Raven/Johnson Biology 8e
Chapter 12
1. A true-breeding plant is one that—
a. produces offspring that are different from the parent
b. forms hybrid offspring through cross-pollination
c. produces offspring that are always the same as the parent
d. can only reproduce with itself
The correct answer is c—
A. Answer a is incorrect. A true breeding plant will produce offspring that are identical to the
parent
The correct answer is c—
B. Answer b is incorrect. True breeding involves self-fertilization, not fertilization between
two different plants.
The correct answer is c—produces offspring that are always the same as the parent
C. Answer c is correct. The offspring are genetically identical to the parent.
The correct answer is c—
D. Answer d is incorrect. True breeding only means that the offspring produced by selffertilization are identical. These plants can be crossed with other true-breeding plants.
2. What property distinguished Mendel’s investigation from previous studies?
a. Mendel used true-breeding pea plants.
b. Mendel quantified his results.
c. Mendel examined many different traits.
d. Mendel examined the segregation of traits.
The correct answer is b—
A. Answer a is not correct. Other early researchers had used pea plants.
The correct answer is b—Mendel quantified his results.
B. Answer b is correct. The fact that Mendel determined the ratio of different phenotypic
traits led to our current understanding of inheritance.
The correct answer is b—
C. Answer c is not correct. Other early researchers had examined a variety of traits.
The correct answer is b—
D. Answer d is not correct. The segregation of traits had been observed by others.
3. A monohybrid cross—
a. is the same as self-fertilization
b. examines a single variant of a trait
c. produces a single offspring
d. examines two variants of a single trait
The correct answer is d—
A. Answer a is not correct. Monohybrid crosses can be made between two individuals.
Raven/Johnson Biology 8e
Chapter 12
The correct answer is d—
B. Answer b is not correct. A monohybrid cross examines two versions of a single trait.
The correct answer is d—
C. Answer c is not correct. The term monohybrid refers to the trait being examined, not
the number of offspring.
The correct answer is d—examines two variants of a single trait
D. Answer d is correct. A monohybrid cross examines a single trait with two versions
(e.g., purple versus white flowers).
4. What was the appearance of the F1 generation of a monohybrid cross of purple (PP) and
white (pp) flower pea plants?
a. All the F1 plants had white flowers.
b. The F1 plants had a light purple or blended appearance.
c. All the F1 plants had purple flowers.
d. The most of the F1 (3/4) had purple flowers, but 1/4 of the plants had white.
The correct answer is c—
A. Answer a is incorrect. The purple flower color is the dominant trait, so the F1 generation
(Pp) cannot appear white.
The correct answer is c—
B. Answer b is incorrect. The phenotype for flower color did not blend. Rather, it provided
evidence for the principle of segregation.
The correct answer is c—All the F1 plants had purple flowers.
C. Answer c is correct. The genotype of the F1 generation would be Pp, meaning that these
individuals would be heterozygous and would have the appearance of the dominant
phenotype.
The correct answer is c—
D. Answer d is incorrect. A plant with white flowers would have to be homozygous
recessive. All of the F1 individuals from this monohybrid cross would be heterozygous.
5. The F1 plants from the previous question are allowed to self-fertilize. What will the
phenotypic ratio be for this F2?
a. All purple
b. 1 purple:1 white
c. 3 purple:1 white
d. 3 white:1 purple
The correct answer is c—
A. Answer a is incorrect. The F1 is all purple, but segregation will occur in the F2.
The correct answer is c—
B. Answer b is incorrect. The phenotypic ratio for a test cross of the F1 heterozygote would
be 1 purple:1 white, but not for a self-cross.
Raven/Johnson Biology 8e
Chapter 12
The correct answer is c—3 purple:1 white
C. Answer c is correct. Self-crossing a heterozygous F1 would produce a ratio of 3 dominant
to 1 recessive.
The correct answer is c—
D. Answer d is incorrect. The ratio is correct in terms of numbers (3:1), but the white trait is
recessive and the purple trait is dominant (see previous problem) so the ratio is reversed.
6. Which of the following is NOT a part of Mendel’s five-element model?
a. Traits have alternative forms (what we now call alleles).
b. Parents transmit discrete traits to their offspring.
c. If an allele is present it will be expressed.
d. Traits do not blend .
