Introduction to Genetic Analysis

Anthony J. F. Griffiths, Susan R. Wessler, Sean B. Carroll

Chapter 13

The Genetic Control of Development - all with Video Answers

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Chapter Questions

Problem 1

In Figure $13-2,$ the transplantation of certain regions of embryonic tissue induces the development of structures in new places. What are these special regions called, and what are the substances they are proposed to produce?

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Problem 2

In Figure $13-5,$ two different methods are illustrated for visualizing gene expression in developing animals. Which method would allow one to detect where within a cell a protein is localized?

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Problem 3

Figure 13 -7 illustrates the expression of the Ultrabithorax (Ubx) Hox protein in developing flight appendages. What is the relationship between where the protein is expressed and the phenotype resulting from the loss of its expression (shown in Figure $13-1$ )?

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Problem 4

In Figure $13-11,$ what is the evidence that vertebrate $\mathrm{Hox}$ genes govern the identity of serially repeated structures?

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Problem 5

As shown in Figure $13-14,$ what is the fundamental distinction between a pair-rule gene and a segment-polarity gene?

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Problem 6

In Table $13-1,$ what is the most common function of proteins that contribute to pattern formation? Why is this the case?

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Problem 7

In Figure $13-20,$ which gap protein regulates the posterior boundary of eve stripe $2 ?$ Describe how it does so in molecular terms.

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Problem 8

As shown in Figure $13-22,$ how many different transcription factors govern where the Distal-less (Dll) gene will be expressed?

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Problem 9

As shown in Figure $13-26,$ the Sonic hedgehog gene is expressed in many places in a developing chicken. Is the identical Sonic hedgehog protein expressed in each tissue? If so, how do the tissues develop into different structures? If not, how are different Sonic hedgehog proteins produced?

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Problem 10

Engrailed, even-skipped, hunchback, and Antennapedia. To a Drosophila geneticist, what are they? How do they differ?

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Problem 11

Describe the expression pattern of the Drosophila gene eve in the early embryo.

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Problem 12

Contrast the function of homeotic genes with that of pair-rule genes.

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Problem 13

When an embryo is homozygous mutant for the gap gene $K r,$ the fourth and fifth stripes of the pair-rule gene $f t z$ (counting from the anterior end) do not form normally. When the gap gene $k n i$ is mutant, the fifth and sixth $f t z$ stripes do not form normally. Explain these results in regard to how segment number is established in the embryo.

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Problem 14

Some of the mammalian Hox genes have been shown to be more similar to one of the insect Hox genes than to the others. Describe an experimental approach that would enable you to demonstrate this finding in a functional test in living flies.

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Problem 15

The three homeodomain proteins Abd-B, Abd-A, and Ubx are encoded by genes within the Bithorax complex of Drosophila. In wild-type embryos, the $A b d$ - $B$ gene is expressed in the posterior abdominal segments, $A b d-A$ in the middle abdominal segments, and $U b x$ in the anterior abdominal and posterior thoracic segments. When the $A b d-B$ gene is deleted, $A b d-A$ is expressed in both the middle and the posterior abdominal segments. When $A b d-A$ is deleted, $U b x$ is expressed in the posterior thorax and in the anterior and middle abdominal segments. When $U b x$ is deleted, the patterns of $A b d-A$ and $A b d-B$ expression are unchanged from wild type. When both $A b d-A$ and $A b d-B$ are deleted, $U b x$ is expressed in all segments from the posterior thorax to the posterior end of the embryo. Explain these observations, taking into consideration the fact that the gap genes control the initial expression patterns of the homeotic genes.

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Problem 16

What genetic tests allow you to tell if a gene is required zygotically or if it has a maternal effect?

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Problem 17

In considering the formation of the A-P and D-V axes in Drosophila, we noted that, for mutations such as bcd, homozygous mutant mothers uniformly produce mutant offspring with segmentation defects. This outcome is always true regardless of whether the offspring themselves are $b c d^{+} / b c d$ or $b c d / b c d .$ Some other maternaleffect lethal mutations are different, in that the mutant phenotype can be "rescued" by introducing a wild-type allele of the gene from the father. In other words, for such rescuable maternal-effect lethals, $m u t^{+} / m u t$ animals are normal, whereas $m u t / m u t$ animals have the mutant defect. Explain the difference between rescuable and nonrescuable maternal-effect lethal mutations.

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Problem 18

Suppose you isolate a mutation affecting A-P patterning of the Drosophila embryo in which every other segment of the developing mutant larva is missing.
a. Would you consider this mutation to be a mutation in a gap gene, a pair-rule gene, a segment-polarity gene, or a segment-identity gene?
b. You have cloned a piece of DNA that contains four genes. How could you use the spatial-expression pattern of their mRNA in a wild-type embryo to identify which represents a candidate gene for the mutation described?
c. Assume that you have identified the candidate gene. If you now examine the spatial-expression pattern of its mRNA in an embryo that is homozygous mutant for the gap gene Krüppel, would you expect to see a normal expression pattern? Explain.

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Problem 19

How does the Bicoid protein gradient form?

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Problem 20

In an embryo from a homozygous Bicoid mutant female, which class(es) of gene expression is (are) abnormal?
a. Gap genes
b. Pair-rule genes
c. Segment-polarity genes
d. Hox genes

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Problem 21

a. The eyeless gene is required for eye formation in Drosophila. It encodes a homeodomain. What would you predict about the biochemical function of the Eyeless protein?
b. Where would you predict that the eyeless gene is expressed in development? How would you test your prediction?
c. The Small eye and Aniridia genes of mice and humans, respectively, encode proteins with very strong sequence similarity to the fly Eyeless protein, and they are named for their effects on eye development. Devise one test to examine whether the mouse and human genes are functionally equivalent to the fly eyeless gene.

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Problem 22

Gene $X$ is expressed in the developing brain, heart, and lungs of mice. Mutations that selectively affect gene $X$ function in these three tissues map to three different regions $(\mathrm{A}, \mathrm{B}, \text { and } \mathrm{C}, \text { respectively }) 5^{\prime}$ of the $X$ coding region.
a. Explain the nature of these mutations.
b. Draw a map of the $X$ locus consistent with the preceding information.
c. How would you test the function of the $\mathrm{A}, \mathrm{B},$ and $\mathrm{C}$ regions?

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Problem 23

Why are regulatory mutations at the mouse Sonic hedgehog gene dominant and viable? Why do coding muta-
tions cause more widespread defects?

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Problem 24

A mutation occurs in the Drosophila doublesex gene that prevents Tra from binding to the $d$ s $x$ RNA transcript. What would be the consequences of this mutation for Dsx protein expression in males? In females?

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Problem 25

You isolate a $g l p-1$ mutation of $C .$ elegans and discover that the DNA region encoding the spatial control region (SCR) has been deleted. What will the GLP-1 protein expression pattern be in a four-cell embryo in mutant heterozygotes? In mutant homozygotes?

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Problem 26

Assess the validity of Monod and Jacob's remark that "anything found to be true of $E$ coli must also be true of Elephants."
a. Compare the structures and mechanisms of action of animal Hox proteins and the Lac repressor. In what ways are they similar?

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