CHAPTER 19
SEX AND GENETICS
 2009 Garland Science Publishing
The Benefits of Sex
19-1
Organisms that reproduce sexually ________________________.
(a)
must be haploid, unlike organisms that reproduce asexually
(b)
can reproduce only with a partner that carries the same alleles
(c)
create zygotes that are genetically identical to each other
(d)
undergo a sexual reproductive cycle that involves an alternation of haploid
cells with the generation of diploid cells
19-2
Which of the following statements is true?
(a)
Another name for the fertilized egg cell is the zygote.
(b)
Diploid organisms reproduce only sexually.
(c)
All sexually reproducing organisms must have two copies of every
chromosome.
(d)
Gametes have only one chromosome.
19-3
Which of the following statements is false?
(a)
Asexual reproduction typically gives rise to offspring that are genetically
identical.
(b)
Mutations in somatic cells are passed on to individuals of the next
generation.
(c)
Sexual reproduction allows for a wide variety of gene combinations.
(d)
Gametes are specialized sex cells.
19-4
Somatic cells ___________________________.
(a)
are not necessary for sexual reproduction in all eucaryotic organisms
(b)
are used to produce germ line cells when organisms reach sexual maturity
(c)
leave no progeny
(d)
do not contain sex chromosomes
19-5
Which of the following statements about the benefits of sexual reproduction is
false?
(a)
Sexual reproduction permits enhanced survival because the gametes that
carry alleles enhancing survival in harsh environments are used
preferentially during fertilization.
(b)
Unicellular organisms that can undergo sexual reproduction have an
increased ability to adapt to harsh environments.
(c)
Sexual reproduction reshuffles genes, which is thought to help species
survive in novel or varying environments.
(d)
19-6
Sexual reproduction can speed the elimination of deleterious alleles.
For each of the following sentences, fill in the blanks with the best word or phrase
selected from the list below. Not all words or phrases will be used; each word or
phrase may be used more than once.
To reproduce sexually, an organism must create haploid
__________________ cells, or __________________, from
diploid cells via a specialized cell division called
__________________. During mating, the father’s haploid cells,
called __________________ in animals, fuse with the mother’s
haploid cells, called __________________. Cell fusion produces a
diploid cell called a __________________, which undergoes many
rounds of cell division to create the entire body of the new
individual. The cells produced from the initial fusion event include
__________________ cells that form most of the tissues of the
body as well as the __________________-line cells that give rise
to the next generation of progeny.
allele
bivalent
eggs
gametes
germ
meiosis
mitosis
pedigree
pollen
somatic
sperm
zygote
19-7
Why is sexual reproduction more beneficial to a species living in an unpredictable
environment than to one living in a constant environment?
19-8
Is the following statement true or false? Explain.
Somatic cells leave no progeny and thus, in an evolutionary sense,
exist only to help create, sustain, and propagate the germ cells.
19-9
Sexual reproduction is a large drain on the limited resources of an individual.
Nonetheless, sexual reproduction is common. In fact, to allow sexual
reproduction, organisms have evolved many elaborate anatomical structures,
cellular processes, and chemical signals. For example, flowers exist entirely to
further the goal of sexual reproduction, and many plants have enlisted the help of
bees and birds to aid in the dissemination of their germ cells. Describe one reason
why most multicellular organisms have evolved to reproduce sexually instead of
relying solely on asexual reproduction.
Meiosis and Fertilization
19-10 Indicate whether each of the following is true for meiosis, mitosis, both, or
neither.
A.
formation of a bivalent
B.
C.
D.
E.
genetically identical products
condensation of chromosomes
segregation of all paternal chromosomes to one cell
involvement of DNA replication
19-11 Meiosis is a highly specialized cell division in which several events occur in a
precisely defined order. Please order the meiotic events listed below.
1.
loss of cohesins near centromeres
2.
chromatid pairing
3.
chromosome condensation
4.
chromosome replication
5.
degradation of cohesins bound to chromosome arms
6.
formation of chiasmata (chiasmata = plural of chiasma)
7.
homolog pairing
8.
alignment of chromosomes at the metaphase plate
19-12 For each of the following sentences, choose one of the options enclosed in square
brackets to make a correct statement.
