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Genetics Chapter 24 Questions
Terms in this set (21)
C1. In the case of multicellular animals, what are the four types of
cellular processes that occur so a fertilized egg can develop into an
adult organism? Briefly discuss the role of each process.
The four processes are cell division, cell differentiation, cell movement,
and cell death. Cell division is needed to produce a multicellular organism. In other
words, cell division is needed for growth. Cell differentiation is needed to create different
cell types, each of which is differentiated to carry out its own specialized function. Cell
movement is needed during embryonic development to create an embryo with the proper
organization of cells and germ layers. And finally, cell death is needed to create certain
bodily structures. For example, in the early embryonic development of mammals, the
hand is initially a flattened, oval structure. The fingers are formed when cell death occurs
in the regions between each finger.
C2. The arrangement of body axes of the fruit fly are shown in Figure
24.5g. Are the following statements true or false with regard to
body axes in the mouse?
A. Along the anteroposterior axis, the head is posterior to the tail.
B. Along the dorsoventral axis, the vertebrae of the back are dorsal
to the stomach.
C. Along the dorsoventral axis, the feet are dorsal to the hips.
D. Along the proximodistal axis, the toes on the hind legs are distal
to the hips of the hind legs.
A. False, the head is anterior to the tail.
C. False, the feet are ventral to the hips.
C3. If you observed fruit flies with the following developmental
abnormalities, would you guess that a mutation has occurred in a
segmentation gene or a homeotic gene? Explain your guess.
A. Three abdominal segments were missing.
B. One abdominal segment had legs.
C. A fly with the correct number of segments had two additional
thoracic segments and two fewer abdominal segments.
A. This would likely be a mutation in a segmentation gene, because
mutations in certain segmentation genes have fewer segments. In this case, it could be a
loss-of-function mutation in a gap gene.
B. This would likely be a mutation in a homeotic gene because the
characteristics of one segment have been converted to the characteristics of a different
C. This would likely be a mutation in a homeotic gene because the
characteristics of two segments have been converted to the characteristics of two different
C4. Which of the following statements are true with regard to
positional information in Drosophila?
A. Morphogens are a type of molecule that conveys positional
B. Morphogenetic gradients are established only in the oocyte,
prior to fertilization.
C. Cell adhesion molecules also provide a way for a cell to obtain
B. False, because gradients are also established after fertilization during
C5. Discuss the morphological differences between the parasegments
and segments of Drosophila. Discuss the evidence, providing
specific examples, that suggests the parasegments of the embryo
are the subdivisions for the organization of gene expression.
A parasegment is only a transient demarcation that divides the
developing embryo. The segments become permanent regions that develop their own
morphological characteristics. The expression of certain genes, such as even-skipped,
occur in parasegments. This expression occurs prior to the formation of the segments.
C6. Here are schematic diagrams of mutant larvae.
The left side of each pair shows a wild-type larva, with gray boxes
showing the sections that are missing in the mutant larva. Which
type of gene is defective in each larva: a gap gene, a pair-rule gene,
or a segment-polarity gene?
A. This is a mutation in a pair-rule gene (runt).
B. This is a mutation in a gap gene (knirps).
C. This is a mutation in a segment-polarity gene (patched).
C7. Describe what a morphogen is and how it exerts its effects. What
do you expect will happen when a morphogen is expressed in the
wrong place in an embryo? List five examples of morphogens that
function in Drosophila.
A morphogen is a molecule, such as a transcription factor, that
influences the morphological fate of a cell or group of cells. A morphogen exerts its
effects by influencing a genetic hierarchy that ultimately leads to the expression of genes
that govern cell locations and morphologies. If a morphogen is expressed in the wrong
place, an abnormal morphology results. An example is the mutation called Antennapedia,
in which a leg is found in place of an antenna. Examples of morphogens in Drosophila
include Bicoid, Hunchback, Giant, Krüppel, and the products of homeotic genes such as
Antennapedia and bithorax.
C8. What is the meaning of positional information? Discuss three
different ways that cells can obtain positional information. Which
APPLICATION AND EXPERIMENTAL QUESTIONS 611
of these three ways do you think is the most important for the
formation of a segmented body pattern in Drosophila?
Positional information refers to the phenomenon whereby the spatial
locations of morphogens and cell adhesion molecules (CAMs) provide a cell with
information regarding its position relative to other cells. In Drosophila, the formation of a
segmented body pattern relies initially on the spatial location of maternal gene products.
