Mastering Concepts
10.1
1. Describe the relationships among chromosomes, DNA, genes, and alleles.
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 a 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.
3. Define the P, F1, and F2 generations.
The P generation is the parental or starting generation. F1 and F2 refer to the first and
second generations of offspring, respectively.
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
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 does the law of segregation reflect the events of meiosis?
The law of segregation says that the two alleles of each gene are packaged into separate
gametes. This law is a consequence of the events of anaphase I in meiosis, during which
homologous chromosomes split up and move into separate cells.
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?
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 complicates the inheritance of
linked genes; sometimes allele combinations 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. They are a result of crossing over. In
contrast, a parental chromatid carries the same combination of alleles that was inherited
from a parent.
3. How do biologists use crossover frequencies to map genes on chromosomes?
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?
Incomplete dominance and codominance produce phenotypes that are intermediate
between or combinations of those produced by homozygous dominant or homozygous
recessive individuals.
2. Differentiate between pleiotropy and epistasis.
Pleiotropy occurs when one gene contributes to multiple phenotypes. The protein
encoded by the gene may enter several different biochemical pathways or affect more
than one body part or process. Epistasis occurs when one gene’s product masks the
effects of another gene.
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
flawed metabolic pathway. In this way, different genotypes can produce the same
phenotype (failure of the metabolic pathway to operate properly).
4. How can epistasis decrease the number of phenotypes observed in a population?
In epistasis, one gene affects the expression of another. The gene interaction may cause
some phenotypes to appear to be missing from a population.
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. What is the role of the SRY gene in sex determination?
In human sex determination, the Y chromosome’s SRY gene encodes a protein that acts as
a master switch. The SRY protein turns on other genes, which direct the undeveloped
testes to secrete the male sex hormone testosterone and to develop male structures. SRY
also turns on a gene encoding a protein that causes embryonic female structures to
disassemble. If a functional SRY gene is not present, an embryo will develop as a female.
3. 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.
4. 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 observing which individuals have a disorder over multiple generations in the same
family, it is often possible to determine whether a disorder is autosomal or sex-linked and
to determine whether it is dominant or recessive.
2. How do the pedigrees differ for autosomal dominant, autosomal recessive, and Xlinked recessive conditions?
In autosomal dominant disorders, pedigrees show affected individuals in every
generation, and all affected individuals have at least one affected parent. In autosomal
recessive conditions, affected individuals can have unaffected parents, so the condition
may skip generations. Autosomal conditions affect males and females equally. In
contrast, X-linked recessive conditions appear in pedigrees where more males are
affected than females; also, affected males can have normal parents, but affected females
must also have an affected father.
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 a polygenic trait?
A polygenic trait is one that is controlled by multiple genes.
10.10
1. How did researchers in this study use a breeding experiment to demonstrate that Bt
resistance alleles in pink bollworms are recessive?
If the resistant alleles are recessive, then matings between heterozygous (susceptible)
moths and homozygous recessive (resistant) moths should yield offspring with a 50%
chance of being susceptible and 50% chance of being resistant. In the breeding
experiments, approximately 50% of the offspring thrived in the presence of Bt, while
50% either died or were quite small. These results supported the hypothesis that the
resistance alleles are recessive.
2. What do you predict will happen to the incidence of resistance alleles in pink bollworm
populations if farmers choose not to plant the required refuge?
Without the refuge strip, resistant moths will only have other resistant moths to mate
with. Allele frequencies will shift toward the recessive (resistance) allele in future
generations.
Write It Out
1. What advantages do pea plants and fruit flies have for studies of inheritance? Why
aren’t humans equally suitable?
Both peas and fruit flies are easy to grow, develop rapidly, produce many offspring, and
have many traits that appear in two easily distinguishable forms. In addition, it is easy to
control genetic crossing in pea plants and fruit flies. Humans cannot be used because
they take longer to reach sexual maturity, do not produce an abundance of offspring, and
cannot be forced to mate to suit the objectives of an experiment.
2. Some people compare a homologous pair of chromosomes to a pair of shoes. Explain
the similarity. How would you extend the analogy to the sex chromosomes for females
and for males?
Shoes come in many varieties, including sandals, boots, and sneakers. Each shoe is paired
with a matching shoe, which has the same size and general appearance, yet the two
members of the pair are not identical. Similarly, homologous chromosomes are the same
length and shape, with the same genes in the same places, yet the alleles are not identical.
The sex chromosomes of males are not homologous, however, and would be like an adult
size 11 sneaker paired with a child’s size 3 sandal. In a female, the shoes would be
homologous and would match.
3. In an attempt to breed winter barley that is resistant to barley mild mosaic virus,
agricultural researchers cross a susceptible domesticated strain with a resistant wild
strain. The F1 plants are all susceptible, but when the F1 plants are crossed with each
other, some of the F2 individuals are resistant. Is the resistance allele recessive or
dominant? How do you know?