Starting with a single diploid cell, mitosis produces [two/four]
[identical/different] [haploid/diploid] cells, whereas meiosis yields
[two/four] [identical/different] [haploid/diploid] cells. This is
accomplished in meiosis because a single round of chromosome
[replication/segregation] is followed by two sequential rounds of
[replication/segregation]. Mitosis is more like meiosis [I/II] than
meiosis [I/II]. In meiosis I, the kinetochores on sister chromatids
behave [independently/coordinately] and thus attach to
microtubules from the [same/opposite] spindle. The cohesinmediated glue between [chromatids/homologs] is regulated
differently near the centromeres than along the chromosome arms.
Cohesion is lost first at the [centromeres/arms] to allow
segregation of [chromatids/homologs] and is lost later at the
[centromeres/arms] to trigger segregation of
[chromatids/homologs].
19-13 In the absence of recombination, how many genetically different types of gamete
can an organism with five homologous chromosome pairs produce?
(a)
5
(b)
10
(c)
32
(d)
64
19-14 Which of the following statements most correctly describes meiosis?
(a)
Meiosis involves two rounds of DNA replication followed by a single cell
division.
(b)
(c)
(d)
Meiosis involves a single round of DNA replication followed by four
successive cell divisions.
Meiosis involves four rounds of DNA replication followed by two
successive cell divisions.
Meiosis involves a single round of DNA replication followed by two
successive cell divisions.
19-15 A diploid cell containing 32 chromosomes will make a haploid cell containing
___ chromosomes.
(a)
8
(b)
16
(c)
30
(d)
64
19-16 Imagine a diploid sexually reproducing organism, Diploidus sexualis, that
contains three pairs of chromosomes. This organism is unusual in that no
recombination between homologous chromosomes occurs during meiosis.
A.
Assuming that the chromosomes are distributed independently during
meiosis, how many different types of sperm or egg cells can a single
individual of this species produce?
B.
What is the likelihood that two siblings of this species will be genetically
identical? You can assume that the homologous chromosomes of each
parent are different from one another and from their counterparts in the
other parent.
19-17 You have received exactly half of your genetic material from your mother, who
received exactly half of her genetic material from her mother (your grandmother).
A.
Explain why it is unlikely that you share exactly one-quarter of your
genetic material with your grandmother, and instead it is more accurate to
say that in general people receive an average of one-quarter of their
genetic endowment from each grandparent.
B.
Consider a gene on chromosome 3 that you received from your
grandmother. Is it likely you received an entire chromosome 3 from your
grandmother? Why or why not?
C.
What portion of your genetic material do you share with your sibling?
Your aunt? Your cousin?
19-18 You examine a worm that has two genders: males that produce sperm and
hermaphrodites that produce both sperm and eggs. The diploid adult has four
homologous pairs of chromosomes that undergo very little recombination. Given
a choice, the hermaphrodites prefer to mate with males, but just to annoy the
worm, you pluck a hermaphrodite out of the wild and fertilize its eggs with its
own sperm. Assuming that all the resulting offspring are viable, what fraction do
you expect to be genetically identical to the parent worm? Assume that each
chromosome in the original hermaphrodite is genetically distinct from its
homolog.
(a)
(b)
(c)
(d)
All
None
1/16
1/256
19-19 Which of the following statements about meiosis is true?
(a)
During meiosis, the paternal chromosomes pair with the maternal
chromosomes before lining up at the metaphase plate.
(b)
Unicellular organisms that have a haploid state undergo meiosis instead of
mitosis during cell division.
(c)
Meiosis produces four genetically identical cells.
(d)
In general, meiosis is faster than mitosis.
19-20 Figure Q19-20 is a diagram of chromosomes during meiosis.
Figure Q19-20
A.
B.
