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. 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. Why does sexual reproduction persist even though it requires more energy than asexual
reproduction?
Sexual reproduction produces the variation needed for a population to survive a changing
environment. The mutations giving rise to variation in asexual populations does give
variation, 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 male and female.
2. What is a karyotype?
A karyotype is a chart that places the homologous chromosomes in size order, matching
shape and banding patterns, after halting the cell in metaphase and staining the resulting
chromosomes for better visibility.
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. What is the difference between somatic and germ cells?
Germ cells can undergo meiosis and produce haploid gametes; somatic cells are body
cells that reproduce by mitosis.
2. 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.
3. What are the roles of meiosis, gamete formation, and fertilization in a sexual life
cycle?
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.
4. What is a zygote?
A zygote is a fertilized egg cell that is the start of a new diploid organism.
9.4
1. What happens during interphase of meiosis?
In interphase the cell grows and synthesizes all molecules necessary for cell division;
DNA replicates and each chromosome is transformed 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 germ cell?
Meiosis I divides the homologous pairs of chromosomes into two separate haploid cells.
Meiosis II then divides the sister chromatids of each duplicated chromosome in the two
cells to produce four non-identical 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 events in metaphase I enable a human to produce over 8 million
genetically different gametes.
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. What is random fertilization?
During random fertilization, any one of the 8,388,608 possible combinations of gametes
in a woman’s eggs can be fertilized by any one the 8,388,608 possible combinations of
gametes in a man’s sperm, producing over 70 trillion offspring combinations.
4. 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 cell. Overall,
the processes have a similar progression of stages, and similar mechanisms move
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 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 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 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
portioned into cells during meiosis I or II. Some gametes will have too few
chromosomes, whereas others will have too many.
3. How can deletions, duplications, inversions, and translocations cause illness?
Deletions, duplications, inversions, and translocations can delete, duplicate, or damage
genes, which may affect the normal production of proteins. Chromosome duplication
may be the least harmful, because the spare genes can mutate while the working genes
continue their normal functions. Chromosome deletions may mean that an individual
cannot make some proteins. Inversion produces symptoms because chromosomes may
not align properly, causing fertility problems, miscarriage, or birth defects. In
translocation, chromosomes may be missing parts, or genes can be broken. Affected
individuals may lack proteins, produce harmful proteins, or have fertility problems.
4. How do inversions and translocations cause fertility problems?
Both inversions and translocations result in faulty chromosomes that are missing
segments or have segments in which genes are out of order. Because an abnormal
chromosome may not successfully match with its homolog during meiosis, an affected
person may be infertile. Even if gametes are produced, embryonic development may be
abnormal, and the pregnancy often will end in a miscarriage.
9.8
1. What are the stages of sperm development in humans?
The stages of sperm development in humans are:
- mitosis in a diploid spermatogonium produces diploid primary spermatocytes;
- meiosis I in a diploid primary spermatocyte produces haploid secondary
spermatocytes;
- meiosis II in secondary spermatocytes produces haploid spermatids;
- spermatids mature into haploid sperm cells
2. What are the stages of development of an egg cell in humans?
The stages of human egg development are:
- mitosis in a diploid oogonium produces diploid primary oocytes;
- meiosis I in a diploid primary oocyte produces a secondary oocyte and a much
smaller polar body (both haploid);
- after fertilization, meiosis II in the secondary oocyte produces a large haploid
ovum and another small polar body, which is discarded.
3. How does gamete production in plants differ from that in animals?
In plants, gamete production occurs in multicellular haploid gametophytes, which
produce haploid gametes by mitosis. The gametes unite in fertilization, and the zygote
develops into a multicellular diploid sporophyte plant. Meiosis in cells of the sporophyte
plant produces haploid spores that germinate into the haploid gametophyte generation.
9.9
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 tease apart 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 experimental groups with 2 potential father plants in each group. Then
you could vary between the groups the ratios of the two and observe the offspring to see
if they contain the DNA of the more common plant in each group.
