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1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

14. Genetic Control of Development

Developmental Patterning Genes

14. Genetic Control of Development

Developmental Patterning Genes

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Plant HOX genes

Kylia Goodner

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Okay. So now let's talk about plant development and plant genes. So far we've all talked about animals, but plants actually contain their own set of homeotic genes or Hox genes. And so, in Arabidopsis, which is typically the model organism used to study plants. There are 3 classes of Hox genes, very easily labeled Class A, B, and C. Now, in the plant, class A forms the sepals. If you have a segment that's expressing A and B, that's going to be the petals. You have a set of region expressing B and C, that's going to be the stamen. And if you have a thing that's expressing only C, that's going to be the carpels.

If you're like me and aren't really quite up to date on your plant anatomy, let's refresh this. So here's the plant, obviously you know the petals. Right? There's lots of things here, you don't need to know this, we're not in a plants class. Right? But we, the things that we are interested in, we're interested in sepals, petals, stamens, and carpels. So here's the stamen, so this thing here is the stamen. Right? We have petals, which we did here. We have the carpels, which is actually going to be, this green thing here is the carpel, and the sepal is actually these little baby leaves down here, the sepal. So these are the 4 things we're worried about. So if we look over here at this plant, you can see the classes are down here. So Class A is blue, B is yellow, and C is red. Class A forms the sepal, which you remember are these little baby leaves down here. So if we look at where only A is expressed, that's the sepals. If we look at where A and B are expressed, so anything with blue and yellow on it, that's going to be the petals, which are things we're most familiar with. If we look at B and C, so yellow and red, that's going to be the stamen. Remember the stamen is this thing here, so that's there. And then finally, Class C forms the carpal, which is only red, and that is here, carpal stamen, petal, and sepals. Just in case you're not familiar with plant development, hopefully that helps, but this is plant development. They use these Hox genes as well, they're just labeled class A, B, and C, instead of the ones that are labeled, you know, that way in fruit flies. So, with that, let's now move on.

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Textbook Question

In this chapter, we have focused on large-scale as well as the inter- and intracellular events that take place during embryogenesis and the formation of adult structures. In particular, we discussed how the adult body plan is laid down by a cascade of gene expression, and the role of cell–cell communication in development. Based on your knowledge of these topics, answer several fundamental questions:

How do we know that eye formation in all animals is controlled by a binary switch gene?

Textbook Question

How is positional information provided along the anterior–posterior axis in Drosophila? What are the functions of bicoid and nanos?

Textbook Question

Early development in Drosophila is atypical in that pattern formation takes place in a syncytial blastoderm, allowing free diffusion of transcription factors between nuclei. In many other animal species, the fertilized egg is divided by cellular cleavages into a larger and larger number of smaller and smaller cells.

How must the model that describes Drosophila development be modified for describing animal species whose early development is not syncytial?

Textbook Question

Early development in Drosophila is atypical in that pattern formation takes place in a syncytial blastoderm, allowing free diffusion of transcription factors between nuclei. In many other animal species, the fertilized egg is divided by cellular cleavages into a larger and larger number of smaller and smaller cells.

What constraints does the formation of a syncytial blastoderm impose on the mechanisms of pattern formation?

Textbook Question

Consider the even-skipped regulatory sequences in Figure 18.9.

Explain what you expect to see happen to even-skipped stripe 2 if it is expressed in a Krüppel mutant background. What about a hunchback mutant background? A giant mutant background? A bicoid mutant background?

Textbook Question

Consider the even-skipped regulatory sequences in Figure 18.9.

Consider the binding sites for gap proteins and Bicoid in the stripe 2 enhancer module. What sites are occupied in parasegments 2, 3, and 4, and how does this result in expression or no expression?

Textbook Question

Consider the even-skipped regulatory sequences in Figure 18.9.

How are the sharp boundaries of expression of eve stripe 2 formed?

Textbook Question

What is the phenotype of maternal-effect mutations?

Textbook Question

What aspects of development do maternal-effect genes control?

Textbook Question

When are gene products from these genes made, and where are they located?

Textbook Question

What are maternal-effect genes?

Textbook Question

Why do loss-of-function mutations in Hox genes usually result in embryo lethality, whereas gain-of-function mutants can be viable? Why are flies homozygous for the recessive loss-of-function alleles and viable?