On the diagram, indicate which label lines correspond to the following
items: (1) sister chromatids, (2) homologous chromosomes, (3) bivalent,
(4) chiasma.
On the figure, draw as small circles (oooo) the cohesin “glue” that is
released in meiotic division I, and draw as small crosses (xxxx) the
cohesin glue that is released in meiotic division II.
19-21 During recombination ________________________.
(a)
sister chromatids undergo crossing-over with each other
(b)
chiasmata hold chromosomes together
(c)
one crossover event occurs for each pair of human chromosomes
(d)
the synaptonemal complex keeps the sister chromatids together until
anaphase II
19-22 After the first meiotic cell division ________________________.
(a)
two haploid gametes are produced
(b)
cells are produced that contain the same number of chromosomes as
somatic cells
(c)
the number of chromosomes will vary depending on how the paternal and
maternal chromosomes align at the metaphase plate
(d)
DNA replication occurs
19-23 Meiosis includes a recombination checkpoint that is analogous to the checkpoints
in cell cycle progression. Double-stranded breaks in the DNA initiate
recombination in meiosis. The broken end of a DNA molecule finds the
corresponding sequence on a homologous chromosome and exchanges a
chromosomal segment with its homolog, thereby repairing the break. Ongoing
recombination sends a negative regulatory signal that prevents cells from entering
meiotic division I.
A.
Mutations in several genes inactivate the recombination checkpoint. What
do you predict will happen if a cell proceeds through meiotic division I
before completing recombination?
B.
What will happen if a cell fails to initiate recombination and proceeds
through meiotic division I? Meiotic division II?
19-24 In some fungi, cell division during meiosis gives rise to an ordered spore sac
containing a row of four haploid spores, as shown in Figure Q19-24A. The
position of each spore within the sac reflects its relation to its neighbors; in other
words, spores that result from the same meiosis II division are positioned next to
each other. You notice that a strain of the fungus produced by crossing a darkcolored strain with a light-colored strain gives rise mostly to spore sacs as shown
in Figure Q19-24B, with a few spore sacs like those in Figure Q19-24C. Indicate
whether the following statements are correct. Explain your reasoning.
Figure Q19-24
A.
B.
C.
D.
E.
Meiosis I and meiosis II in the fungus occur in the reverse order from that
which occurs in humans.
Recombination in the fungus can occur during prophase I.
Recombination in the fungus cannot occur during prophase II.
The spore sacs in panel C result from recombination between the
centromere and the gene responsible for spore color.
Recombination has occurred between the gene responsible for spore color
and the end of the chromosome arm.
19-25 In mammals, there are two sex chromosomes, X and Y, which behave like
homologous chromosomes during meiosis. Normal males have one X
chromosome and one Y chromosome, and normal females have two X
chromosomes. Males with an extra Y chromosome (XYY) are found occasionally.
Which of the following could give rise to such an XYY male? Explain your
answer.
(a)
nondisjunction in the first meiotic division of spermatogenesis; normal
meiosis in the mother
(b)
nondisjunction in the second meiotic division of spermatogenesis; normal
meiosis in the mother
(c)
nondisjunction in the first meiotic division of oogenesis; normal meiosis
in the father
(d)
nondisjunction in the second meiotic division of oogenesis; normal
meiosis in the father
19-26 Which of the following would not lead to aneuploidy during meiosis?
(a)
sister chromatids segregating inappropriately
(b)
non-sister chromatids segregating inappropriately
(c)
a reciprocal rearrangement of parts between nonhomologous
chromosomes (e.g., the left arm of chromosome 2 exchanging places with
the right arm of chromosome 3)
(d)
an extra set of chromosomes produced during S phase (e.g., if paternal
chromosome 3 were replicated twice)
19-27 A single nondisjunction event during meiosis ___________________.
(a)
will block recombination
(b)
will occur only during meiosis II
(c)
cannot occur with sex chromosomes
(d)
will involve the production of two normal gametes if it occurs during
meiosis II
19-28 During fertilization in humans, _______________________.