Write It Out
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. What is the evidence that sexual reproduction has been successful over evolutionary
time?
The prevalence of sexual reproduction in every eukaryotic kingdom is evidence that
sexual reproduction has been successful over evolutionary time.
3. Some fungi reproduce asexually while 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.
4. Sketch the relationship between mitosis, meiosis, and fertilization in a sexual life
cycle.
[Answer will be visual and based on Figure 9.5.]
5. What is the difference between haploid and diploid cells? Are your skin cells haploid
or diploid? What about germ cells? Gametes?
Haploid cells have one set of DNA, while diploid cells have two. Skin and germ cells are
diploid, while gametes are haploid.
6. 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 spermatogonia 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.
7. 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.
8. Huntington disease is caused by a faulty gene on chromosome 4. Before researchers
discovered the actual gene in 1993, people at risk for the disease were tested for the
presence of a nearby “marker” on chromosome 4. The marker was not the gene itself, but
it reliably predicted who would develop the disease later in life. Sequences farther away
from the disease gene, however, were not good predictors. How do the events of crossing
over explain this observation? Create a sketch to accompany your answer.
The sketch should show that crossing over occurs at chiasmata and only genes in that
local area will be swapped. Since markers close to the disease gene are likely to be
contained within that crossing over region, they are good indicators.
9. Draw all possible metaphase I chromosomal arrangements for a cell with a diploid
number of 8. How many unique gametes are possible for this species?
[Drawings should yield 16 possible arrangements]
10. 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.
11. 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 non-twin siblings because they start
as two different zygotes.
12. 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.
13. List some examples of chromosomal abnormalities, and explain how each relates to
an error in meiosis.
Examples would include: extra or missing sex chromosomes and extra autosomes, which
result from failure of chromosomes to separate correctly in meiosis; chromosomal
deletions, chromosomal duplications, chromosomal inversions, or chromosomal
translocations would occur from failures in homologous chromosomes to align properly
during prophase I or from failure during crossing over.
14. Define the following terms: crossing over, synapsis, gamete, autosome,
nondisjunction, and homologous pair.
Crossing over is the exchange of like genes between sister chromatids of a homologous
pair, which is the pair of chromosomes with the same shape and genes inherited from the
two parents. Synapsis is the gene-by-gene alignment of homologous chromosomes. A
gamete is a haploid sex cell. An autosome is a chromosome that is alike in both sexes.
Nondisjunction is the failure of chromosomes to properly separate during meiosis.
15. How does spermatogenesis differ from oogenesis, and how are the processes similar?
Spermatogenesis results in four equally-sized, very small spermatids that must undergo a
maturation process. Oogenesis results in one very large egg and three polar bodies since
no cytokinesis occurs. The processes are similar in that they involve an initial mitotic
division and then two meiotic divisions to produce gametes that are haploid.
16. Describe how a plant life cycle may include a multicellular haploid and a diploid
phase.
The life cycle of a sexually reproducing plant includes an alteration of generations.
Meiosis occurs in the diploid (sporophyte) generation and yields haploid spores. The
spores divide mitotically to form the haploid (gametophyte) generation. Gametophytes
produce haploid gametes by mitosis, not meiosis. A sperm cell fertilizes an egg cell to
form a diploid zygote, which divides mitotically and develops into the sporophyte.
Pull It Together
1. Fit the following terms into this concept map: chromatid, centromere, nondisjunction,
fertilization, 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 the first or second division leads to”
2. What happens in meiosis I and meiosis II?
In meiosis I, homologous chromosomes are divided equally into two daughter cells. DNA
replication does not occur between meiosis I and meiosis II. During meiosis II, sister
chromatids separate and each the daughter cell divides, producing four non-identical
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 chromatid strands,
which scrambles the maternal and paternal genes into four blended chromatids.. The
second source of variation is found in 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 two sets of homologous chromosomes in a diploid cell come from?
The two sets of homologous chromosomes come from DNA replication.