Answers to Open-Ended Questions
Hoefnagels Essentials 2/e
Chapter 10
Answers to Mastering Concepts
10.1
1. How are chromosomes, DNA, genes, and alleles related?
Chromosomes consist of DNA and associated proteins. The DNA in a chromosome is
divided into genes, which are sequences of nucleotides that encode proteins. Alleles are
different versions of one gene.
2. How do meiosis, fertilization, diploid cells, and haploid cells interact in a sexual life
cycle?
Specialized diploid cells undergo meiosis, a type of cell division that produces haploid
cells. Haploid cells, in turn, combine during fertilization to form the diploid zygote,
which is the first cell of the next generation.
10.2
1. Why did Gregor Mendel choose pea plants as his experimental organism?
Mendel chose pea plants because they are easy to grow, develop quickly, produce many
offspring, and have many traits that appear in two forms that are easy to distinguish. It
also is easy to hand-pollinate pea plants, so an investigator can control which plants mate
with one another.
2. Distinguish between dominant and recessive; heterozygous and homozygous;
phenotype and genotype; wild-type and mutant.
Dominant alleles appear in a phenotype whenever they are present; recessive alleles
contribute to the phenotype only if no dominant alleles are present. An individual is
homozygous for a gene if both alleles are identical; in a heterozygous individual, the two
alleles for a gene are different. An organism’s phenotype is its appearance; the genotype
is the alleles an individual possesses. The wild type allele is the most common form of a
gene in a population; a mutant allele arises when a gene undergoes a mutation.
10.3
1. What is a monohybrid cross, and what are the genotypic and phenotypic ratios
expected in the offspring of the cross?
A monohybrid cross is a mating between two individuals that are each heterozygous for
one gene. The genotypic ratio expected in a monohybrid cross is 1:2:1 (homozygous
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dominant:heterozygous:homozygous recessive); the phenotypic ratio is 3:1
(dominant:recessive).
2. How are Punnett squares helpful in following inheritance of single genes?
Punnett squares show the genotypes of each parent as well as the genotypes of potential
offspring. Phenotypic and genotypic ratios of offspring can be predicted from the data in
Punnett squares.
3. What is a testcross, and why is it useful?
A testcross is a mating between a homozygous recessive individual and an individual of
unknown genotype. The genotype of the unknown parent can be deduced from the ratio
of phenotypes among the offspring.
4. How do Punnett squares reflect the events of meiosis?
Punnett squares are diagrams in which the possible gametes for the two parents are
written along two edges of the square (the blanks inside the Punnett square represent all
possible fertilization events). The alleles inside each parent’s gametes reflect the
movement of homologous chromosomes into separate cells during meiosis I. That is,
during metaphase I, homologous chromosomes align randomly at the center of the cell.
Then, during anaphase I and telophase I, the homologous chromosomes separate from
each other and move to separate cells. Because the chromosomes lined up randomly
during metaphase I, each gamete has an equal probability of receiving either allele.
10.4
1. What is a dihybrid cross, and what is the phenotypic ratio expected in the offspring of
the cross?
In a dihybrid cross, two individuals that are each heterozygous for two genes are mated.
The phenotypic ratio that is expected is 9:3:3:1 (dominant for both genes: dominant for
one gene and recessive for the other: recessive for one gene and dominant for the other:
recessive for both genes).
2. How does the law of independent assortment reflect the events of meiosis?
The law of independent assortment states that the segregation of alleles for one gene does
not affect the segregation of alleles for another gene on a separate chromosome. This law
is a consequence of the events of metaphase I in meiosis, during which each homologous
pair of chromosomes aligns independently of other chromosome pairs during metaphase I
of meiosis.
3. How can the product rule be used to predict the results of crosses in which multiple
genes are studied simultaneously?
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The product rule allows you to estimate the odds that an offspring will have a certain
combination of alleles for multiple genes by multiplying the probability that each
separate event will occur.
10.5
1. How do patterns of inheritance differ for unlinked versus linked pairs of genes?
The inheritance pattern of unlinked genes is predictable since allele combinations are not
affected by crossing over. When pairs of genes are linked, they are carried on the same
chromosome and are inherited together. Crossing over can occur between linked genes,
creating allele combinations that differ from those in either parent.