(a)
a wave of Ca2+ ions is released in the fertilized egg’s cytoplasm
(b)
only one sperm binds to the unfertilized egg
(c)
a sperm moves in a random fashion until it encounters an egg
(d)
several sperm pronuclei compete in the cytoplasm to fuse with the egg
nucleus
19-29 Do you agree or disagree with the following statement? Explain your answer.
If a diploid organism has 16 chromosomes (and thus 8 pairs of
homologous chromosomes), that organism can produce only 28
genetically different gametes.
Mendel and the Laws of Inheritance
19-30 Which of the following statements about Mendel’s experiments is false?
(a)
The pea plants could undergo both cross-fertilization and self-fertilization.
(b)
The true-breeding strains were homozygous for the traits that Mendel
examined.
(c)
The egg can carry either the allele from the maternal or the paternal
chromosome.
(d)
All traits that Mendel studied were recessive.
Questions 19-31 to 19-34 use the following information and Figure Q19-31. These
questions may be used independently, or as a group.
Figure Q19-31 diagrams one of Mendel’s experiments using the round and wrinkled seed
traits.
Figure Q19-31
19-31 Which of the following could be considered a true-breeding strain for the seed
shape phenotype? (See Figure Q19-31.)
(a)
all of the round-seeded plants produced in the F2 generation
(b)
all of the wrinkle-seeded plants produced in the F2 generation
(c)
all of the round-seeded plants produced in the F1 generation
(d)
half of the round-seeded plants produced in the F2 generation
19-32 If you crossed the round-seeded plants obtained in the F1 generation with a truebreeding strain of round-seeded plants, how many wrinkle-seeded plants would
you expect to obtain in the next generation? (See Figure Q19-31.)
(a)
none
(b)
25%
(c)
75%
(d)
all
19-33 If you crossed the round-seeded plants obtained in the F1 generation with a truebreeding strain of wrinkle-seeded plants, how many round-seeded plants would
you expect to obtain in the next generation? (See Figure Q19-31.)
(a)
25%
(b)
50%
(c)
75%
(d)
100%
19-34 Which of the following statements about the round-seeded pea plants obtained in
the F2 generation is false? (See Figure Q19-31.)
(a)
These plants are phenotypically identical for seed shape.
(b)
Two-thirds of these plants are expected to be heterozygous for the seed
shape allele.
(c)
We expect 25% of these plants to be homozygous for the seed shape
allele.
(d)
If these plants were crossed to wrinkle-seeded plants, some of these plants
would produce only round-seeded plants.
19-35 Which of the following reasons was essential for Mendel to disprove the theory of
blended inheritance?
(a)
The traits that Mendel examined all involved genes that did not display
linkage.
(b)
The traits that Mendel examined all involved the reproductive structures of
the pea plant.
(c)
Mendel pioneered techniques permitting the fusion of male and female
gametes from the same plant to produce a zygote.
(d)
The traits that Mendel examined involved an allele that was dominant and
an allele that was recessive.
19-36 Which of the following reasons was essential for Mendel’s law of independent
assortment?
(a)
(b)
(c)
(d)
All the traits that Mendel examined involved genes that did not display
linkage.
Several of the phenotypes that Mendel examined involved color.
Mendel observed chromosomal segregation in pea plant cells.
Mendel carried out his experiments on plants and not on fungi.
19-37 Is the following statement true or false? Explain.
The phenotype of an organism reflects all of the alleles carried by
that individual.
19-38 With respect to gene E on the chromosome drawn in Figure Q19-38, which gene
is least likely to behave according to Mendel’s law of independent assortment?
Explain your answer.
Figure Q19-38
19-39 Cystic fibrosis results from mutations in a single gene that lies on chromosome 7.
Only homozygous mutant (ff) individuals are sick; homozygous wild-type (FF)
and heterozygous (Ff) individuals are healthy. A healthy married couple has one
child with cystic fibrosis and the wife is pregnant with a second child.
A.
What is the genotype of the mother? The father?
B.
What is the chance that the second child will have cystic fibrosis?