The correct answer is c—
A. Answer a is incorrect. The fact that traits have alternative forms was critical for Mendel’s
observations.
The correct answer is c—
B. Answer b is incorrect. Mendel concluded that parents transmitted discrete units of
information that determined the traits of the offspring.
The correct answer is c—If an allele is present it will be expressed.
C. Answer c is correct. Alleles are not always expressed. Some alleles are dominant while
others are recessive.
The correct answer is c—
D. Answer d is incorrect. Mendel did not observe blending of traits.
7. A heterozygous individual is one that carries—
a. two completely different sets of genes
b. two identical alleles for a particular gene
c. only one functional allele
d. two different alleles for a given gene
The correct answer is d—
A. Answer a is incorrect. The concept of heterozygosity relates to a single gene, not the
entire genome.
The correct answer is d—
B. Answer b is incorrect. A heterozygous individual has two different versions of a single
gene.
The correct answer is d—
C. Answer c is incorrect. A heterozygous individual has two alleles; however, one is
dominant while the other is recessive.
The correct answer is d—two different alleles for a given gene
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Chapter 12
D. Answer d is correct. A heterozygous individual has a dominant and a recessive copy of a
single allele.
8. An organism’s ___________ is determined by its _____________.
a. genotype; phenotype
b. phenotype; genotype
c. alleles; phenotype
d. F1 generation; alleles
The correct answer is b—
A. Answer a is incorrect. The genotype is the controlling factor that determines phenotype.
The correct answer is b—phenotype; genotype
B. Answer b is correct. The phenotype of an individual is determined by the alleles that
make up that individual’s genotype.
The correct answer is b—
C. Answer c is incorrect. The alleles are the genes that determine the phenotype of the
individual
The correct answer is b—
D. Answer d is incorrect. The F1 generation is the product of reproduction and so would
depend on the genetic makeup of the gametes.
9. Which of the following represent the phenotype for the recessive human trait, albinism?
a. Absence of the pigment melanin
b. Presence of a nonfunctional allele for the enzyme tyrosinase
c. Absence of the enzyme tyrosinase from the individual’s cells
d. Both a and c
The correct answer is d—
A. Answer a is incorrect. Although albinism is associated with a lack of pigment, that is not
the only phenotypic trait.
The correct answer is d—
B. Answer b is incorrect. The allele for the enzyme tyrosinase is nonfunctional; however,
this represents the genotype of the individual, not phenotype.
The correct answer is d—
C. Answer c is incorrect. The absence of the enzyme is part of the phenotype; however, it
does not represent the total phenotype.
The correct answer is d—Both a and c
D. Answer d is correct. The phenotype represents all observed effects of the genotype; that
is, the absence of the enzyme and the resulting absence of pigment.
10. A dihybrid cross between a plant with long smooth leaves and a plant with short hairy
leaves produces a long smooth F1. If this F1 is allowed to self-cross to produce an F2, what
would you predict for the ratio of F2 phenotypes?
Raven/Johnson Biology 8e
a.
b.
c.
d.
Chapter 12
9 long smooth:3 long hairy:3 short hairy:1 short smooth
9 long smooth:3 long hairy:3 short smooth:1 short hairy
9 short hairy:3 long hairy:3 short smooth:1 long smooth
1 long smooth:1 long hairy:1 short smooth:1 short hairy
The correct answer is b—
A. Answer a is incorrect. The double-recessive phenotype (short hairy) is switched with
recessive for length and dominant for texture (short smooth).
The correct answer is b—9 long smooth:3 long hairy:3 short smooth:1 short hairy
B. Answer b is correct. The phenotypic ratio should be 9 dom dom:3 dom rec:3 rec dom:1
rec rec.
The correct answer is b—
C. Answer c is incorrect. The double-dominant phenotype (long smooth) is switched with
the double recessive (short hairy).
The correct answer is b—
D. Answer d is incorrect. This 1:1:1:1 ratio would be seen in a test cross not in a self-cross of
the F1.