These gene products lead to the sequential activation of the segmentation genes.
Positional information can come from morphogens that are found within the oocyte, from
morphogens secreted from cells during development, and from cell-to-cell contact.
Although all three are important, morphogens in the oocyte have the greatest impact on
the overall body structure.
C9. Gradients of morphogens can be preestablished in the oocyte.
Also, later in development, morphogens can be secreted from cells.
How are these two processes similar and different?
The two processes are similar in that they set up concentration gradients
that lead to the spatial activation of genes in particular regions of the embryo. When the
gradients are established in the oocyte, the morphogen becomes incorporated into cells
during the cleavage and cell division stage of embryogenesis. In this case, the morphogen
is already inside. Later in development, when a morphogen is secreted from a particular
cell or group of cells, the morphogen must bind to cell surface receptors to elicit its
C10. Discuss how the anterior portion of the anteroposterior axis is
established. What aspects of oogenesis are critical in establishing
this axis? What do you think would happen if the bicoid mRNA
was not trapped at the anterior end but instead diffused freely
throughout the oocyte?
The anterior portion of the antero-posterior axis is established by the
action of Bicoid. During oogenesis, the mRNA for Bicoid enters the anterior end of the
oocyte and is sequestered there to establish an anterior (high) to posterior (low) gradient.
Later, when the mRNA is translated, the Bicoid protein in the anterior region establishes
a genetic hierarchy that leads to the formation of anterior structures. If Bicoid was not
trapped in the anterior end, it is likely that anterior structures would not form.
C11. Describe the function of the Bicoid protein. Explain how its ability
to exert its effects in a concentration-dependent manner is a
critical feature of its function.
The Bicoid protein is a morphogen that causes the anterior portions of
the embryo to form properly. It functions as a transcription factor. Other genes, such as
hunchback, are stimulated by Bicoid in a concentration-dependent manner. In this case,
hunchback is stimulated only in the anterior portion of the embryo. In a general sense, the
concentration dependence of gene activation leads to the activation of genes in particular
regions of the embryo, leading to the stimulation of a few broad bands and eventually to a
C12. With regard to development, what are the roles of the maternal
effect genes versus the zygotic genes? Which types of genes are
needed earlier in the development process?
Maternal effect gene products influence the formation of the main body
axes, including the antero-posterior, dorso-ventral, and terminal regions. They are
expressed during oogenesis and needed very early in development. Zygotic genes,
particularly the three classes of the segmentation genes, are necessary after the axes have
been established. The segmentation genes are expressed after fertilization.
C13. Discuss the role of homeotic genes in development. Explain
what happens to the phenotype of a fly when a gain-of-function
homeotic gene mutation causes the protein to be expressed in an
abnormal region of the embryo. What are the consequences of a
loss-of-function mutation in such a gene?
A homeotic gene governs the final fate of particular segments in the
adult animal. A gain-of-function mutation is due to an aberrant expression of a homeotic
gene in the wrong place or the wrong time. This causes the region to develop
inappropriate characteristics. A loss-of-function mutation usually causes a segment to
develop characteristics that are normally found in the anterior adjacent segment.
C14. Describe the molecular features of the homeobox and
homeodomain. Explain how these features are important in the
function of homeotic genes.
The coding sequence of homeotic genes contains a 180-bp consensus
sequence known as a homeobox. The protein domain encoded by the homeobox is called
a homeodomain. The homeodomain contains three conserved sequences that are folded
into α-helical conformations. The arrangement of these α helices promotes the binding of
the protein to the major groove of the DNA. Helix III is called the recognition helix
because it recognizes a particular nucleotide sequence within the major groove. In this
way, homeotic proteins are able to bind to DNA in a sequence-specific manner and
thereby activate particular genes.
C15. What would you predict to be the phenotype of a Drosophila larva
whose mother was homozygous for a loss-of-function allele in the
Because the nanos gene product plays a role in the development of
posterior structures, such a larva would probably develop with two anterior ends.
C16. Based on the photographs in Figure 24.13, in which segments
would the Antp gene normally be expressed?
It would normally be expressed in the three thoracic segments that have
legs (T1, T2, and T3).
C17. If a mutation in a homeotic gene produced the following
phenotypes, would you expect the mutation to be a loss-offunction
or a gain-of-function allele? Explain your answer.
