Mastering Concepts
10.1
1. Describe the relationships among chromosomes, DNA, genes, and alleles.
Chromosomes contain tightly packed DNA and associated proteins. DNA are the strands
of genetic material that contains genes, sequences of nucleotides that code for amino
acids. Those genes come in varieties called alleles.
2. How do meiosis, fertilization, diploid cells, and haploid cells interact in a sexual life
cycle?
Meiosis in the adult organism creates the haploid gamete cells that combine during
fertilization to form the diploid zygote cell. That cell undergoes mitosis to make the cells
that are necessary for growth into the adult form.
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 alternate 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 both heterozygous for
one gene. The genotypic ratio expected in a monohybrid cross is 1:2:1; the phenotypic
ratio is 3:1.
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 test cross, and why is it useful?
A test cross 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 in the F1 generation.
4. How does the law of segregation reflect the events of meiosis?
The law of segregation reflects the movement of homologous chromosomes into separate
cells during meiosis I.
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 heterozygous for two genes are mated. The
phenotypic ratio that is expected is 9:3:3:1.
2. How does the law of independent assortment reflect the events of meiosis?
The law of independent assortment reflects that 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?
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 either parent. The inheritance pattern of non-linked genes
is more predictable since it is not affected by crossing over. The inheritance of non-linked
genes can be visualized using a Punnett Square.
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. In contrast, parental chromatids carry
the same combinations of alleles that were inherited from the parents. Crossing over has
not altered them.
3. How do biologists use crossover frequencies to map genes on chromosomes?
The farther apart genes are on a chromosome, the more frequently they will cross over.
By comparison, genes that are close together on a chromosome are less likely to be
separated. Analysis of how often the traits appear together helps to establish 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. What is pleiotropy?
Pleiotropy occurs when a gene produces multiple phenotypic expressions. Pleiotropy
results when the protein encoded by a gene enters several different biochemical pathways
or 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
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?
Sex determination is the factor (genetic or environmental) that decides if an organism is
male or female.
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. 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 its X chromosome will be expressed. Recessive alleles on an X
chromosome of a female may be masked by dominant alleles on its homologous X
chromosome.
4. Why does X inactivation occur in female mammals?
X inactivation happens to one of the two copies of a gene on the homologous X
chromosomes. Only females have two copies of the X chromosome.
10.8
1. How are pedigrees helpful in determining a disorder’s mode of inheritance?
Pedigrees track a trait through multiple generations and allow the pattern of transmission
and inheritance to be studied. Pedigrees also may help predict the appearance of the trait
in future generations.
2. How do the pedigrees differ for autosomal dominant, autosomal recessive, and Xlinked recessive conditions?
Autosomal dominant pedigrees show affected individuals in every generation and all
affected individuals have at least one affected parent. Autosomal recessive conditions
show a pedigree in which affected individuals can have normal parents, and the condition
often skips generations. 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?
Environment can affect a phenotype in a variety of ways. Temperature can influence
gene expression of temperature-sensitive alleles; infectious agents can intensify a genetic
disorder; upbringing and nourishment will affect temperament and physical health.
2. What is a polygenic trait?
A polygenic trait is one that is controlled by many genes.
10.10
1. Explain the logic of planting non-Bt-crop buffer strips around fields planted with Bt
crops.
The buffer strip creates a feeding zone for non-Bt-resistant larvae. These moths are
likely to mate with the Bt-resistant varieties surviving in the field. Since both Bt-resistant
and non-Bt-resistant moths survive in the population, this strategy should reduce the
likelihood that Bt-resistance will increase significantly in the population.
2. How did researchers in this study use a breeding experiment to demonstrate that Bt
resistance alleles in pink bollworms are recessive?
Researchers knew that if the resistant allele was recessive then matings between moths
heterozygous for resistance with moths fully resistant (homozygous recessive) should
show a phenotypic ratio in the offspring of approximately 1:1. Experiments revealed that
indeed approximately 50% of the offspring thrived while 50% either died or were quite
small.