19-40 Sickle-cell anemia is caused by a mutant allele of a hemoglobin gene. Individuals
with two mutant alleles have sickle-cell anemia. Individuals homozygous and
heterozygous for the mutant gene are more resistant to malaria than those with
two wild-type alleles. Is this mutation dominant, recessive, or co-dominant?
19-41 Your friend has obtained some pea seeds from the Abbey of St. Thomas in Brno,
where Gregor Mendel worked. He is very excited because not only did he obtain
some yellow and green pea seeds from true-breeding plants (like the ones used in
Mendel’s famous experiment), he was also able to obtain some purple pea seeds
from a true-breeding plant. First, your friend takes the true-breeding yellow and
green pea seeds, repeats the cross that Gregor Mendel did, and obtains the same
results: he sees 100% yellow-seeded pea plants in the F1 generation, and 75%
yellow-seeded pea plants and 25% green-seeded pea plants in the F2 generation.
His results are illustrated in Figure Q19-41A. Your friend then decides to set up
two more crosses. For cross #2, he crosses the true-breeding purple-seeded pea
plants to the true-breeding yellow-seeded pea plants. The results from this cross
are shown in Figure Q19-41B. Next, for cross #3, he crosses the true-breeding
purple-seeded pea plants to the true-breeding green-seeded pea plants. These
results are shown in Figure Q19-41C.
Figure Q19-41
Given these results, if you were to take the purple-seeded pea plants produced in
the F1 generation in cross #2 and cross them to the purple-seeded pea plants
produced in the F1 generation of cross #3, what do you expect that the phenotype
of the progeny would look like? Explain your answer.
19-42 You are given two true-breeding strains of hamster. One strain has white fur color
and the other has a dark brown fur color. When you cross the white fur strain to
the dark brown fur strain, you obtain F1 progeny that have a light brown fur color.
When you cross the F1 progeny with each other, 25% of the F2 generation have
white fur, 25% have dark brown fur, and 50% have light brown fur. How many
genes crucial for fur coloration differ between the two starting strains? Explain
your answer.
Extra credit: Propose a molecular mechanism for how fur color is determined in
this species of hamster.
19-43 Loss-of-function mutations ________________________.
(a)
cause the production of proteins that are active in inappropriate
circumstances
(b)
will usually show a phenotype when heterozygous
(c)
are only present in a population at barely detectable levels
(d)
are usually recessive
Genetics as an Experimental Tool
19-44 Conditional alleles are mutant gene versions encoding proteins that can function
normally at the permissive condition but are defective at the restrictive condition.
One commonly used condition is temperature. Conditional alleles are especially
useful to geneticists because they permit the study of essential genes. At the
permissive temperature, the organism lives normally. When the organism is
shifted to the non-permissive temperature, the effect of inactivating the gene can
be studied. Which of the three types of mutation shown in Figure Q19-44 is most
likely to lead to a conditional allele? Explain your answer.
Figure Q19-44
19-45 You performed a genetic screen in C. elegans and found two mutations (#1 and
#2) that cause antisocial behavior. You want to learn whether these are mutations
in the same gene or different genes, so you cross mutant #1 and mutant #2
together to perform a complementation test. Imagine that the mutations are in
different genes: mutation #1 is in gene M and mutation #2 in gene N.
Figure Q19-45
A.
B.
C.
D.
What is the outcome of a complementation test if #1 and #2 are both
recessive mutations (m and n, respectively)? Fill in the empty oval in
Figure Q19-45A and indicate whether the offspring exhibits social or
antisocial behavior.
What is the outcome if #1 is dominant and #2 is recessive (M* and n)? Fill
in the empty oval in Figure Q19-45B and indicate whether the offspring
exhibits social or antisocial behavior.
For the case in A, would the phenotype of the offspring differ if the two
mutations were in the same gene?
For the case in B, would the phenotype of the offspring differ if the two
mutations were in the same gene? Are complementation tests useful for
analyzing dominant mutations?
19-46 Do you agree or disagree with the following statement? Explain your answer.