2. What is the difference between recombinant and parental chromatids, and how do they
arise?
Recombinant chromatids are chromosomes that have a mixture of maternal and paternal
alleles instead of alleles from just a single parent. Crossing over produces recombinant
alleles. In contrast, a parental chromatid carries the same combination of alleles that was
inherited from a parent.
3. Explain how to use crossover frequencies to make a linkage map.
The farther apart two linked genes are on a chromosome, the more frequently they will
cross over. Analyzing crossover frequencies for multiple pairs of traits reveals which
genes are close together and which are far apart. This information is used to deduce
linkage maps, which show the relative positions of genes on chromosomes.
10.6
1. How do incomplete dominance and codominance increase the number of phenotypes
observed in a population?
Incomplete dominance and codominance produce phenotypes that are intermediate
between or combinations of those produced by homozygous dominant or homozygous
recessive individuals.
2. What is pleiotropy?
Pleiotropy occurs when a single gene has multiple effects on the phenotype. The gene
may encode a protein that enters several different biochemical pathways or that affects
more than one body part or process.
3. How can the same phenotype stem from many different genotypes?
Each gene encodes one protein, but many different proteins may interact in a single
metabolic pathway. A mutation in a gene encoding any of these proteins may produce a
similar phenotype (i.e., failure of the metabolic pathway).
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4. This section explains two ways that a person can have blood that tests as type O.
Explain how each relates to specific alleles of genes I and H.
Gene I determines the combination of A and/or B molecules on red blood cells; gene H
encodes a protein that is required to link the molecules to the cell surface. The most
obvious way to have a blood type of O is to be homozygous for allele i. The second way
is to be homozygous for allele h; in that case, any combination of alleles for gene I will
produce blood that tests as type O, because no protein will be available to link any A or B
markers on the cell surface.
10.7
1. What determines a person’s sex?
One pair of chromosomes, the sex chromosomes, determines a person’s sex. A female
has two X chromosomes; a male has an X and a Y chromosome.
2. Why do males and females express recessive X-linked alleles differently?
Each female has a pair of X chromosomes, whereas a male has only one X chromosome.
Any trait a male has on his X chromosome will be expressed. Recessive alleles on an X
chromosome of a female may be masked by dominant alleles on her other X
chromosome.
3. Why does X inactivation occur in female mammals?
X inactivation prevents each cell of a female from receiving a “double dose” of X
chromosome genes.
10.8
1. How are pedigrees helpful in determining a disorder’s mode of inheritance?
By documenting family relationships and observing which individuals have a disorder
over multiple generations, it is often possible to determine whether a disorder is
autosomal or sex-linked and to determine whether it is dominant or recessive.
2. For each of the pedigrees in figure 10.23, determine the genotype of individual #1 in
the first row.
In part (a), autosomal dominant, individual #1 is homozygous recessive because she does
not have achondroplasia. If she were heterozygous or homozygous dominant, she would
have the condition. In part (b), individual #1 is heterozygous; she is a carrier of albinism,
an autosomal recessive trait. In part (c), individual #1 is also heterozygous. One of her X
chromosomes carries the recessive allele encoding red-green colorblindness, and the
other does not.
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10.9
1. How can the environment affect a phenotype?
The environment can directly affect the expression of some genes. For example,
temperature can influence the expression of temperature-sensitive alleles. The
environment can also affect the phenotype in other ways, as when infectious agents
intensify a genetic disorder. Many aspects of the phenotype, including temperament and
physical health, reflect not only genes but also upbringing, nutrition, and many other
environmental variables.
2. What is polygenic inheritance, and how is it different from codominance?
A polygenic trait is one that is controlled by multiple genes. Codominance reflects the
relationships among multiple alleles of the same gene.
Write It Out
1. Select one gene mentioned in the chapter, then explain the link between an organism’s
genotype (for that gene) and its corresponding phenotype. Make sure to use the term
protein in your answer.
Cystic fibrosis arises when an individual inherits two recessive alleles of a particular gene
on chromosome 7. The recessive alleles encode nonfunctional proteins, which affect the
function of the lungs.
2. List three genes (mentioned in this chapter or not) that do not affect physical
appearance. Do these genes contribute to an organism’s phenotype?