3. 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 would only have other resistant moths to mate
with; allele frequencies would shift toward the recessive (resistance) allele in future
generations. If this occurs then Bt as a pesticide in corn will no longer be useful.
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 all kinds of varieties: sandals, boots, sneakers, but they are paired with
their matching shoe which will be the same size, and have straps or laces, rubber treads or
uppers all in the same places and of the same materials. Similarly, homologous
chromosomes are the same length and shape with the same genes in the same places. 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 of the plants of the F2 generation.
4. Given the relationship between genes, alleles, and proteins, how can a recessive allele
appear to “hide” in a heterozygote?
A recessive allele often encodes a nonfunctional protein. A heterozygous individual has
one dominant and one recessive allele, but the recessive allele appears to “hide” because
the cell has enough of the normal protein (encoded by the dominant allele), to function
properly.
5. Chapter 8 explained the roles of oncogenes and tumor suppressor genes in cancer. Both
types of genes encode proteins that regulate the cell cycle. Determine the function of each
type of protein, then explain why oncogenes are dominant and tumor suppressor genes
are recessive.
The proteins in normal proto-oncogenes stimulate cell division. Only one copy of the
mutant oncogene allele is necessary to express cancer, which means the oncogene allele
is dominant. Tumor suppressor genes promote apoptosis or prevent cell division. Two
mutated versions of this gene must be present to express cancer, and so the gene variation
is recessive.
6. Many plants are polyploid (see chapter 9); that is, they have more than two sets of
chromosomes. How would having four (rather than two) copies of a chromosome more
effectively mask expression of a recessive allele?
The extra chromosomes will provide additional opportunities for a dominant allele to
mask the expression of a recessive allele.
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. 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.
9. In a dihybrid cross, the predicted phenotype ratio is 9:3:3:1; the “9” represents the
proportion of plants expressing at least one dominant allele for both traits. How would
you use test crosses to determine whether these plants are homozygous dominant or
heterozygous for one or both genes?
A test cross is a mating with a homozygous recessive individual. In this case, you would
obtain a plant that was homozygous recessive for both alleles. If a plant is homozygous
dominant for both genes, all of the offspring will have the dominant phenotype for both
traits. If the plant is heterozygous for either gene, about half the offspring will exhibit the
recessive phenotype for that trait.
10. A white woman with fair skin, blond hair, and blue eyes and a black man with dark
brown skin, hair, and eyes have fraternal twins. 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.
11. The radish has nine groups of traits. Within each group, dihybrid crosses do not yield
a 9:3:3:1 phenotypic ratio. Instead, such crosses yield an overabundance of phenotypes
like those of the parents. What does this information reveal about the chromosomes of
this plant?
The information reveals that at least some of the genes are located on the same
chromosome.
12. How does gene linkage interfere with Mendel’s law of independent assortment? Why
doesn’t the inheritance pattern of linked genes disprove Mendel’s law?
Within each linkage group, dihybrid crosses did not produce the proportions of offspring
that Mendel’s law of independent assortment predicts. Scientists eventually realized that
each linkage group was simply a set of genes transmitted together on the same
chromosome. This observation does not disprove Mendel’s law of independent
assortment, which applies only when genes are located on different chromosomes.
13. 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.
14. 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.
15. Explain how each of the following appears to disrupt Mendelian ratios: 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 and equally expressed
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.
16. Suppose a single trait is controlled by a gene with four codominant alleles. A person
can inherit any combination of two of the four alleles. How many phenotypes are
possible for this trait?
If the alleles are labeled A, B, C, and D, the following allele combinations are possible:
AA, AB, AC, AD, BB, BC, BD, CC, CD, and DD. Ten phenotypes are possible.
