Answers to Open-Ended Questions
Hoefnagels Essentials 2/e
Chapter 9
Answers to Mastering Concepts
9.1
1. How do asexual and sexual reproduction differ?
Asexual reproduction requires only one parent and produces offspring that are identical,
except for mutations. Sexual reproduction requires two parents and produces genetically
variable offspring.
2. What are the advantages of asexual reproduction?
Asexual reproduction is adaptive when the environment is unchanging or when it is
difficult to acquire mates.
3. Why does sexual reproduction persist even though it requires more energy than asexual
reproduction?
Sexual reproduction produces the variation that allows a population to persist in a
changing environment. Mutations generate some variation among asexual organisms, but
not quickly enough or in great enough degrees to be successful in a rapidly changing
environment.
9.2
1. What are autosomes and sex chromosomes?
Autosomes are chromosomes that are the same regardless of sex, whereas the sex
chromosomes determine whether an individual is genetically male or female.
2. Draw a karyotype for a cell with a diploid number of 8.
The drawing should include four homologous pairs of chromosomes. The members of
each pair should not be attached to each other, but they should look alike. Each of the
four pairs should have a unique combination of size, centromere location, and banding
pattern.
3. How are the members of a homologous pair similar and different?
A homologous pair is composed of two chromosomes that are similar in size, centromere
location, and banding pattern. They carry the same sequence of genes (but not
necessarily the same alleles) for the same traits. Each member of a homologous pair
comes from a different parent.
Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of
McGraw-Hill Education.
9.3
1. How do haploid and diploid nuclei differ?
A diploid cell (2n) has two full sets of chromosomes, with one set coming from each
parent. A haploid cell (n) has only one set of chromosomes.
2. What are the roles of meiosis, gamete formation, and fertilization in sexual life cycles?
In meiosis, a diploid cell divides twice to produce four haploid nuclei, reducing the
chromosome number by half. Meiosis also creates new allele combinations in the
haploid nuclei. Gamete formation packages the haploid chromosomes into reproductive
cells (sperm and egg cells). Fertilization merges haploid gametes from two parents,
producing a new diploid cell with half its chromosomes coming from each parent.
3. What is a zygote?
A zygote is a fertilized egg cell; it is the first cell of a diploid organism.
4. What is the difference between somatic cells and germ cells?
Somatic cells are body cells that do not directly participate in reproduction. Germ cells
participate directly in reproduction by undergoing meiosis and producing haploid
gametes.
9.4
1. What happens during interphase?
In interphase the cell grows and synthesizes all molecules necessary for cell division.
DNA replicates, transforming each chromosome into a pair of connected sister
chromatids. Chromatin begins to condense, and spindle proteins are produced.
2. How do the events of meiosis I and meiosis II produce four haploid cells from one
diploid cell?
Meiosis I divides the homologous pairs of chromosomes from one diploid cell into two
separate haploid cells. In each of these cells, meiosis II then divides the sister chromatids
of each chromosome. The overall result is four non-identical haploid daughter cells.
9.5
1. How does crossing over shuffle genes?
Crossing over shuffles alleles when part of one chromosome switches places with part of
its homolog. The result is chromosomes with new combinations of alleles from both
parents.
Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of
McGraw-Hill Education.
2. Explain how to arrive at the estimate that one human couple can produce over 70
trillion unique offspring.
During metaphase I, 23 homologous pairs of chromosomes (consisting of four
chromatids) align at the equator of the cell. Each homologous pair can align in either of
two ways. For 23 homologous pairs, there are 223 (8,388,608) possible alignments. Then,
during fertilization, any one of the woman’s 8,388,608 possible eggs can be fertilized by
any one of a man’s 8,388,608 possible sperm. The square of 8,388,608 therefore gives
the number of possible offspring combinations -- about 70.3 trillion.
3. How are identical twins different from fraternal twins?
Identical (monozygotic) twins arise from a single zygote and are therefore genetically
identical to each other. Fraternal (dizygotic) twins arise from two separate fertilized eggs
and are therefore as similar (or dissimilar) as any two non-identical siblings.
9.6
1. In what ways are mitosis and meiosis similar?
Mitosis and meiosis are similar in that both divide the chromosomes of a eukaryotic cell.
Overall, the processes follow a similar progression of stages, and similar mechanisms
move the chromosomes.
2. In what ways are mitosis and meiosis different?
Mitosis occurs in somatic cells at any life cycle stage, whereas meiosis is restricted to
germ cells at particular times in the life cycle. Mitosis involves only one cell division;
meiosis involves two. Mitosis yields two daughter cells; meiosis yields four. Cytokinesis
occurs once for every DNA replication event of mitosis; cytokinesis occurs twice after
one DNA replication event of meiosis. Crossing over occurs only in meiosis; homologous
chromosomes do not align with one another in mitosis. Daughter cells produced by
mitosis are identical; daughter cells produced by meiosis are genetically variable.
Daughter cells from mitosis are used for growth and repair and in asexual reproduction;
daughter cells from meiosis are used in sexual reproduction.
9.7
1. What is polyploidy?
Polyploidy means having extra full sets of chromosomes; for instance 3n (triploid), 4n
(tetraploid), or 6n (hexaploid).
2. How can nondisjunction during meiosis lead to gametes with extra or missing
chromosomes?
Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of
McGraw-Hill Education.
If the chromosomes don’t separate properly at anaphase I or II, chromatids are unequally
distributed into daughter cells. Some gametes will have too few chromosomes, whereas
others will have too many.