Textbook Question

What is the phenotype associated with zygotic gene mutations?

Textbook Question

What are zygotic genes, and when are their gene products made?

Textbook Question

Does the maternal genotype contain zygotic genes?

Textbook Question

Compare and contrast the specification of segmental identity in Drosophila with that of floral organ specification in Arabidopsis. What is the same in this process, and what is different?

Textbook Question

List the main classes of zygotic genes. What is the function of each class of these genes?

Textbook Question

Actinomycin D is a drug that inhibits the activity of RNA polymerase II. In the presence of actinomycin D, early development in many vertebrate species, such as frogs, can proceed past the formation of a blastula, a hollow ball of cells that forms after early cleavage divisions, but development ceases before gastrulation (the stage at which cell layers are established). What does this tell you about maternal versus zygotic gene activity in early frog development?

Textbook Question

You observe that a particular gene is being transcribed during development. How can you tell whether the expression of this gene is under transcriptional or translational control?

Textbook Question

The homeotic mutation Antennapedia causes mutant Drosophila to have legs in place of antennae and is a dominant gain-of-function mutation. What are the properties of such mutations? How does the Antennapedia gene change antennae into legs?

Textbook Question

The bicoid gene is a coordinate maternal–effect gene. If loss of bicoid function in the egg leads to lethality during embryogenesis, how are females homozygous for bicoid produced? What is the phenotype of a male homozygous for bicoid loss-of-function alleles?

Textbook Question

The bicoid gene is a coordinate maternal–effect gene. A female that is homozygous for a loss-of-function bicoid allele is mated to a wild-type male. What are the phenotypes of their progeny?

Textbook Question

The bicoid gene is a coordinate maternal–effect gene.

A female Drosophila heterozygous for a loss-of function bicoid allele is mated to a male that is heterozygous for the same allele. What are the phenotypes of their progeny?

Textbook Question

The Drosophila homeotic mutation spineless aristapedia (ssᵃ) results in the formation of a miniature tarsal structure (normally part of the leg) on the end of the antenna. What insight is provided by (ssᵃ) concerning the role of genes during determination?

Textbook Question

Given that maternal Bicoid activates the expression of hunchback (see Figure 18.7), what would be the consequence of adding extra copies of the bicoid gene by transgenic means to a wild-type female with two copies, thus creating a female fly with three or four copies of the bicoid gene? How would hunchback expression be altered? What about the expression of other gap genes and pair-rule genes?

Textbook Question

Embryogenesis and oncogenesis (generation of cancer) share a number of features including cell proliferation, apoptosis, cell migration and invasion, formation of new blood vessels, and differential gene activity. Embryonic cells are relatively undifferentiated, and cancer cells appear to be undifferentiated or dedifferentiated. Homeotic gene expression directs early development, and mutant expression leads to loss of the differentiated state or an alternative cell identity. M. T. Lewis [(2000). Breast Can. Res. 2:158–169] suggested that breast cancer may be caused by the altered expression of homeotic genes. When he examined 11 such genes in cancers, 8 were underexpressed while 3 were overexpressed compared with controls. Given what you know about homeotic genes, could they be involved in oncogenesis?

Textbook Question

What phenotypes do you expect in flies homozygous for loss-of-function mutations in the following genes: Krüppel, odd-skipped, hedgehog, and Ultrabithorax?

Textbook Question

The specification of the anterior–posterior axis in Drosophila embryos is initially controlled by various gene products that are synthesized and stored in the mature egg following oogenesis. Mutations in these genes result in abnormalities of the axis during embryogenesis. These mutations illustrate maternal effect. How do such mutations vary from those produced by organelle heredity? Devise a set of parallel crosses and expected outcomes involving mutant genes that contrast maternal effect and organelle heredity.

Textbook Question

The pair-rule gene fushi tarazu is expressed in the seven even-numbered parasegments during Drosophila embryogenesis. In contrast, the segment polarity gene engrailed is expressed in the anterior part of each of the 14 parasegments. Since both genes are active at similar times and places during development, it is possible that the expression of one gene is required for the expression of the other. This can be tested by examining expression of the genes in a mutant background—for example, looking at fushi tarazu expression in an engrailed mutant background, and vice versa. Given the hierarchy of gene action during Drosophila embryogenesis, what might you predict to be the result of these experiments?