17. What is the role of the Y chromosome in human sex determination?
The Y chromosome contains the SRY gene that acts as a switch for other sex determining
genes that then activate in the embryo so that it develops as a male and dismantles all
female embryonic structures.
18. Do you agree with the statement that all alleles on the Y chromosome are dominant?
Why or why not?
No. One might be tempted to answer “yes” because the Y chromosome is not
homologously paired, so all alleles on the Y chromosome are expressed. However,
recessive alleles are still “recessive” even if no dominant allele can mask them. A
recessive allele encodes a nonfunctional protein. If an allele on the Y chromosome
encodes a nonfunctional protein, then the allele is recessive.
19. Suppose a fetus has X and Y chromosomes but lacks receptors for the protein
encoded by the SRY gene. Will the fetus develop as a male or as a female? Explain your
answer.
The fetus will develop as a female. Without SRY protein receptors, the signal to develop
as a male will never be received.
20. How are X-linked genes inherited differently in male and female humans?
Whereas a female inherits two X chromosomes, a male inherits his single X chromosome
from his mother. A male expresses every allele (dominant or recessive) on his X
chromosome because he lacks a second allele that could mask the expression of recessive
alleles.
21. 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. Which X chromosome is
inactivated is a random event. This prevents female mammals with two X chromosomes
from expressing more X-linked genes than a male.
22. Rett syndrome is a severe X-linked recessive disorder that affects mostly female
children. How does X inactivation explain this observation?
Because the disorder is severe most males die as a result of inheriting the recessive allele.
Females who are heterozygous, however will have the dominant allele inactivated in
some cells, leaving the recessive allele to be expressed. The effects may not be lethal
since the recessive allele is inactivated in about half of the cells, but the disease will be
severely debilitating.
23. The cells of a track runner are collected before an Olympic competition. Technicians
examining the cells discover two Barr bodies in each nucleus. How is this unusual? What
would you expect to find if you constructed a karyotype of the runner’s chromosomes?
Cells of a female with an extra X chromosome have an extra Barr body, not just one as in
a normal female. Perhaps she is XXX, and so the karyotype should show three and not
two X chromosomes.
24. 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.
25. Why are male calico cats rare?
In cats, the genes encoding black and orange fur are located only on the X chromosome.
Calico cats result from the random inactivation of black and orange alleles. Male calico
cats are unusual because they would have to be XXY.
26. 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.
27. 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?
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 impossible to trace
parentage.
28. Explain the following “equation”:
Genotype + Environment = Phenotype
Genotype represents what proteins will be produced and how they will interact with each
other, but the environment often affects how those proteins will express themselves or
when the genes will be activated and inactivated. The combination of all these factors
will determine the actual physical expression, or phenotype.
29. Mitochondria and chloroplasts contain DNA that encodes some proteins essential to
life. These organelles are inherited via the female parent’s egg. Do you expect these
genes to follow Mendelian laws of inheritance? Explain your answer.
Mendelian laws of inheritance rely on the separation of homologous pairs (law of
independent assortment) and alleles within a gene pair (law of segregation). Both of
these separation events are the result of spindle fibers separating chromosomes in the
stages of meiosis. Chloroplasts and mitochondria do not undergo meiosis and so their
DNA is not subject to the Mendelian laws of inheritance.
Genetics Problems
1. Holstein cattle suffer from the condition citrullinemia, in which homozygous recessive
calves die within a week of birth because they cannot break down ammonia that is
produced when amino acids are metabolized. If a cow that is heterozygous for the
citrullinemia gene is inseminated by a bull that is homozygous dominant, what is the
probability that a calf inherits citrullinemia?
No calves will inherit citrullinemia; each has a 50% chance of being a carrier
2. Wild-type canaries are yellow. A dominant mutant allele of the color gene, designated
W, causes white feathers. Inheriting two dominant alleles is lethal to the embryo. If a
yellow canary is crossed to a white canary, what is the probability that an offspring will
be yellow? What is the probability that it will be white?
