Unit 12: Developmental Genetics
How a single fertilized egg's genes lay out a whole body plan, segment by segment, and what happens when one gene in that plan misfires.
6 topics · 21 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Maternal Effect Genes and Early Embryo Patterning
- How it worksHow does a gradient of bicoid protein deposited by the mother establish the head-to-tail axis of a fly embryo?
- Worked exampleTrace what happens to a fly embryo's anterior structures when the mother's bicoid gene is completely nonfunctional.
- Common mistakeWhy doesn't a maternal effect gene's phenotype depend on the embryo's own genotype at all?
- Going furtherWhat happens to the head-to-tail axis of a fly embryo when bicoid mRNA is injected into the posterior end instead of the anterior end?
Your turn1 of 2
A mother fly carries a completely nonfunctional bicoid gene, so no bicoid protein gradient is ever deposited in her eggs. What happens to the anterior (head and thorax) structures of her offspring's embryos, regardless of the offspring's own bicoid genotype?The Segmentation Gene Cascade
- How it worksHow do gap genes, pair-rule genes, and segment polarity genes refine a broad gradient into individual stripes?
- Worked exampleTrace how a mutation in a pair-rule gene deletes alternating segments in a fly larva.
- Ties togetherHow does the segmentation cascade turn the bicoid gradient from a maternal effect gene into a segmented body plan?
Your turn1 of 2
A fly larva carries a mutation in one particular pair-rule gene, and as a result, every OTHER body segment fails to form, producing a larva with alternating segments missing. Why does a pair-rule gene mutation specifically delete alternating segments rather than one contiguous block?Homeotic Genes and the Hox Cluster
- How it worksHow does each Hox gene's expression domain along the embryo assign that segment its correct identity?
- Worked exampleFind which body segment develops abnormally when the Hox gene normally expressed there is experimentally deleted.
- Common mistakeWhy doesn't a homeotic gene mutation stop a body segment from forming, only change what it becomes?
- Worked exampleTrace which Hox gene's expression boundary must shift to explain a homeotic transformation of one segment into another's identity.
- Ties togetherHow does the Hox cluster's colinear gene order connect to the segmentation cascade that first divides the embryo into stripes?
Your turn1 of 2
A Hox gene that normally specifies a thoracic segment's identity is experimentally deleted. The segment still forms, with the correct size and boundaries, but it develops the WRONG structures for its position. Why doesn't losing a homeotic gene stop the segment from forming at all, only change what it becomes?Homeotic Mutations: Antennapedia and Bithorax
- How it worksHow does an Antennapedia mutation switch on the leg-identity Hox gene in the head segment by mistake?
- Worked exampleTrace how a bithorax mutation duplicates the wing-bearing thoracic segment into a fly with two pairs of wings.
- Going furtherWhat happens to a homeotic phenotype when the misexpressed Hox gene is only partially active instead of fully on?
Your turn1 of 2
An Antennapedia mutation causes a fly to grow a fully formed leg where its antenna should be, on its head. How does a single gene mutation produce such a dramatic structural swap?Cell Signaling and Induction in Development
- How it worksHow does an inducing cell secrete a signaling molecule that changes gene expression in a responding cell nearby?
- Worked exampleTrace how the Spemann organizer induces the formation of a second body axis when transplanted into a host embryo.
- Common mistakeWhy doesn't every cell that receives the same inductive signal respond in exactly the same way?
Your turn1 of 2
When Spemann and Mangold transplanted a small group of cells (the organizer) from one embryo into a different location on a host embryo, the host developed a second, complete body axis, including structures made from the HOST's own tissue rather than the transplanted cells. How does one small group of organizer cells accomplish this?Programmed Cell Death in Development
- How it worksHow does a genetically programmed apoptosis pathway remove the webbing between developing fingers?
- Going furtherWhat happens to limb development when the genes controlling apoptosis fail to activate on schedule?
- Common mistakeWhy isn't apoptosis just uncontrolled cell damage the way necrosis is?
Your turn1 of 2
A human hand begins development as one solid, paddle-shaped structure, but the fingers separate before birth as the webbing between them disappears. How does a genetically programmed process remove exactly that webbing tissue and no other part of the hand?