Textbook Question

The pair-rule gene fushi tarazu is expressed in the seven even-numbered parasegments during Drosophila embryogenesis. In contrast, the segment polarity gene engrailed is expressed in the anterior part of each of the 14 parasegments. Since both genes are active at similar times and places during development, it is possible that the expression of one gene is required for the expression of the other. This can be tested by examining expression of the genes in a mutant background—for example, looking at fushi tarazu expression in an engrailed mutant background, and vice versa. Based on your prediction, can you predict the phenotype of the fushi tarazu and engrailed double mutant?

Textbook Question

In contrast to Drosophila, some insects (e.g., centipedes) have legs on almost every segment posterior to the head. Based on your knowledge of Drosophila, propose a genetic explanation for this phenotype, and describe the expected expression patterns of genes of the Antennapedia and bithorax complexes.

Textbook Question

One of the most interesting aspects of early development is the remodeling of the cell cycle from rapid cell divisions, apparently lacking G1 and G2 phases, to slower cell cycles with measurable G1 and G2 phases and checkpoints. During this remodeling, maternal mRNAs that specify cyclins are deadenylated, and zygotic genes are activated to produce cyclins. Audic et al. [(2001). Mol. and Cell. Biol. 21:1662–1671] suggest that deadenylation requires transcription of zygotic genes. Present a diagram that captures the significant features of these findings.

Textbook Question

The bristles that develop from the epidermis in Drosophila are evenly spaced, so that two bristles never occur immediately adjacent to each other. How might this pattern be established during development?

Textbook Question

A number of genes that control expression of Hox genes in Drosophila have been identified. One of these homozygous mutants is extra sex combs, where some of the head and all of the thorax and abdominal segments develop as the last abdominal segment. In other words, all affected segments develop as posterior segments. What does this phenotype tell you about which set of Hox genes is controlled by the extra sex combs gene?

Textbook Question

You are traveling in the Netherlands and overhear a tulip breeder describe a puzzling event. Tulips normally have two outer whorls of brightly colored petal-like organs, a third whorl of stamens, and an inner (fourth) whorl of carpels. However, the breeder found a recessive mutant in his field in which the outer two whorls were green and sepal-like, whereas the third and fourth whorls both contained carpels. What can you speculate about the nature of the gene that was mutated?

Textbook Question

The apterous gene in Drosophila encodes a protein required for wing patterning and growth. It is also known to function in nerve development, fertility, and viability. When human and mouse genes whose protein products closely resemble apterous were used to generate transgenic Drosophila [Rincon-Limas et al. (1999). Proc. Nat. Acad. Sci. (USA) 96:2165–2170], the apterous mutant phenotype was rescued. In addition, the whole-body expression patterns in the transgenic Drosophila were similar to normal apterous.

What do these results indicate about the molecular nature of development?

Textbook Question

The apterous gene in Drosophila encodes a protein required for wing patterning and growth. It is also known to function in nerve development, fertility, and viability. When human and mouse genes whose protein products closely resemble apterous were used to generate transgenic Drosophila [Rincon-Limas et al. (1999). Proc. Nat. Acad. Sci. (USA) 96:2165–2170], the apterous mutant phenotype was rescued. In addition, the whole-body expression patterns in the transgenic Drosophila were similar to normal apterous.

What is meant by the term rescued in this context?

Textbook Question

A powerful approach to identifying genes of a developmental pathway is to screen for mutations that suppress or enhance the phenotype of interest. This approach was undertaken to elucidate the genetic pathway controlling C. elegans vulval development. In a complementary experiment, a gain-of-function let-23 mutant with a multi-vulva phenotype was also mutagenized. What types of mutations will suppress the multi-vulva phenotype?

Textbook Question

A powerful approach to identifying genes of a developmental pathway is to screen for mutations that suppress or enhance the phenotype of interest. This approach was undertaken to elucidate the genetic pathway controlling C. elegans vulval development. A lin-3 loss-of-function mutant with a vulva-less phenotype was mutagenized. Based on your knowledge of the genetic pathway, what types of mutations will suppress the vulva-less phenotype?

Textbook Question

In Arabidopsis, flower development is controlled by sets of homeotic genes. How many classes of these genes are there, and what structures are formed by their individual and combined expression?

