CK-12 Biology for High School FlexBook® 2.0
Answer Key
Chapter 3: Genetics
3.1 Pea Plants
Review
Questions
1.
2.
3.
4.
What is the blending theory of inheritance? Why did Mendel question this theory?
List the seven characteristics that Mendel investigated in pea plants.
How did Mendel control pollination in pea plants?
What are hybrids?
Answers
1. The blending theory of inheritance states that offspring have a blend, or mix, of
the characteristics of their parents. Mendel noticed plants in his own garden that
weren’t a blend of the parents, so he questioned this theory.
2. Mendel studied seed form and color, flower color, pod form and color, and stem
place and size.
3. Mendel was interested in the offspring of two different parent plants, so he had to
prevent self-pollination. To do so, he removed the anthers from the flowers of
some of the plants in his experiments. Then he pollinated them by hand with
pollen from other parent plants of his choice.
4. Hybrids are the offspring of a cross-pollination experiment.
3.2 Mendel's First Experiment
Review
Questions
1. Describe in general terms Mendel’s first set of experiments.
2. State Mendel's first law.
3. Assume you are investigating the inheritance of stem length in pea plants. You
cross-pollinate a short-stemmed plant with a long-stemmed plant. All of the
offspring have long stems. Then, you let the offspring self-pollinate. Describe the
stem lengths you would expect to find in the second generation of offspring.
Answers
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1. Mendel first experimented with just one characteristic of a pea plant at a time. He
started with color and crossed two different color plants. The F1 generation
results from cross-pollination of two parent (P) plants, and contained all purple
flowers. The F2 generation results from self-pollination of F1 plants, and
contained 75% purple flowers and 25% white flowers.
2. The law of segregation states that there are two factors controlling a given
characteristic, one of which dominates the other, and these factors separate and
go to different gametes when a parent reproduces.
3. The F2 would have 75% long-stemmed plants and 25% short-stemmed plants.
3.3 Mendel's Second Experiment
Review
Questions
1. What was Mendel investigating with his second set of experiments? What was
the outcome?
2. State Mendel’s second law.
3. If a purple-flowered, short-stemmed plant is crossed with a white-flowered,
long-stemmed plant, would all of the purple-flowered offspring also have
short stems? Why or why not?
Answers
1. With his second set of experiments, Mendel wondered whether different
characteristics are inherited together. He found that different characteristics were
not inherited together.
2. The law of independent assortment states that factors controlling different
characteristics are inherited independently of each other.
3. No, all of the purple-flowered offspring would not have short stems. From
Mendel’s second set of experiments, the independent assortment of factors
would result in purple-flowered offspring having either short stems or long stems.
3.4 Mendel's Laws
Review
Questions
1. If Darwin knew of Mendel’s work, how might it have influenced his theory of
evolution? Do you think this would have affected how well Darwin’s work was
accepted?
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2. Explain Mendel’s laws in genetic terms, that is, in terms of chromosomes, genes,
and alleles.
3. Explain the relationship between genotype and phenotype. How can one
phenotype result from more than one genotype?
Answers
1. Darwin may have been able to link genetics and evolution, demonstrating how
traits are inherited over time and how this can influence evolution. This may have
made for a much stronger argument, and help the acceptance of his work, to
some. Answers can vary, but must be supported.
2. Mendel’s first law states that there are two alleles controlling a given
characteristic, one of which dominates the other, and these alleles separate and
go to different gametes when a parent reproduces. His second law states that
genes controlling different characteristics are inherited independently of each
other. Genes are located on chromosomes, and the genes encode the traits
Mendel studied. Each gene has multiple alleles, and these alleles are Mendel’s
factors. As chromosomes segregate during meiosis, alleles and genes end up in
unique combinations within gametes.
3. The alleles an individual inherits make up the individual’s genotype. The two
alleles may be the same or different. The expression of an organism’s genotype
produces its phenotype. As the dominant allele is expressed over the recessive
allele, a heterozygote (Bb for example) will express the dominant allele and have
the dominant phenotype, just like a homozygote (for the dominant allele, BB).
3.5 Probability
Review
Questions
1. Define probability. Apply the term to a coin toss.
2. How is gamete formation like tossing a coin?
3. With a BB homozygote, what is the chance of a gamete having the B allele? The
b allele?
Answers
1. Probability is the likelihood, or chance, that a certain event will occur. When you
toss a coin, the chance of a head turning up is 50 percent. This is because a coin
has only two sides, so there is an equal chance of a head or tail turning up on
any given toss.
2. During meiosis, homologous chromosomes, and the alleles they carry, segregate
and go to different gametes. Therefore, using Hh for example, the H and h alleles
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segregate and go to different gametes. As a result, half the gametes produced by
the Hh parent will have the H allele and half will have the h allele. Based on the
rules of probability, any given gamete of this parent has a 50% chance of having
the H allele and a 50% chance of having the h allele, just like getting heads or
tails when tossing a coin.
3. 100% B allele, 0% b allele.
3.6 Punnett Squares
Review
Questions
1. What is a Punnett square? How is it used?
2. Draw a Punnett square of an Ss x ss cross. The S allele codes for
long stems in pea plants and the s allele codes for short stems. If S is dominant
to s, what percentage of the offspring would you expect to have each phenotype?