The resistance allele is recessive because it was not expressed in the F1 generation but
was expressed in some plants of the F2 generation.
4. How did Mendel use evidence from monohybrid and dihybrid crosses to deduce his
laws of segregation and independent assortment? How do these laws relate to meiosis?
From his series of monohybrid crosses, Mendel concluded that genes occur in alternative
forms (alleles) and that each individual inherits two alleles for each gene. His law of
segregation states that two alleles of the same gene separate as they are packaged into
gametes. This law reflects meiosis because homologous chromosomes are pulled into
separate cells during meiosis I. From his series of dihybrid crosses, Mendel developed
the law of independent assortment, which states that during gamete formation, the
segregation of the alleles of one gene does not influence the segregation of the alleles for
another gene. This law reflects meiosis (as long as the two genes being studied reside on
different chromosomes) because the orientation of each homologous pair of
chromosomes does not affect the orientation of other homologous pairs during meiosis I.
5. How does crossing over "unlink" genes?
Crossing over separates alleles that occurred together on the same chromatid, so that
alleles that were previously linked are no longer transmitted together.
6. Consider two genes that are near each other on a chromosome. After a germ cell
undergoes meiosis, are the resulting gametes likely or unlikely to contain a recombinant
chromatid for these two genes? Explain.
If the two genes are close together on the same chromosome, they are unlikely to be
separated during crossing over. The gametes are therefore unlikely to contain a
recombinant chromatid for these two genes,
7. Springer spaniels often suffer from canine phospho-fructokinase (PFK) deficiency. The
dogs lack an enzyme that is crucial in extracting energy from glucose molecules.
Affected pups have extremely weak muscles and die within weeks. A DNA test is
available to identify male and female dogs that are carriers. Why would breeders wish to
identify carriers if these dogs are not affected?
It would be beneficial because breeders could prevent carriers from mating, thus reducing
the incidence of this disease in the dogs.
8. Explain how each of the following produces phenotypic ratios other than those Mendel
observed: incomplete dominance, codominance, pleiotropy, epistasis.
Incomplete dominance: The heterozygote’s phenotype is intermediate between those of
the two homozygotes. This goes against the idea that two alleles should produce only
two phenotypes, with one allele dominant over the other. Instead of a 3:1 phenotypic
ratio, the ratio is 1:2:1.
Codominance: The heterozygote fully expresses two different alleles. This goes against
the idea that two alleles should produce only two phenotypes, with one allele dominant
over the other. Instead of a 3:1 phenotypic ratio, the ratio is 1:2:1.
Pleiotropy: One gene has multiple phenotypic expressions. Mendel’s laws imply that
each gene controls only one trait. One allele can therefore change the phenotype in
multiple ways.
Epistasis: One gene affects the expression of another gene. Entire classes of phenotypes
corresponding to one gene can seem to disappear if the allele of the other gene changes.
9. Which gene on the Y chromosome triggers the development of male characteristics?
What would happen if a male inherited a nonfunctional allele for this gene?
The SRY gene encodes a protein that stimulates the development of male structures in an
embryo. A male that inherits a nonfunctional SRY allele would be genetically male but
would appear female.
10. An individual that is genetically male develops as a female. Is this individual more or
less likely to express an X-linked recessive disorder than an average female?
The individual is genetically male, so he has only one X chromosome. Like all other
males, he is more likely to express an X-linked recessive disorder than an average female.
11. Would you expect dominant X-linked traits to affect women as often as men? Explain
your answer.
The simple answer is yes, because females and males would need only one affected X to
express the disorder. However, X inactivation means that at least some of a heterozygous
female’s cells are likely to express the normal X. Therefore, an X-linked dominant
disorder is likely to be less severe in females than in males.
12. What does X inactivation accomplish?
In X inactivation, all but one X chromosome is shut off in each cell, a process that
happens early in the embryonic development of a mammal. This prevents female
mammals with two X chromosomes from receiving a “double dose” of X-linked genes
relative to a male.
13. A family has an X-linked dominant form of congenital generalized hypertrichosis
(excessive hairiness). Although the allele is dominant, males are more severely affected
than females. Moreover, the women in the family often have asymmetrical, hairy patches
on their bodies. How does X chromosome inactivation explain this observation?
A female is a mosaic for X-linked genes because the maternal or paternal X chromosome
is inactivated at random in each cell.
14. X inactivation explains the large color patches in calico cat fur and the smaller
patches in tortoiseshell cat fur. In which type of cat do you expect X inactivation occurs
earlier in development? Why?