Textbook Question

The Hoxd9–13 genes are thought to specify digit identity (see Figure 18.18). You wish to examine the effect of loss-of-function alleles in developing limbs. How would you construct a mouse in which the function of Hoxd9–13 is retained during anterior–posterior embryonic patterning but is absent from developing limbs?

Textbook Question

The Hoxd9–13 genes are thought to specify digit identity (see Figure 18.18). What would be the consequence of ectopically expressing Hoxd10 throughout the developing mouse limb bud? What about Hoxd11? What about both Hoxd10 and Hoxd11?

Textbook Question

The floral homeotic genes of Arabidopsis belong to the MADS-box gene family, while in Drosophila, homeotic genes belong to the homeobox gene family. In both Arabidopsis and Drosophila, members of the Polycomb gene family control expression of these divergent homeotic genes. How do Polycomb genes control expression of two very different sets of homeotic genes?

Textbook Question

Three-spined stickleback fish live in lakes formed when the last ice age ended 10,000 to 15,000 years ago. In lakes where the sticklebacks are prey for larger fish, they develop 35 bony plates along their body as armor. In contrast, sticklebacks in lakes where there are no predators develop only a few or no bony plates. Loss-of-function mutations in the coding region of the homologous gene in humans result in loss of hair, teeth, and sweat glands, as in the toothless men of Sind (India). What does this suggest about hair, teeth, and sweat glands in humans?

Textbook Question

Three-spined stickleback fish live in lakes formed when the last ice age ended 10,000 to 15,000 years ago. In lakes where the sticklebacks are prey for larger fish, they develop 35 bony plates along their body as armor. In contrast, sticklebacks in lakes where there are no predators develop only a few or no bony plates. In crosses between fish of the two different morphologies, the lack of bony armor segregates as a recessive trait that maps to the ectodermal dysplasin (Eda) gene. Comparisons between the Eda-coding regions of the armored and nonarmored fish revealed no differences. How can you explain this result?

Textbook Question

Vulval development in C. elegans is dependent on the response of some of the central epidermal progenitor cells in the region of the developing vulva to a chemical signal from the gonad. Signaling from the gonad is blocked by action of the vulvaless mutant let-23 so that none of the central progenitor cells form vulval structures. In the vulvaless mutant, n300, the central progenitor cells do not form.

What phenotype (vulva formed or vulvaless) would you expect from the double mutant? Why?

Textbook Question

Vulval development in C. elegans is dependent on the response of some of the central epidermal progenitor cells in the region of the developing vulva to a chemical signal from the gonad. Signaling from the gonad is blocked by action of the vulvaless mutant let-23 so that none of the central progenitor cells form vulval structures. In the vulvaless mutant, n300, the central progenitor cells do not form.

Which gene is likely to act earlier in the vulval developmental pathway?

Textbook Question

The flowering jungle plant Lacandonia schismatica, discovered in southern Mexico, has a unique floral structure. Petal-like organs are in the outer whorls surrounding a number of carpels, and stamens are in the center of the flower. Closely related species are dioecious; female plants bear flowers that resemble those of Lacandonia, but without the central stamens. What type of mutation could have resulted in the evolution of Lacandonia flowers?

Textbook Question

Homeotic genes are thought to regulate each other. Are similar interactions observed between Hox genes?

Textbook Question

Homeotic genes are thought to regulate each other. What aspect of the phenotype of apetala2 agamous double mutants indicates that these two genes act antagonistically?

Textbook Question

Basidiomycota is a monophyletic group of fungi that includes most of the common mushrooms. You are interested in the development of the body plan of mushrooms. How would you identify the genes required for patterning during mushroom development?

Textbook Question

In Drosophila, loss-of-function Ultrabithorax mutations result in the posterior thoracic segments differentiating into body parts with an identity normally found in the anterior thoracic segments. When the Ultrabithorax gene was cloned, it was shown to encode a transcription factor and to be expressed only in the posterior region of the thorax. Thus, Ultrabithorax acts to specify the identity of the posterior thoracic segments. Similar genes were soon discovered in other animals, including mice and humans. You have found that mice possess two closely related genes, Hoxa7 and Hoxb4, which are orthologs (see Genetic Analysis 14.2 for definition) of Ultrabithorax. You wish to know whether the two mouse genes act to specify the identity of body segments in mice.

How will you determine whether the mouse genes have redundant functions?