Three examples of genes that do not affect physical appearance are the gene that confers
ABO blood type (usually denoted I); the gene that encodes the phenylalanine-converting
enzyme (the absence of which leads to the disease PKU); and the gene that encodes a
clotting factor protein (the absence of which leads to hemophilia). These three genes
affect the individual’s characteristics, even if their effects are not outwardly apparent.
Other observation techniques (e.g., blood analysis) reveal these phenotypes.
3. A woman with fair skin, blond hair, and blue eyes gives birth to fraternal twins; the
father has dark brown skin, dark hair, and brown eyes. One twin has blond hair, brown
eyes, and light skin, and the other has dark hair, brown eyes, and dark skin. What
Mendelian law does this real-life case illustrate?
This scenario represents Mendel’s principle of independent assortment. Different genes
confer each of these traits. When cells inside the mother and father underwent meiosis,
the alleles for these genes moved independently of one another (i.e., the movements of
the chromosome with the “hair color” gene did not affect the movements of the
chromosome with the “eye color” gene). Each parent therefore produced gametes with
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different combinations of alleles. The twins each happened to inherit a different set of
alleles for these genes.
4. If two different but linked genes are located very far apart on a chromosome, how may
the inheritance pattern create the appearance of independent assortment?
Since the genes are very far apart on the chromosome, they have a high probability of
being separated by crossing over. Recombinant chromatids may be common, giving the
appearance that the genes are on separate chromosomes.
5. Explain how each of the following appears to disrupt Mendelian ratios: incomplete
dominance, codominance, pleiotropy. In each case, what is happening at the level of the
protein?
(1) In incomplete dominance, the heterozygote’s phenotype is intermediate between those
of the two homozygotes; the number of phenotypes is therefore three instead of two.
Incomplete dominance happens when multiple alleles encode functional proteins, whose
effects appear to blend in the phenotype. (2) In codominance, the heterozygote’s
phenotype includes the full expression of two different alleles; the number of possible
phenotypes is therefore three instead of two. Codominance happens when multiple alleles
encode functional proteins, whose effects both contribute fully to the phenotype. (3) In
pleiotropy, one gene encodes a protein that affects the phenotype in multiple ways that
seem unrelated to each other. For example, a single defective connective tissue protein
causes the long limbs, heart valve problems, and other symptoms of Marfan syndrome.
Mendelian ratios imply that each gene controls only one trait.
6. Many men with "Y chromosome infertility" are unable to produce sperm. Do you think
this condition is typically inherited? Explain your answer.
Since an affected man cannot produce sperm, he cannot pass on his defective Y
chromosome on to the next generation. The condition is therefore likely to arise
spontaneously, not be inherited.
7. In a condition called androgen insensitivity, an individual that is genetically male may
have the external appearance of a female. Is this individual more or less likely to express
an X-linked recessive disorder than an average female?
The individual has inherited only one X chromosome and is therefore more likely to
express an X-linked recessive disorder than an average female with two X chromosomes.
8. Calico cats have large patches of orange and black fur; in tortoiseshell cats, the patches
are smaller. In which type of cat does X inactivation occur earlier in development? How
do you know?
In cats, the genes encoding black and orange fur are located only on the X chromosome.
After one X chromosome is inactivated in a cell, all daughter cells from that cell line will
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also carry the same inactive and active X chromosomes. If X inactivation occurs early in
development, then the cat is calico (large patches of cells with the same inactivated X
chromosome). If X inactivation occurred later in development the cat would be
tortoiseshell (small patches of cells with the same inactivated X chromosome).
9. Pedigree charts can sometimes be difficult to construct and interpret. People may
refuse to supply information, and adoption or serial marriages can produce blended
families. Artificial insemination may involve anonymous sperm donors. Many traits are
strongly influenced by the environment. How does each of these factors complicate the
use of pedigrees?
Pedigrees are most useful when the genotypes of all members of a family can be
determined. If people refuse to supply medical information, it can be impossible to tell
who is affected and who is not. Blended families and artificial insemination make it
difficult or impossible to trace parentage. A person’s phenotype may not be a good
predictor of genotype in traits that are strongly influenced by the environment, making it
difficult to determine the genotypes of previous generations.
