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Ch. 18 - Developmental Genetics
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 18 - Developmental GeneticsProblem 21a
Chapter 18, Problem 21a

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

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Understand the role of Hox genes: Hox genes are a group of related genes that control the body plan of an embryo along the head-tail axis. Specifically, Hoxd9–13 genes are involved in specifying digit identity in limb development. Ectopic expression refers to the abnormal expression of a gene in a location where it is not normally expressed.
Analyze the effect of ectopically expressing Hoxd10: Hoxd10 is normally expressed in the proximal regions of the limb bud. If Hoxd10 is ectopically expressed throughout the limb bud, it could lead to a shift in the identity of distal limb structures (such as digits) toward more proximal structures, potentially disrupting normal digit formation.
Consider the effect of ectopically expressing Hoxd11: Hoxd11 is typically expressed in the intermediate regions of the limb bud. Ectopic expression of Hoxd11 throughout the limb bud might result in intermediate limb structures being specified in regions where distal or proximal structures should form, altering the normal pattern of limb development.
Evaluate the combined effect of ectopically expressing both Hoxd10 and Hoxd11: Co-expression of Hoxd10 and Hoxd11 throughout the limb bud could lead to a more severe disruption of limb patterning. The combined effect might result in a loss of distal structures (digits) and an overrepresentation of proximal or intermediate structures, as the normal gradient of Hox gene expression would be disrupted.
Summarize the consequences: The ectopic expression of Hox genes disrupts the spatial and temporal regulation of limb development, which is critical for proper digit identity. The specific consequences depend on the gene(s) expressed and their normal roles in limb patterning. Experimental evidence or further modeling would be needed to confirm the exact phenotypic outcomes.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Hox Genes

Hox genes are a group of related genes that determine the body plan and the identity of segments in an organism during embryonic development. They are crucial for the proper formation of limbs and other structures, as they provide positional information that dictates where specific body parts will develop. In vertebrates, different Hox genes are expressed in specific regions, influencing the morphology of limbs and digits.
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Ectopic Expression

Ectopic expression refers to the abnormal expression of a gene in a location where it is not normally expressed. In the context of developmental biology, ectopic expression of Hox genes can lead to the formation of structures in inappropriate locations, potentially resulting in malformations. For example, ectopically expressing Hoxd10 in the limb bud could disrupt normal digit formation, leading to altered digit identity or number.
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Digit Identity

Digit identity refers to the specific characteristics and functions of individual digits (fingers and toes) in vertebrates, which are determined by the expression of specific Hox genes. Each digit is associated with a particular Hox gene expression pattern, which influences its development and identity. Alterations in the expression of these genes, such as through ectopic expression, can lead to changes in digit identity, resulting in phenotypic variations or abnormalities.
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Related Practice
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?

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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?

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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?

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Textbook Question

The Hoxd9–13 genes are thought to specify digit identity. 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?

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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?

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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?

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