11. A testcross is used to determine if an individual is—
a. homozygous dominant or heterozygous
b. homozygous recessive or homozygous dominant
c. heterozygous
d. true breeding
The correct answer is a—homozygous dominant or heterozygous
A. Answer a is correct. The ratio of phenotypes produced from a testcross allows for the
determination of whether an individual is homozygous dominant or heterozygous.
The correct answer is a—
B. Answer b is incorrect. The phenotypes of these two individuals would be very different.
No testcross would be needed.
The correct answer is a—
C. Answer c is incorrect. The phenotypic ratio resulting from a testcross would establish
whether an individual was heterozygous, but it can also be used to distinguish between
heterozygous and homozygous dominant individuals.
The correct answer is a—
D. Answer d is incorrect. A testcross is used to determine whether an individual is
heterozygous or homozygous dominant. The test for whether an individual is true
breeding is whether self-fertilization results in offspring that are identical to the parent.
12. What is a polygenic trait?
a. A set of multiple phenotypes determined by a single gene.
b. A single phenotypic trait determined by a two alleles.
c. A single phenotypic trait determined by more than one gene.
Raven/Johnson Biology 8e
Chapter 12
d. The collection of traits possessed by an individual.
The correct answer is c—
A. Answer a is incorrect. A polygenic trait is a single phenotypic trait.
The correct answer is c—
B. Answer b is incorrect. An allele is just a variation of a given gene. A polygenic trait
involves multiple genes.
The correct answer is c—A single phenotypic trait determined by more than one gene.
C. Answer c is correct. Polygenic means that more than one gene is involved in determining
a phenotypic trait.
The correct answer is c—
D. Answer d is incorrect. A polygenic trait refers to a single phenotypic trait, not many traits.
13. When a single gene influences multiple phenotypic traits the effect is called—
a. codominance
b. epistasis
c. incomplete dominance
d. pleiotropy
The correct answer is d—
A. Answer a is incorrect. Codominance occurs when the influence of different alleles are
equally expressed in the heterozygous individual.
The correct answer is d—
B. Answer b is incorrect. Epistasis occurs when the phenotype of an individual is influenced
by the interactions between gene products.
The correct answer is d—
C. Answer c is incorrect. Incomplete dominance occurs when the phenotype of the
heterozygous individual represents a blending of the dominant and recessive traits.
The correct answer is d—pleiotropy
D. Answer d is correct. Some genes can influence multiple phenotypic traits. These are
pleiotropic genes.
14. What is the probability of obtaining an individual with the genotype bb from a cross
between two individuals with the genotype Bb?
a. 1/2
b. 1/4
c. 1/8
d. 0
The correct answer is b—
A. Answer a is incorrect. For an individual with the genotype Bb the probability of forming a
b gamete is 1/2, so the probability of bb offspring is (1/2)(1/2) = 1/4. Overall, in this
cross: the progeny should be 1/4 BB, 1/2 Bb, and 1/4 bb.
Raven/Johnson Biology 8e
Chapter 12
The correct answer is b—1/4
B. Answer b is correct. The probability of receiving a b allele from each parent is 1/2. 1/2
(male) x 1/2 (female) = 1/4. Overall, in this cross: the progeny should be 1/4 BB, 1/2 Bb,
and 1/4 bb.
The correct answer is b—
C. Answer c is incorrect. For an individual with the genotype Bb the probability of forming
a b gamete is 1/2, so the probability of bb offspring is (1/2)(1/2) = 1/4. Overall, in this
cross: the progeny should be 1/4 BB, 1/2 Bb, and 1/4 bb.
The correct answer is b—
D. Answer d is incorrect. For an individual with the genotype Bb the probability of forming a
b gamete is 1/2, so the probability of bb offspring is (1/2)(1/2) = 1/4. Overall, in this
cross: the progeny should be 1/4 BB, 1/2 Bb, and 1/4 bb.
15. What is the probability of obtaining an individual with the genotype CC from a cross
between two individuals with the genotypes CC and Cc?
a. 1/2
b. 1/4
c. 1/8
d. 1/16
The correct answer is a—1/2
A. Answer a is correct. The probability of the homozygous dominant parent generating a
gamete carrying a C allele is 1. The probability of producing a C gamete for the
heterozygous parent is 1/2.