The earlier in development that X inactivation occurs, the larger the portion of the body
affected by the X chromosome that is expressed. The relatively large patches that
characterize the calico cat therefore reflect earlier X chromosome inactivation, and the
small patches in the tortoiseshell cat reflect later inactivation.
15. In the following pedigree, is the disorder’s mode of inheritance autosomal dominant,
autosomal recessive, or X-linked recessive? Explain your reasoning.
The mode of inheritance is autosomal dominant. The disorder cannot be X-linked
recessive since individual 7 on line II, who received only one X from her affected father,
expresses the disorder. Notice that the pedigree has no carriers; every individual that
inherits an allele of the disorder expresses the disorder. The allele conferring the disorder
must therefore be dominant.
16. 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.
17. How do heart disease and cancer illustrate diseases that reflect both genetic and
environmental influences?
Heart disease runs in families, suggesting a possible genetic connection, but the risk of
heart disease also increases in smokers and people who do not get enough exercise.
Chapter 8 described that cancer reflects heritable genetic mutations and environmental
risk factors such as exposure to UV radiation or cigarette smoke.
18. Design an experiment using twins to determine the degree to which autism is genetic
or environmental.
One possible approach would be to track the incidence of autism in two groups, each
consisting of multiple sets of identical twins. In one group, the twins should be separated
at birth and reared in different homes; in the other group, the twins should remain
together and be reared in the same home. This design allows the researchers to determine
the effect of environment while holding genetics constant. If autism is 100% genetic, then
identical twins should always have the same phenotype for autism (either both have it or
both don’t), regardless of environment. If autism is 100% environmental, then twins
should be no more likely to share the autism phenotype than any other two people.
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 a breeder mates two
dogs that are heterozygous for the hair allele. Draw a Punnett square to 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).
3. A species of ornamental fish comes in two colors; red is dominant and gray is
recessive. Emily owns a red fish, and she wants to know its genotype. Therefore, she
mates her pet 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 GgDd). (a) If 32 offspring
are born, how many of the offspring are expected to be homozygous recessive for both
genes? (b) What proportion of the offspring will have the dominant phenotype for both
traits? (Assume that the traits assort independently.)
(a) 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 ggdd is ¼ x ¼ = 1/16. If the
lizards have 32 offspring, two should therefore be homozygous recessive for both genes.
(b) About 75% of the offspring will have the dominant phenotype for gene G (GG or
Gg). Likewise, about 75% of the offspring will have the dominant phenotype for gene D
(DD or Dd). The probability of offspring with the dominant phenotype for both traits is
therefore ¾ x ¾ = 9/16.
5. A fern with genotype AABbCcddEe mates with another fern with genotype
aaBbCCDdee. 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 (AaBbCcDdEe) is therefore 1 x ½ x ½ x ½ x ½ = 1/16.
6. Genes Q, R, and S are on the same chromosome. The crossover frequency between S
and Q is 5%, the crossover frequency between Q and R is 30%, and the crossover
frequency between R and S is 35%. Use this information to create a linkage map for the
chromosome.
The largest crossover frequency indicates the two genes that are farthest apart. The
smallest frequency indicates the two closest genes. So the map is _S_Q___________R_.
7. Three babies are born in the hospital on the same day. Baby X has type B blood; Baby
Y has type AB blood; Baby Z has type O blood. Use the information in the table below to
determine which baby belongs to which couple. (Assume that all individuals are
homozygous dominant for the H gene.)
The only couple that could have a baby with genotype AB is Logan/Leslie, so baby Y
must belong to couple 1. Of the remaining couples, only Jordan/Taylor could have a baby
with blood type B, so baby X must belong to couple 3. Baby Z therefore belongs to
Sam/Casey (couple 2).
8. 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.
9. A woman is a carrier for red-green color blindness. Her genotype is XcX, where Xc
indicates the chromosome with the allele conferring color blindness and X indicates the
chromosome with the normal color vision allele. Very early in development, the Xc
chromosome was inactivated in the first cell of her right eye. At the same time, the
normal X chromosome was inactivated in the first cell of her left eye. Is she color blind in
her left eye, her right eye, both eyes, or neither eye?
In the right eye, Xc is inactivated, so color vision is normal. In the left eye, the normal X
is inactivated; the left eye is color blind because Xc is expressed.
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. Explain the effects of a mutation, using allele, dominant, recessive, genotype, and
phenotype in your answer.
A mutation creates a new allele of a gene. Typically, a nonfunctional allele will be
recessive, and a functional allele will be dominant. Any mutation to the nucleotide
sequence of a gene affects the genotype. Only mutations that affect the function of the
protein that the gene encodes affect the phenotype.
3. Add meiosis, gametes, mutations, incomplete dominance, codominance, pleiotropy,
and epistasis 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 “results in 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.” “Epistasis” can lead with “occurs when expression of one gene affects the
expression of another” to “Gene.”