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Good Old Mr./Mrs. Fruit Fly and Transcription Error
Pattern Formation in Bodies
 the development of a spatial organization in which the tissues and organs of an
organism are all in their characteristic places
 we use the fruit fly to study pattern formations because it is VERY OVER STUDIED!
o Positional Formation
 these are the molecular controls that control pattern formation
 pattern formation in animals begins in the early embryo (when the overall 3-D plan
is established
o cells go from undifferentiated to differentiated meaning stem cells to body
cells (more on this to come)
 look up stem cells in the text and write a few notes below
Life Cycle of Drosophila:
 We have been breeding
flies an you should be
very familiar with the
life cycle of a fruit fly.
The space below is so you
can fill in any holes in
regards to this topic. Be
aware the word blastula
is going to be explored
later on when we do
human anatomy.
VERY important term!
Edward B. Lewis
 Many scientists tried/did manipulate embryos to see what outcomes would occur.
o During an amniocentesis, doctors need to be very careful not to disturb the
developing embryo.
o If the embryo is poked, that
area will not form correctly
because the tissue/cell cycle
was disturbed during a very
critical phase of development!
 Lewis studied bizarre fly mutants
and supplied the first concrete
evidence that genes somehow direct
the development of embryos.
Volhard and Wieschaus
 Studied patter formation in flies
 Very hard because there are over
13,000 genes!
 Studies segmentation in flies (their bodies)
o Some of these segmentation genes would certainly be embryonic lethals
 Mutations with phenotypes leading to death at the embryo or larval
stage
 Describe there exp. (right column on page 423) below:
So what is involved with some fly mutants?
 Maternal effect genes
o When this gene is mutant in the mother the resulting offspring is mutant
regardless of their phenotype
o In fruit flies these genes encode proteins or mRNA that are placed in the egg
while it is still in the mother’s ovary.
o When the mother has a mutation in such a gene, she makes a defective gene
product. . . . thus defective eggs.
o Also called egg polarity genes
 Establishes the anterior/posterior axis and the dorsal ventral axis
 Mutations here are also normally lethal
 What does the word bicoid mean? (page 424)
Segmentation Patterns:
 Controlled by segmentation genes
 Sequential activation of three sets of segmentations genes provides for an animal’s
body plan:
o Gap genes:
 Map out the basic subdivisions along the anterior-posterior axis
of an embryo.
 Mutations create gaps in the body
o Pair-rule genes:
 Second set to genes to control segmentation.
 They create “pair segments” and create further sub-divisions in
the embryo.
 Mutations cause half the number of segments to occur
o Segment polarity genes:
 Set the anterior/posterior axis of each segment
 Mutations create mirror image repetition of some segments
Homebox Genes (Homeotic Genes)
 Homeobox-containing, or "hox," genes are responsible for the big decisions of
development rather than the details of engineering. Fruit flies with a particular
mutation in one of their many hox genes, for example, will develop an anatomically
normal leg - in the spot where an antenna should be.
Mutation causes legs to grow from
the wrong segment, in this case,
where the antennae are supposed to
be.
Mutation causes an extra thoracic
segment, and an extra set of
wings.
In humans, a homeotic gene is thought to play a role in the development of irises the part of the eye that opens and closes the pupil dependent on the light intensity.
In rare cases, humans are born without irises, a condition called aniridia.
 The homeobox is a 180-basepair sequence of DNA that has been
found in all homeotic genes (and also in many other regulatory
genes).
 Homoeotic genes are almost identical in very different species. We can study homeotic
genes in the fruit fly to learn about the same genes the control development in a
human or mouse. These groups of genes have remained relatively unchanged
throughout evolutionary history.
Implications
 The discovery of homeobox genes shows that small mutations in a base pair of DNA
can result in profound physical changes - most of these are lethal.
 The conservation of homeobox genes - species from human to worm - shows a link
between these animals, evidence of common ancestry
 Homeobox genes supports the Theory of Evolution and punctuated equilibrium, by
showing that simple mutations can result in large changes of morphology.