Chapter 21 The Genetic Control of Animal Development

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Transvection
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Zygotic Gene Activity in Development

• The differentiation of cell types and the
  formation of organs depend on genes being
  activated in particular spatial and temporal
  patterns.

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Zygotic Gene Expression

• The earliest events in animal development are
  controlled by maternally synthesized factors.
• At some point after fertilization, zygotic gene
  expression, or expression of genes from the
  embryos genome, begins.
• The initial wave of zygotic gene expression is a
  response to maternally synthesized factors. As
  development proceeds, activation of zygotic genes
  leads to complex cascades of gene expression.

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Body Segmentation in Drosophila
Cuticle patterns -Denticle belts
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Homeotic Genes

• Homeotic mutations transform one segment into
  another this is called homeosis.
• Mutation in bithorax (bx) affect two thoracic
  segments.
• Mutations in Antennapedia transform the antennae
  into legs.

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The Homeotic Genes of Drosophila

• The Drosophila homeotic genes form two large
  clusters on one of the autosomes.
• All of the homeotic genes encode helix-turn-helix
  transcription factors with a conserved
  homeodomain region involved in DNA binding.
  These genes control a regulatory cascade of
  target genes that control segment identity.
• The expression pattern of these genes along the
  anterior-posterior axis corresponds to the order
  of the genes along the chromosome.

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The Bithorax and Antennapedia Complexes of
Drosophila
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Organ Formation

• Organ formation requires the organization of many
  different types of cells.
• Organ formation is under genetic control.

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The eyeless Gene

• Mutant flies lack eyes.
• The wild-type eyeless gene encodes a homeodomain
  transcription factor that activates a pathway
  involving thousands of genes.

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Expression of eyeless in Other Tissues Causes
Formation of Extra Eyes
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Mammalian Homologues of eyeless

• The mouse homologue of eyeless, Pax6, produces
  extra eyes when it is inserted into Drosophila.
• In mice, mutations in the eyeless homologue
  reduce the size of the eyes.
• Mutations in the human homologue cause aniridia.

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Two Hybrid
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Genetic Analysis of Development in Vertebrates

• Geneticists can study development in vertebrates
  by applying knowledge gained from the study of
  model invertebrates, by analyzing mutations and
  phenocopies of mutant genes in model vertebrates
  such as mice and zebrafish, and by examining the
  differentiation of stem cells.

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The Mammalian Hox Genes
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Evolution of the Hox Genes

• The four mammalian Hox gene clusters were created
  by quadruplication of a primordial cluster early
  in vertebrate evolution.
• The organization of the HOM/Hox genes has been
  preserved during evolution.
• Within each cluster, the physical order of the
  Hox genes corresponds to their anterior-posterior
  expression pattern.

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T2
A1
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61 amino Acid conservedDNA Binding Sequence
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