female because past outcomes do not affect

future probabilities.

(C) 1/4

Rationale:

This answer suggests the student understands that, regardless of past outcomes, the

probability of two births producing two males is 1/2 × 1/2 = 1/4.

(D) 1/2

Distractor Rationale:

This answer suggests the student may understand that the probability of having a male

offspring is 1/2, but does not understand that the probability of having two males is 1/2 ×

1/2 = 1/4 because future probabilities are multiplied to calculate the overall probability

according to the law of multiplication, which states that if events are independent, the

probability of both events occurring is the product of the events occurring separately.

Aligned to: LO 3.14 CA 3.14: Apply Math to Genetics

5. In radishes, flower color and root shape are determined by single loci alleles that are found on

different chromosomes. Radish flowers may be red, purple, or white. A cross between a red-

flowered plant (RR) and a white-flowered plant (WW) yields purple offspring (RW). The radish

root may be long (L) or short (l), with long being the dominant phenotype.

The data below show the phenotypes of the offspring produced in the F1 generation. Which table

shows the possible offspring for a cross between RRLl and RWll parents?

(A)

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Distractor Rationale:

This answer suggests the student may understand, through creating a Punnett square for

the cross, that purple, long radishes are expected in a much higher percentage of the

offspring and some red, short radishes would also be expected, but does not understand

that producing an offspring with the white phenotype is not possible because it requires two

W alleles, and only one parent in the cross is able to contribute a W allele. The student also

may not understand that although producing offspring of all one phenotype (as seen in

choice C) is not probable, it is still possible, especially in a small sample population,

because the parents carry the alleles necessary to produce those offspring, and, by chance,

that combination of alleles could be the only one that occurs.

(B)

Distractor Rationale:

This answer suggests the student may understand, through creating a Punnett square for

the cross, that some purple, short radishes are expected in the offspring, but does not

understand that producing an offspring with the white phenotype is not possible because it

requires two W alleles, and only one parent in the cross is able to contribute a W allele. The

student also may not understand that although producing offspring of all one phenotype

(as seen in choice C) is not probable, it is still possible, especially in a small sample

population, because the parents carry the alleles necessary to produce those offspring, and,

by chance, that combination of alleles could be the only one that occurs.

(C)

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Rationale:

This answer suggests the student understands that the cross RRLl × RWll can produce

purple, short offspring (RW), and understands that, although it is improbable, it is still

possible to produce only offspring with that phenotype because the parents carry the alleles

necessary to produce those offspring, and, by chance, that combination of alleles could be

the only one that occurs.

(D)

Distractor Rationale:

This answer suggests the student may understand, through creating a Punnett square for

the cross, that the genotypes RRLl and RWll could produce offspring with only the long

phenotype because only one parent needs to donate the dominant allele for length (L) to

produce offspring with the long genotype, but does not understand that the cross could not

produce white offspring (WW) because it requires two W alleles, and only one parent in the

cross is able to contribute a W allele. The student also may not understand that although

producing offspring of all one phenotype (as seen in choice C) is not probable, it is still

possible, especially in a small sample population, because the parents carry the alleles

necessary to produce those offspring, and, by chance, that combination of alleles could be

the only one that occurs.

Aligned to: LO 3.14 CA 3.14: Apply Math to Genetics

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