The correct answer is a—
B. Answer b is incorrect. The probability of the homozygous dominant parent generating a
gamete carrying a C allele is 1. The probability of producing a C gamete for the
heterozygous parent is 1/2. The product rule applies so 1 x 1/2 = 1/2.
The correct answer is a—
C. Answer c is incorrect. The probability of the homozygous dominant parent generating a
gamete carrying a C allele is 1. The probability of producing a C gamete for the
heterozygous parent is 1/2. The product rule applies so 1 x 1/2 = 1/2.
The correct answer is a—
D. Answer d is incorrect. The probability of the homozygous dominant parent generating a
gamete carrying a C allele is 1. The probability of producing a C gamete for the
heterozygous parent is 1/2. The product rule applies so 1 x 1/2 = 1/2.
Raven/Johnson Biology 8e
Chapter 12
Challenge Questions
1. Create a Punnett square for the following crosses and use this to predict phenotypic
ratio for dominant and recessive traits. Dominant alleles are indicated by uppercase
letters and recessive are indicated by lowercase letters.
a. A monohybrid cross between individuals with the genotype Aa and Aa
b. A dihybrid cross between two individuals with the genotype AaBb
c. A dihybrid cross between individuals with the genotype AaBb and aabb
The approach to solving this type of problem is to identify the possible gametes. Each individual
parent will donate one allele to the offspring. Separate the possible gamete combinations into the
boxes along the top and side. Fill in the Punnet square by carrying each letter representing each
allele into the boxes below and to the right for each column and row. As a general rule the
dominant allele (represented by a capital letter) is always listed first. Notice that if an individual
only makes one type of gamete, it is not necessary to use more than one row or column for this
individual.
Answer for 1a—
A
a
A
AA
Aa
a
Aa
aa
Phenotypic ratio: 3 dominant to 1 recessive
Answer for 1b—
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
Phenotypic ratio: 9 dominant dominant to 3 dominant recessive to 3 recessive dominant to 1
recessive recessive
Answer for 1c—
AB
Ab
aB
ab
Raven/Johnson Biology 8e
ab
Chapter 12
AaBb
Aabb
aaBb
aabb
Phenotypic ratio: 1 dominant dominant to 1 dominant recessive to 1 recessive dominant to 1
recessive recessive
2. Use probability to predict the following.
a. What is the probability of obtaining an individual with the genotype rr from
the self-fertilization of a plant with the genotype Rr?
b. What is the probability that a testcross with a heterozygous individual will
produce homozygous recessive offspring?
c. A plant with the genotype Gg is self-fertilized. Use probability to determine the
proportion of the offspring that will have the dominant phenotype.
d. Use probability to determine the proportion of offspring from a dihybrid cross
(GgRr x GgRr) that will have the phenotype ggR_.
Answer for 2a—One half of the gametes produced by this plant will have the recessive allele r.
The probability of obtaining an rr individual is the product of the probabilities of finding a
gamete carrying the r allele. 1/2 x 1/2 = 1/4.
Answer for 2b—The testcross involves the use of a homozygous recessive individual to cross
with an unknown individual. The problem states that the second parent in heterozygous. The
probability of obtaining a recessive allele from the homozygous recessive individual is 1. The
probability of obtaining a recessive allele from a heterozygous individual is 1/2. 1 x 1/2 = 1/2.
The probability is that 1/2 of the offspring from a testcross with a heterozygous individual will be
homozygous recessive.
Answer for 2c—The dominant phenotype (G_) will occur for individuals for individuals with the
genotypes GG, Gg, or gG. There is a 1/4 probability of combining a G gamete with another G
gamete. Likewise, there is a 1/4 probability of combining G with g or g with G. The rule of
addition applies—1/4 + 1/4 + 1/4 = 3/4.
Answer for 2d—When considering dihybrid crosses it is important to remember that the
dominant phenotype will occur 3/4 of the time, and the recessive phenotype only appears 1/4 of
the time (the ratios of a simple monohybrid cross). Assuming that the alleles behave
independently, then the rule of multiplication applies. There is a 1/4 chance of obtaining the
combination gg because this is the recessive phenotype. There is a 3/4 chance of obtaining the
combination R_ since this is the dominant phenotype. 3/4 x 1/4 = 3/16.