10. Explain the following “equation”:
Genotype + Environment = Phenotype
Genotype represents what proteins can be produced, but the environment often affects
which genes are expressed (and when they are expressed). The combination of all these
factors will determine the actual physical expression, or phenotype.
Genetics Problems
1. In rose bushes, red flowers (FF or Ff) are dominant to white flowers (ff). A true
breeding red rose is crossed with a white rose; two flowers of the F1 generation are
subsequently crossed. What will be the most common genotype of the F2 generation?
A true-breeding red rose has genotype FF; the white rose has genotype ff. Therefore all
flowers of the F1 generation have genotype Ff. When the F1 flowers are mated together,
the predicted offspring genotypes are 25% FF, 50% Ff, and 25% ff. The most common
genotype in the F2 generation is therefore Ff.
2. In Mexican hairless dogs, a dominant allele confers hairlessness. However, inheriting
two dominant alleles is lethal; the fetus dies before birth. Suppose two dogs that are
heterozygous for the hair allele mate. Predict the genotypic and phenotypic ratios of the
puppies that are born.
The parents are heterozygous (Hh). The Punnett square predicts a genotypic ratio of 25%
HH, 50% Hh, and 25% hh. However, the HH offspring are never born. As a result, twothirds of the puppies that are born will be Hh (hairless), and one-third will be hh (hairy).
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3. A species of ornamental fish comes in two colors; red is dominant and gray is
recessive. Emily mates her red fish with a gray fish. If 50 of the 100 babies are red, what
is the genotype of Emily's fish?
If Emily’s fish has genotype RR, then all of the baby fish will have genotype Rr (red). If
her fish has genotype Rr, then about half of the baby fish will have genotype Rr (red) and
about half will have genotype rr (gray). Since only half of the babies are red, Emily’s red
fish must have genotype Rr.
4. Two lizards have green skin and large dewlaps (genotype Gg Dd). If they mate and 32
offspring are born, how many of the offspring are expected to be homozygous recessive
for both genes? (Assume that the traits assort independently.)
Consider the genes one at a time. About 25% of the offspring of two lizards with
genotype Gg should be homozygous recessive (gg). Likewise, about 25% of the offspring
of two lizards with genotype Dd should be homozygous recessive (dd). According to the
product rule, the probability of offspring with genotype gg dd is ¼ x ¼ = 1/16. If the
lizards have 32 offspring, two should therefore be homozygous recessive for both genes.
5. A fern with genotype AA Bb Cc dd Ee mates with another fern with genotype aa Bb
CC Dd ee. Assume the genes assort independently. What proportion of the offspring will
be heterozygous for all genes? Hint: Use the product rule.
Consider the genes one at a time, then use the product rule. Gene A: 100% heterozygous.
Gene B: 50% heterozygous. Gene C: 50% heterozygous. Gene D: 50% heterozygous.
Gene E: 50% heterozygous. The probability of an offspring being heterozygous for all
genes (Aa Bb Cc Dd Ee) is therefore 1 x ½ x ½ x ½ x ½ = 1/16.
6. In fraggles, males are genotype XY and females are XX. Silly, a male fraggle, has a
rare X-linked recessive disorder that makes him walk backwards. He mates with Lilly,
who is a carrier for the disorder. What proportion of their male offspring will walk
backwards?
50% of the male offspring will walk backwards, and 50% will walk normally.
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Pull It Together
1. Which cells in the human body are haploid? Which cells are diploid?
Gametes are haploid cells and nearly all other cells are diploid.
2. What is the difference between genotype and phenotype?
A genotype describes the genetic makeup of an individual and a phenotype describes the
observable expression of its genetic makeup.
3. Add meiosis, gametes, mutations, incomplete dominance, codominance, and pleiotropy
to this concept map.
“Meiosis” connects with the phrase “produces” to “Gametes,” which leads with the word
“are” to “Haploid cells.” “Genes” connects with the phrase “can undergo” to
“Mutations,” which leads with “produce new” to “Alleles.” “Pleiotropy” connects with
“is when one gene has multiple effects on the” to “Phenotype.” “Codominance” can lead
with “occurs when multiple alleles for a gene are” to “Dominant.” “Incomplete
dominance” can lead with “occurs when heterozygotes have an intermediate” to
“Phenotype.”
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