3. What letter should replace the question marks (?) in this Punnett square? Explain
how you know.
4. How do the Punnett squares for a monohybrid cross and a dihybrid cross differ?
5. What are the genotypes of gametes of a AaBb self-pollination?
6. Mendel carried out a dihybrid cross to examine the inheritance of the
characteristics for seed color and seed shape. The dominant allele for yellow
seed color is Y, and the recessive allele for green color is y. The dominant allele
for round seeds is R, and the recessive allele for a wrinkled shape is r. The two
plants that were crossed were F1 dihybrids RrYy. Identify the ratios of traits
that Mendel observed in the F2 generation. Create a Punnett square to help you
answer the question.
Answers
1. A Punnett square is a chart that allows you to easily determine the expected
percentage of different genotypes in the offspring of two parents.
2. 50% long stems, 50% short stems
3. The ? should all be replace by "a." As one parent only contributes an "A," and as
offspring in each column and row have an "a," the "a" must come from the
second parent, which must be homozygous aa.
4. A monohybrid cross Punnett square is a very simple 4 box square. A dihybrid
cross Punnett square is more complex at 16 boxes.
5. The genotypes of the gametes of a AaBb self-pollination cross are AB, Ab, aB,
ab.
6. 9 yellow, round seeds; 3 yellow, wrinkled seeds; 3 green, round seeds; 1 green,
wrinkled seeds.
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3.7 Non-Mendelian Inheritance
Review
Questions
1. A classmate tells you that a person can have type AO blood. Do you agree?
Explain.
2. Mendelian inheritance does not apply to the inheritance of alleles that result in
incomplete dominance and codominance. Explain why this is so.
3. Describe the relationship between environment and phenotype.
4. Mendel investigated stem length, or height, in pea plants. What if he had
investigated human height instead? Why would his results have been harder to
interpret?
Answers
1. No, a person cannot have type AO blood. There is no such blood type. Type O
blood only exists with a recessive genotype.
2. Incomplete dominance shows an offspring phenotype between that of the two
parents, which is unlike Mendel’s observations. Also, codominance occurs when
both alleles are expressed equally in the phenotype of the heterozygote. Two
dominant alleles is also unlike Mendel’s observations.
3. For some characteristics, such as human height, the environment interacts with
alleles to determine the phenotype.
4. Human height has a wide range of phenotypes. Firstly, it would be difficult to
control the crosses of humans and wait for the phenotype to develop. Secondly,
the wide range of phenotypes (as human height is a polygenic characteristic)
would have been difficult to interpret genetically with what was known in
Mendel’s time.
3.8 Human Genome
Review
Questions
1. Describe the human genome.
2. What has the Human Genome Project achieved?
3. Describe the makeup of the human genome.
Answers
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1. The human genome is all the DNA of Homo sapiens. Humans have about 3
billion bases of information, divided into roughly 20,000 to 22,000 genes, which
are spread among non-coding sequences and distributed among 24 distinct
chromosomes (22 autosomes plus the X and Y sex chromosomes).
2. The Human Genome Project has determined the DNA sequence of the entire
human genome.
3. The human genome consists of protein-coding exons, associated introns and
regulatory sequences, genes that encode other RNA molecules, and other DNA
sequences.
3.9 Human Chromosomes
Review
Questions
1.
2.
3.
4.
Describe human chromosomes.
Compare and contrast human autosomes and sex chromosomes.
What is SRY?
Why are females the "default sex" of the human species?
Answers
1. Chromosomes are coiled structures made of DNA and proteins called histones.
Chromosomes are the form of the genetic material of a cell during cell division.
The human genome has 23 pairs of chromosomes located in the nucleus of
somatic cells: 22 pairs are autosomes, and 1 pair is the sex chromosomes, which
may be two X-chromosomes, or one X and one Y-chromosome.
2. Autosomes are chromosomes that contain genes for characteristics that are
unrelated to sex. These chromosomes are the same in males and females. The
sex chromosomes are related to sex determination (actually only the Y
chromosome contains genes that determine sex).
3. SRY is a gene on the Y-chromosome known as the sex-determining region Y,
and triggers an embryo to develop into a male. Without a Y chromosome, an
individual develops into a female.
4. Without a Y chromosome, an individual develops into a female. As females have
two X-chromosomes, and no Y-chromosome, they lack the SRY gene, so the
female sex automatically develops.
3.10 Genetic Linkage
Review
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Questions
1. What are linked genes?
2. Explain how you would construct a linkage map for a human chromosome. What
data would you need?
3. People with red hair usually have very light skin. What might be a genetic
explanation for this observation?
4. How often does crossing-over occur between non-linked genes? Explain your
answer.
Answers
1. Linked genes are genes located on the same chromosome.
2. Frequencies of crossing-over are used to construct a linkage map. Linkage is
assessed by determining how often crossing-over occurs between two genes on
the same chromosome. The lower the frequency of crossing over, the closer
together on the same chromosome the genes are presumed to be.