A trait that is found at a low frequency in the population has to be
a recessive trait.
19-47 You are studying a diploid yeast strain that normally uses glucose as an energy
source but can use maltose when no glucose is present. You are interested in
understanding how this yeast strain metabolizes maltose as an alternative energy
source. You isolate the genes involved in maltose metabolism by screening for
yeast that cannot grow when maltose is the sole energy source. You find six
different mutants, all of which are recessive, and name these alleles mal1, mal2,
mal3, mal4, mal5, and mal6. Next, you isolate gametes from the homozygous
diploid mutant yeast strains and perform crosses between the different strains to
do complementation analysis, because you wish to determine whether the
mutations are likely to affect the same or different genes. Your results are shown
in Table Q19-47.
Table Q19-47 Complementation analysis of mal genes.
In how many genes are you likely to have isolated mutations? Which alleles seem
to affect the same genes? Explain your answer.
19-48 Gene A is located near gene B on chromosome 13 in humans. A mutation in the
germ line of an individual with the haplotype AB generates gametes with the
genotype Ab. Many descendents of this founder individual carry the b mutation,
which predisposes carriers to high blood pressure. Initially, all descendents who
inherit the b mutation also inherit the neighboring A allele. Through the
generations, fewer and fewer descendents with the b mutation carry the A allele,
and instead they have the a allele. (Individuals with A and a are equally healthy
and fit.) Explain how the b and A alleles are separated.
19-49 Shown in Figure Q19-49 is a genetic pedigree of a family with several members
affected by a heritable disease. Affected individuals are shown in black and
healthy individuals are shown in white. Males are shown as boxes and females as
circles. Can a single mutation explain the pattern of inheritance? Is the mutation
responsible for the disease dominant or recessive? Is the mutation carried on the
X chromosome, the Y chromosome, or an autosome?
Figure Q19-49
19-50 The single nucleotide polymorphisms found in the human population
__________________.
(a)
are important for genetic mapping because they represent mutations in
genes important for human disease
(b)
are rarely found among blood relatives
(c)
can be linked into haplotype blocks
(d)
arose mainly during the past 10,000 years
How We Know: Using SNPs to Get a Handle on Human Disease
19-51 You are trying to map a human gene thought to be involved in cat allergies.
Because you know this gene is on chromosome 20, you decide to examine the
linkage of several SNPs located on chromosome 20 with respect to the gene
involved in cat allergies. You have obtained DNA from 10 individuals, and you
know whether they are allergic to cats. Your SNP results are shown in Table Q1951.
Table Q19-51 SNP tests for chromosome 20. (+ indicates the presence of SNP.)
A.
B.
Which SNP is most likely to be tightly linked to the gene involved in cat
allergies? Explain your answer.
Of the SNPs tested above, which is likely to be the next closest to the gene
responsible for the allergic state? Why?
19-52 Any two human beings typically have an estimated 0.1% difference in their
nucleotide sequences, which is equivalent to about 3 million nucleotide
differences. These differences are the basis of the SNPs used to construct genetic
linkage maps. Some of these SNPs actually lie in the region of the DNA that
codes for the protein, yet they have no effect on the phenotype of individuals
carrying the SNP on both homologous chromosomes. Explain how some SNPs
can lie within the portion of the DNA that codes for the protein and yet have no
discernible effect on the protein’s activity.
19-53 Finding co-inheritance of a SNP variant and a disease tells scientists that
____________________.
(a)
everybody who carries this SNP will get the disease
(b)
sequences within the SNP cause the disease
(c)
a gene important for causing the disease is linked to the SNP
(d)
SNPs on other chromosomes will not be co-inherited with the disease
19-54 You decide to carry out genetic association studies and identify a SNP variant that
is found significantly more often in individuals who have schizophrenia than in
those who are not affected. This SNP is found within an intron of the SZP gene.
A.
B.
Can you deduce that an abnormality of the SZP gene is a cause of
increased risk of schizophrenia?
Can you say whether the SNP variant itself is a cause?