Answers to Mastering Concepts Questions
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. How can asexually reproducing organisms acquire new genetic information?
Mutations can create new gene variants, and some asexually reproducing organisms can
acquire new genetic information by exchanging DNA. For example, conjugation can
increase genetic diversity in bacteria and Paramecium.
3. What circumstances select for asexual and sexual reproduction?
Asexual reproduction is adaptive when the environment is always the same or when it is
hard to acquire mates. Sexual reproduction produces the genetic variation that is adaptive
in a 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 male or female.
2. What is a karyotype?
A karyotype is a chart that places the chromosomes in size order.
3. How are the members of a homologous pair similar and different?
A homologous pair of chromosomes 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.
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. 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 that is the start of a new 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
can undergo meiosis and produce haploid gametes.
9.4
1. What happens during interphase of meiosis?
In interphase, the cell grows and synthesizes all molecules necessary for cell division;
DNA replicates, so that each chromosome consists of 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 into two separate haploid cells.
Meiosis II divides the sister chromatids of each duplicated chromosome to produce four
nonidentical haploid daughter cells with unduplicated chromosomes.
9.5
1. How does crossing over shuffle genes?
Crossing over shuffles genes when part of one chromosome switches places with part of
its homologous chromosome.
2. Explain how different chromosome alignments during metaphase I can result in over 8
million genetically different gametes in a human.
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 possible alignments (more than
8,300,000).
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 nonidentical 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 cell. Overall,
the processes have a similar progression of stages, in which a spindle aligns the
chromosomes in the center of the cell and then pulls them apart.
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 for
every 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 in growth, repair, and asexual
reproduction; daughter cells from meiosis are used in sexual reproduction.
9.7
1. What is polyploidy?
Polyploidy means having extra 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?
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.
9.8
1. How did the researchers use chloroplast DNA to learn about the evolutionary history
of tetraploid Tragopogon species?
Because chloroplasts only come from the egg, chloroplast DNA is all maternal and can
be used to determine the “parentage” of the new species. Sequencing the chloroplast
DNA and comparing it to the diploid species’ sequences revealed the maternal and
paternal contributions to the evolutionary history of each tetraploid species.
2. The Soltises suggest that because T. dubius is so much more common than the other
two diploid species, its pollen is also the most abundant. How would you test the
hypothesis that the most common plant is most likely to be the father of a tetraploid
hybrid?
You could set up experiments with two species of potential father plants in each group.
Then you could vary the ratios of the two species and observe the offspring to see if they
contain the DNA of the more common plant in each group.
Answers to Write It Out Questions
1. Distinguish between asexual reproduction and sexual reproduction.
In asexual reproduction, an organism replicates its genetic material and produces
identical offspring. Sexual reproduction, in contrast, is the production of genetically
variable offspring whose genetic makeup comes from two parents.
2. Some fungi reproduce asexually if nutrients are abundant, but switch to sexual
reproduction when conditions are not as good. Explain this observation.
Asexual reproduction produces essentially identical offspring that are as well-suited to
the environment as their parent. When conditions are favorable, producing identical
clones means high reproductive success. Sexual reproduction produces variable
offspring. When conditions are not favorable, producing genetically variable offspring
increases reproductive success because at least some of the offspring may survive the
new conditions better than the parents.
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, and gametes are haploid.
5. Define the following terms: crossing over, gamete, diploid, haploid, autosome,
homologous pair.
Crossing over is the exchange of alleles between homologous chromosomes. A gamete is
a haploid sex cell that contains half the number of chromosomes as a somatic cell. A
diploid cell contains two sets of chromosomes, with one set coming from each parent; a
haploid cell contains one set of chromosomes. An autosome is a chromosome that is alike
in both sexes. A homologous pair of chromosomes is two chromosomes, one coming
from each parent, that are alike in size, banding pattern, and gene arrangement.
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 that may be
haploid or diploid, depending on 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.
[Drawings should yield 16 possible arrangements.]
8. A dog has 39 pairs of chromosomes. Considering only the orientation of homologous
chromosomes during metaphase I, how many genetically different puppies are possible
from the mating of two dogs? Is this number an underestimate or an overestimate? Why?
There are 2n = 239 (= 549,755,813,888) possible combinations of homologous
chromosomes during metaphase I of meiosis. That is how many different gametes each
parent can produce. Random fertilization means that the number of possible genetically
different puppies is about 3 x 1023. This is an underestimate because it does not account
for crossing over during meiosis in each parent.
9. What is the difference between monozygotic and dizygotic twins?
Monozygotic twins are genetically identical because they come from the same zygote.
Dizygotic, or fraternal twins, are no more alike than no twin siblings because they start as
two different zygotes.
10. 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 results from two separate
sperm fertilizing two separate egg cells.
11. Many male veterans of the Vietnam War claim that their children born years later
have birth defects caused by a contaminant in the herbicide Agent Orange used as a
defoliant in the conflict. What types of cells would the chemical have to have affected in
these men to cause birth defects years later? Explain your answer.
The germ cells were probably affected since they eventually give rise to sperm cells,
which will fertilize an egg and result in the offspring. Since somatic cells are not involved
in reproduction, mutations to body cells are not passed to offspring.
Answers to Pull It Together Questions
1. Fit the following terms into this concept map: chromatid, centromere, nondisjunction,
fertilization, zygote, and mitosis.
“Fertilization” leads to “Zygote” with “produces a”. “Diploid cells” leads to “mitosis”
with “replicate by means of”. “Chromosomes” leads to “Chromatids” with “duplicate,
forming pairs of”. “Chromatids” leads to “Centromere” with “identical pairs attach at
the”. “Chromatids” also leads to “Nondisjunction” with “failing to detach during
anaphase II is a type of”
2. What happens in meiosis I and meiosis II?
In meiosis I, homologous chromosomes are divided equally into two daughter cells.
During meiosis II, sister chromatids separate and each daughter cell divides, producing
four nonidentical cells.
3. What two processes in meiosis I generate genetic variation among gametes?
During meiosis I, variation is first created by crossing over of the homologous
chromosomes, which scrambles the maternal and paternal genes into four blended
chromatids. The second source of variation occurs during the random alignment of the
paired chromosomes in metaphase I.
4. Why must diploid organisms produce haploid gametes?
Diploid organisms create haploid gametes so that when the male and female gametes
combine, the resulting zygote has the same number of chromosomes as each of the
parents. If gametes were diploid, then the chromosome number would double with each
generation.
5. Where do the members of each pair of homologous chromosomes in a diploid cell
come from?
Each set of chromosomes comes from one parent.