Textbook Question

In land plants, there is an alternation of generations between a haploid gametophyte generation and a diploid sporophytic generation. Both generations are typically multicellular and may be free-living. The male (pollen) and female (embryo sac) gametophytes are the haploid generation of flowering plants.

How would you conduct a screen to identify genes required for male gametophyte development?

Textbook Question

In land plants, there is an alternation of generations between a haploid gametophyte generation and a diploid sporophytic generation. Both generations are typically multicellular and may be free-living. The male (pollen) and female (embryo sac) gametophytes are the haploid generation of flowering plants.

How would you conduct a screen to identify genes required for female gametophyte development in Arabidopsis?

Textbook Question

Zea mays (maize, or corn) was originally domesticated in central Mexico at least 7000 years ago from an endemic grass called teosinte. Teosinte is generally unbranched, has male and female flowers on the same branch, and has few kernels per 'cob,' each encased in a hard, leaf-like organ called a glume. In contrast, maize is highly branched, with a male inflorescence (tassel) on its central branch and female inflorescences (cobs) on axillary branches. In addition, maize cobs have many rows of kernels and soft glumes. George Beadle crossed cultivated maize and wild teosinte, which resulted in fully fertile F₁ plants. When the F₁ plants were self-fertilized, about 1 plant in every 1000 of the F₂ progeny resembled either a modern maize plant or a wild teosinte plant. What did Beadle conclude about whether the different architectures of maize and teosinte were caused by changes with a small effect in many genes or changes with a large effect in just a few genes?

Textbook Question

In Drosophila, recessive mutations in the fruitless gene (fru) result in males courting other males; and recessive mutations in the Antennapedia gene (Ant⁻) lead to defects in the body plan, specifically in the thoracic region of the body, where mutants fail to develop legs. The two genes map 15 cM apart on chromosome 3. You have isolated a new dominant Antdᵈ mutant allele that you induced by treating your flies with X-rays. Your new mutant has legs developing instead of antennae on the head of the fly. You cross your newly induced dominant Antᵈ mutant (a pure-breeding line) with a homozygous recessive fru mutant (which is homozygous wild type at the Ant⁺ locus), as diagrammed below: Provide a molecular explanation for the reason your new Antᵈ mutant is dominant and for its novel phenotype.

Textbook Question

In Drosophila, recessive mutations in the fruitless gene (fru) result in males courting other males; and recessive mutations in the Antennapedia gene (Ant⁻) lead to defects in the body plan, specifically in the thoracic region of the body, where mutants fail to develop legs. The two genes map 15 cM apart on chromosome 3. You have isolated a new dominant Antdᵈ mutant allele that you induced by treating your flies with X-rays. Your new mutant has legs developing instead of antennae on the head of the fly. You cross your newly induced dominant Antᵈ mutant (a pure-breeding line) with a homozygous recessive fru mutant (which is homozygous wild type at the Ant⁺ locus), as diagrammed below: Your cross results in the following phenotypic proportions: Legs on head, normal courting behavior 75 Normal head, abnormal courting behavior 25 Legs on head, abnormal courting behavior 0 Normal head, normal courting behavior 0 Provide a genetic explanation for these results and describe a test for your hypothesis

Textbook Question

In Drosophila, recessive mutations in the fruitless gene (fru) result in males courting other males; and recessive mutations in the Antennapedia gene (Ant⁻) lead to defects in the body plan, specifically in the thoracic region of the body, where mutants fail to develop legs. The two genes map 15 cM apart on chromosome 3. You have isolated a new dominant Antdᵈ mutant allele that you induced by treating your flies with X-rays. Your new mutant has legs developing instead of antennae on the head of the fly. You cross your newly induced dominant Antᵈ mutant (a pure-breeding line) with a homozygous recessive fru mutant (which is homozygous wild type at the Ant⁺ locus), as diagrammed below: What phenotypes, and in what proportions, do you expect in the F₂ obtained by interbreeding F₁ animals?

Multiple Choice

Which genes are the first genes that control patterning of the offspring during early development?

Multiple Choice

Areas with higher bicoid expression will develop into which body pattern?

Multiple Choice

Activation of the segmentation genes occurs in which of the following orders?

Multiple Choice

Which of the following HOX clusters are responsible for forming the abdominal in Drosophila development?

Multiple Choice

In Arabidopsis, which class of HOX genes are responsible for forming the plant carpels?

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