A. An abdominal segment has antennae attached to it.
B. The most anterior abdominal segment resembles the most posterior
C. The most anterior thoracic segment resembles the most posterior
When a loss-of-function mutation occurs in a homeotic gene, the
segment(s) where the homeotic gene is normally expressed will have the characteristics
of the adjacent anterior segment(s). For a gain-of-function mutation, a homeotic gene is
expressed in the wrong place. The segment where it is incorrectly expressed will have
inappropriate characteristics (like legs where the antennae should be). Therefore, the
phenotypic consequence of a gain-of-function mutation is that one or more segments will
have characteristics of segments that would normally occur elsewhere in the fly. (Note: It
is possible that a gain-of-function mutation could resemble a loss-of-function mutation if
the gain-of-function mutation resulted in the abnormal expression of a homeotic gene in
the segment that is adjacent and posterior to the segment where the homeotic gene is
A. Gain-of-function allele.
B. Either a loss-of-function or gain-of-function mutation could explain this
phenotype; a loss-of-function mutation is more likely because it is easier to mutate a gene
and eliminate its function.
C. Gain-of-function allele.
C18. Explain how loss-of-function mutations in the following categories
of genes would affect the morphologies of Drosophila larvae:
A. Gap genes
B. Pair-rule genes
C. Segment-polarity genes
A. When a mutation inactivates a gap gene, a contiguous section of the larva
B. When a mutation inactivates a pair-rule gene, some regions that are
derived from alternating parasegments are missing.
C. When a mutation inactivates a segment-polarity gene, portions are missing
at either the anterior or posterior end of the segments.
C19. What is the difference between a maternal effect gene and a
zygotic gene? Of the following genes that play a role in Drosophila
development, which would be maternal effect genes and which
would be zygotic? Explain your answer.
A. nanos C. bicoid
B. Antp D. lab
A maternal effect gene is expressed in the nurse cells surrounding a
developing oocyte, and the gene products (i.e., mRNA or protein) are transferred from the
nurse cells to the oocyte. Because the nurse cells are diploid, the phenotype of the
resulting individual (after oocyte fertilization) actually depends on the diploid genotype
of the nurse cells (which is the same as the diploid genotype of the mother). Zygotic
genes are expressed after fertilization. (Note: Some genes are both maternal effect and
zygotic; they are expressed in the nurse cell and after fertilization. For this question,
however, let's assume that a gene is likely to be one or the other, but not both.)
A. Nanos is a maternal effect gene; the nanos mRNA accumulates in the
B. Antp is a zygotic gene. It is a homeotic gene that is turned on in embryonic
development after the segmentation genes have been expressed.
C. Bicoid is a maternal effect gene; the bicoid mRNA accumulates in the
D. Lab is a zygotic gene. It is a homeotic gene that is turned on in embryonic
development after the segmentation genes have been expressed.
C20. Cloning of mammals (such as Dolly) is described in Chapter 20 .
Based on your understanding of animal development, explain why
an enucleated egg is needed to clone mammals. In other words,
what features of the oocyte are essential for animal development?
Proper development in mammals is likely to require the products of
maternal effect genes that play a key role in initiating embryonic development. The adult
body plan is merely an expansion of the embryonic body plan, which is established in the
oocyte. Because the starting point for the development of an embryo is the oocyte, this
explains why an enucleated oocyte is needed to clone mammals.
C21. Discuss the similarities and differences between the bithorax
and Antennapedia complexes in Drosophila and the Hox gene
complexes in mice.
Drosophila has eight homeotic genes located in two complexes
(Antennapedia and bithorax) on one chromosome (chromosome 3). The mouse has four
Hox complexes designated HoxA (on chromosome 6), HoxB (on chromosome 11), HoxC
(on chromosome 15), and HoxD (on chromosome 2). There are a total of 38 genes in the
four complexes and 13 gene types. Among the first six genes, five of them are
homologous to genes found in the Antennapedia complex of Drosophila. Among the last
seven, three of them are homologous to the genes of the bithorax complex. Like the
bithorax and Antennapedia complexes in Drosophila, the arrangement of Hox genes
along the mouse chromosomes reflects their pattern of expression from the anterior to the
posterior end. With regard to differences, the mouse has a larger number of homeotic
genes, and gene knockouts do not always lead to transformations that resemble the
anterior adjacent segment.
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