3. Draw a diagram to show how nondisjunction of all chromosomes during meiosis I in
one parent could lead to polyploid offspring. Use 2n=6 for the starting cells; assume the
other parent’s gamete contributes the normal number of chromosomes.
The diagram should show all three homologous pairs of chromosomes failing to separate
during anaphase I of meiosis. The products will be one cell containing all six
chromosomes (a total of 12 chromatids) and one cell with no chromosomes. When the
cell containing all of the chromosomes undergoes meiosis II, it will generate gametes
containing six chromosomes. If this abnormal gamete were fertilized by a normal haploid
gamete containing three chromosomes, the zygote would be triploid: it would have nine
chromosomes instead of the six in a normal diploid cell.
4. Explain why few children are born with extra autosomes (with the exception of
trisomy 21).
Most embryos with extra chromosomes cease developing before birth. Embryos with
extra chromosomes account for about half of all spontaneous abortions (miscarriages) in
early pregnancy.
5. Why do sex chromosome abnormalities include XO but not OY?
The X chromosome contains a large amount of genetic material; the Y chromosome is
much smaller. A zygote that has no Y chromosome can survive, but a zygote that has no
X chromosome is missing too much genetic information and will die.
Answers to Write It Out Questions
1. Explain why evolution often selects traits that promote genetic diversity.
Organisms that reproduce asexually are successful as long as the environment stays the
same (or changes slowly). But most habitats on Earth are continuously changing; they
may become hotter, colder, wetter, drier, sunnier, shadier, more acidic, more basic, etc. In
that case, organisms that reproduce sexually are likely to have the greatest reproductive
success because their offspring are genetically variable. At least some of their offspring
are likely to survive as the environment changes.
2. Most cells in a sexually reproducing organism have two sets of chromosomes. Explain
this observation and describe its significance to meiosis.
Sexually reproducing organisms have two parents, each of which contributes one set of
chromosomes. But when a diploid organism reproduces sexually, it cannot simply
combine one of its diploid cells with a diploid cell from a mate. If it did, the chromosome
Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of
McGraw-Hill Education.
number would double in the next generation. Instead, the organism produces haploid
gametes that combine with the haploid gametes from another organism. Meiosis produces
these gametes.
3. Sketch the relationship between mitosis, meiosis, and fertilization in a sexual life
cycle.
[Answer will be visual and based on figure 9.5.]
4. What is the difference between haploid and diploid cells? Are your skin cells haploid
or diploid? What about gametes?
Haploid cells have one set of chromosomes, whereas diploid cells have two sets. Skin
cells are diploid, but gametes are haploid.
5. Some male veterans of the Gulf War in Iraq claim that their children have birth defects
that were caused by toxic substances contaminating the war zone. What types of cells
would a toxin have to have affected in these men to cause birth defects years later?
Explain your answer.
The toxin must have affected the germ cells, since they undergo meiosis and eventually
give rise to sperm cells. Since somatic cells are not involved in reproduction, mutations in
those cells are not passed to offspring.
6. How are mitosis and meiosis different?
Meiosis occurs in germ cells; mitosis occurs in somatic cells. Meiosis yields four haploid
daughter cells from one diploid cell; mitosis yields two daughter cells with the same
number of chromosomes as the starting cell. Meiosis generates genetically variable
daughter cells used in sexual reproduction; mitosis yields genetically identical daughter
cells for growth, repair, and asexual reproduction.
7. Draw all possible metaphase I chromosomal arrangements for a cell with a diploid
number of 8.
[Answers will be visual; drawings should yield 16 possible arrangements.]
8. Is it possible for a boy–girl pair of twins to be genetically identical? Why or why not?
No, a boy-girl pair of twins must be genetically different, because sex is determined by
genes on the sex chromosomes. The boy-girl combination arises when two separate
sperm fertilize two separate egg cells.
9. List some examples of chromosomal abnormalities, and explain how each relates to an
error in meiosis.
Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of
McGraw-Hill Education.
Extra or missing sex chromosomes and extra autosomes result from failure of
chromosomes to separate correctly in meiosis (nondisjunction). After nondisjunction, a
gamete may be missing a chromosome or have an extra one; after fertilization, the
offspring will have a chromosomal abnormality.
Answers to Pull It Together Questions
1. Fit the following terms into this concept map: chromatid, centromere, nondisjunction,
fertilization, and mitosis.
“Fertilization” connects with the phrase “produces a” to “Zygote.” “Mitosis” connects
with the phrase “divides chromosomes during replication of” to “Diploid cells” and
“Haploid cells.” “Chromosomes” connects with the phrase “when replicated, consist of
two” to “Chromatids,” which connects with the phrase “attach to each other at the” to
“Centromere.” “Nondisjunction” connects with the phrase “is the failed separation of” to
“Chromatids.”
2. Create a separate concept map that includes these terms: crossing over, gamete,
autosome, and homologous pair. You may add other terms to the map as well.
Answers will vary, but one possibility is to connect “Homologous pair” with the phrase
“aligns prior to” to “Crossing over.” The term “Crossing over” could connect with the
phrase “occurs in the cells that give rise to” to the term “Gametes.” And the term
“Gametes” could connect with the phrase “contain one copy of each” to the term
“Autosome.”
3. Where do the members of each pair of homologous chromosomes in a diploid cell
come from?
One member of each pair of homologous chromosomes comes from one parent. The
other member of the homologous pair comes from the other parent.
4. What two processes in meiosis I generate genetic variation among gametes?
During prophase I, variation is created by crossing over between the members of each
pair of homologous chromosomes. The second source of variation is the random
alignment of the paired chromosomes during metaphase I.
Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of
McGraw-Hill Education.