50% chance for each color
3. In humans, more than 100 forms of deafness are inherited as recessive alleles on many
different chromosomes. Suppose that a woman who is heterozygous for a deafness gene
on one chromosome has a child with a man who is heterozygous for a deafness gene on a
different chromosome. Does the child face the general population risk of inheriting either
form of deafness or the 25% chance that Mendelian ratios predict for a monohybrid
cross? Explain your answer.
No. The child faces a 25% chance of inheriting both recessive alleles. The chance that
both of those alleles are of the same gene, and lead to a dominant phenotype is much
lower.
4. A man and a woman each have dark eyes, dark hair, and freckles. The genes for these
traits are on separate chromosomes. The woman is heterozygous for each of these genes,
but the man is homozygous. The dominance relationships of the alleles are as follows:
B = dark eyes; b = blue eyes
H = dark hair; h = blond hair
F = freckles; f = no freckles
a. What is the probability that their child will share the parents’ phenotype?
b. What is the probability that the child will share the same genotype as the mother? As
the father?
Use the product rule or a Punnett square to obtain your answers. Which method do you
think is easier?
a. 100%
b. 1/8 chance that a child will have the same genotype as either parent
The product rule is an easier method.
5. Genes J, K, and L are on the same chromosome. The crossover frequency between J
and K is 19%, the crossover frequency between K and L is 2%, and the crossover
frequency between J and L is 21%. 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 _L_K____________J_.
6. A particular gene in dogs contributes to coat color. The two alleles exhibit incomplete
dominance. Dogs with genotype mm have normal pigmentation; genotype Mm leads to
“dilute” pigmentation; genotype MM produces an all-white dog. If a breeder mates a
normal dog with a white dog, what will be the genotypes and phenotypes of the puppies?
If two Mm dogs are mated, what is the probability that a puppy will be all white?
Dilute (Mn). Normal (mm) X All white (MM) = all dilute (Mm) pups
25%. Mm X Mm = 25% normal, 50% dilute, 25% all white
7. Three babies are born in the hospital on the same day. Baby X has type AB blood;
Baby Y has type B 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.)
Baby Z, O blood, belongs to couple 2, because an AB parent cannot produce an O child.
Baby X, AB blood, belongs to couple 1, because an O parent cannot produce an AB
child.
Baby Y, B blood, therefore belongs to couple 3.
8. Consider a woman whose brother has hemophilia A but whose parents are healthy.
What is the chance that she has inherited the hemophilia allele? What is the chance that
the woman will conceive a son with hemophilia?
50% chance the woman inherits the allele; if she did, her son has a 50% chance of having
hemophilia
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. Some cells, like red
blood cells, lack a nucleus, and are therefore not haploid or diploid.
2. What is the difference between a genotype and a phenotype?
A genotype describes the genetic makeup of an individual and a phenotype describes the
expression of its genetic makeup.
3. What is the difference between a dominant and a recessive allele?
Dominant alleles are expressed as functional proteins. Only one dominant allele is
necessary for the effects of that allele to be expressed. A recessive allele encodes a
nonfunctional protein. A recessive allele is only “expressed” if no dominant alleles are
present (and therefore no functional proteins are expressed).
4. Add meiosis, gametes, mutations, incomplete dominance,
codominance, pleiotropy, and epistasis to this concept map.
“Meiosis” leads to “Gametes” with “produces”, which leads to “Haploid cells” with
“are”. “Genes” leads to “Mutation” with “can undergo”, which leads to “Alleles” with
“results in new”. “Pleiotropy” leads to “Phenotype” with “is when genes have multiple
effects on the”. “Codominance” can lead to “Dominant” with “occurs when multiple
alleles for a gene are”. “Incomplete dominance” can lead to “Phenotype” with “occurs
when heterozygotes have an intermediate”. “Epistasis” can lead to “Genes” with “occurs
when expression of one gene affects the expression of other”.