3. The genes for red hair and light skin are usually inherited together, meaning they
are linked, and close together on the same chromosome.
4. Never, crossing-over does not occur between non-linked genes, as they are on
different chromosomes.
3.11 Mendelian Inheritance
Review
Questions
1. Describe the inheritance pattern for a single-gene autosomal dominant trait, such
as free-hanging earlobes.
2. Draw a pedigree for hitchhiker’s thumb. Your pedigree should cover at least two
generations and include both dominant and recessive forms of the trait. Label the
pedigree with genotypes, using the letter H to represent the dominant allele for
the trait and the letter h to represent the recessive allele.
3. Why is a recessive X-linked allele always expressed in males?
4. What is necessary for a recessive X-linked allele to be expressed in females?
5. What is an example of a recessive X-linked trait?
Answers
1. Single-gene autosomal dominant traits are passed from parent to offspring when
at least one dominant allele is inherited. Only when two recessive alleles are
inherited, which can only occur when both parents are heterozygous, will the trait
be absent.
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2. Answers may vary but should be at least two generations and cover both the
dominant and recessive forms of the trait.
3. Because males have just one X chromosome, they have only one allele for any
X-linked trait. If that allele is recessive, there is no dominant allele to be
expressed, so a recessive X-linked allele will always be expressed in males.
4. Because females have two X-chromosomes, for a recessive allele to be
expressed, the female must be homozygous for that allele.
5. An example of a recessive X-linked trait is red-green color blindness.
3.12 Genetic Disorders
Review
Questions
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2.
3.
4.
5.
Describe a genetic disorder caused by a mutation in a single gene.
What causes Down syndrome?
What is nondisjunction?
What is gene therapy?
Explain why genetic disorders caused by abnormal numbers of chromosomes
most often involve the X chromosome.
Answers
1. Marfan syndrome is a genetic disorder caused by a mutation in a single gene. It
is an autosomal dominant disorder caused by a defective protein in connective
tissue. People with Marfan syndrome have heart and bone defects and unusually
long, slender limbs and fingers. (Add: Other disorders may be discussed.)
2. Down syndrome is caused by an extra copy (complete or partial) of chromosome
21.
3. Nondisjunction is the failure of replicated chromosomes to separate properly
during meiosis.
4. Gene therapy is a method to cure genetic disorders by inserting normal genes
into cells with mutant genes.
5. Most chromosomal disorders involve the X chromosome. This is because the X
and Y chromosomes are very different in size, so nondisjunction of the sex
chromosomes occurs relatively often. Also, zygotes with abnormal numbers of
autosomes usually do not survive.
3.13 Biotechnology
Review
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Questions
1.
2.
3.
4.
5.
Define biotechnology.
What is recombinant DNA?
Identify the steps of gene cloning.
What is the purpose of the polymerase chain reaction?
Describe the three steps of PCR.
Answers
1. Biotechnology is the use of technology to change the genetic makeup of living
things for human purposes.
2. Recombinant DNA is the DNA that results when DNA from two organisms is
combined.
3. The four main steps of gene cloning are (1) the use of restriction enzymes to cut
DNA at specific location, (2) the ligation of the isolated gene/DNA with a plasmid
to produce recombinant DNA, (3) transformation of bacteria with the recombinant
DNA, and (4) selection of bacteria with the recombinant DNA.
4. The polymerase chain reaction (PCR) makes many copies of a gene or other
DNA segment.
5. PCR involves three steps: denaturing, annealing, and extension.
a. Denaturing involves heating DNA to break the bonds holding together the
two DNA strands. This yields two single strands of DNA.
b. Annealing involves cooling the single strands of DNA to allow the primers
to bind to the DNA sequence.
c. Extension occurs when Taq polymerase adds nucleotides to the primers,
doubling the amount of DNA molecules.
3.14 Biotechnology Applications
Review
Questions
1. What are transgenic crops?
2. Make a flow chart outlining the steps involved in creating a transgenic crop.
3. Explain how bacteria can be genetically engineered to produce a human protein.
Answers
1. Transgenic crops are genetically modified with new genes that code for traits
useful to humans.
2. (1) The desired gene is ligated into a plasmid. (2) The recombinant DNA is
inserted back into the bacteria. (3) The bacteria is used to insert the gene of
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interest into the chromosome of a plant cell. (4) Plant cells are grown in culture,
and (5) a plant is generated from a cell clone.
3. If a gene of interest for a human protein is ligated into a plasmid, and the plasmid
is transformed into bacteria, the bacteria will treat the transformed gene as if it
belonged in the cell, transcribing the gene and translating the protein. As the
DNA is replicated prior to cell division, the plasmid will be replicated and divided
among the daughter cells. The bacteria will continue to produce the human
protein this way.
3.15 Ethical, Legal, and Social Issues in Biotechnology (ELSI)
Review
Questions
1. Identify three ethical, legal, or social issues raised by biotechnology.
2. State your view on an ELSI issue, and develop a logical argument to support
your view.
Answers
1. Answers will vary. ELSI issues include the following: Are genetically modified
foods safe to eat? Who controls a person’s genetic information? How far should
we go to ensure that children are free of mutations? etc.
2. Answers will vary.
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