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Lecture 5
Candidate Gene Approach - 1
BSE652
19-1-2017
To understand genetic basis of development
“How genes might specify the complex structures found in higher organisms is a major unsolved problem in
biology.” Sydney Brenner, 1974 – “THE GENETICS OF CAENORHABDITIS ELEGANS”
“To discern the genetic contribution clearly, the thing to do is to keep the environment constant and change
the genes.” From the Gene to Behavior, Seymour Benzer, 1971
“This is not easy to do with human beings; they are notoriously uncooperative and unwieldy experimental
subjects, particularly if one must wait generations for the results.” Seymour Benzer, 1971
To understand genetic basis of development
Question
There are 15-25,000 genes in a given multi-cellular organism.
Which genes’ role should one study?
Candidate gene approach
1. Mutate all genes, one gene at a time, to see which developmental process is being affected
Saturation mutagenesis screen
Drosophila as the model organism
Morgan, Sturtevant ………………
A culture was started where the gene is named after the mutant phenotype, white
C. elegans as the model organism
Sidney Brenner, Sulston and Horvitz ………………
Mutants were named in series based on gross phenotype – unc, lin, lon etc.
Zebrafish as the model organism
Chuck Kimmel, Christiane Nüsslein-Volhard
First vertebrate saturated mutagenesis screen, transparent embryo, very helpful for
hematopoiesis research
Mouse as the model organism
Kathryn Anderson, Monica Justice
Very difficult to conduct. However, some key genes were discovered using this approach.
Nature, 1980
Outline of the crossing schemes used in the Heidelberg screen
 Please read about balancer chromosomes
 These are specific for specific
chromosomes and prevent homologous
recombination
 Balancer/balancer animals are dead
 Please remember that there is no meiotic
recombination in drosophila males
 And drosophila stocks cannot be frozen
for later use, it has to be continuously
maintained
The art and design of genetic screens:
drosophila melanogaster
Daniel St. Johnston
VOLUME 3 | MARCH 2002
Compulsory reading
Cuticular patterning defect – the assay for the screen
Wingless
Staufen
Wild type
Small
Dumpy
Long
In C. elegans the tradition was not to name individual phenotype rather to name series. This screen also
identified some mutants with movement defects – Uncoordinated or Unc mutants.
THE GENETICS OF CAENORHABDITIS ELEGANS
S. BRENNER
The lin mutants
Vulval defect mutants identified Notch pathway members – lin-12
Large scale genetic screen in a small vertebrate: zebrafish
Fig. 4. Examples of mutations with specific defects in the
development of zebrafish embryos. AI! embryos shown
are 24 hours-old. (A)Wildtype. (B) cyclops mutant with
partial!y fused eyes (Hatta et a/.. 1993). (C) Wildtype. (D)
cyclops mutant showing the absence of a floor plate
(Hatta et al., 1993). (E) Wildtype, (F) no tail mutant which
lacks a differentiated notochord, has no tai! and
abnormally shaped somites (Halpern et al.,1993). (GI
Wildtype, (H) spade tail mutant accumulates trunk somitic
mesoderm precursor cells in the tail (Ho et al., 1990).
Which genes will escape the scan?
1. Maternally supplied genes i.e. the reason why maternal effect screen had to be conducted separately.
2. Involved in patterning/differentiation of internal structures
3. Only first instance of essential function may be scored
4. Genes having redundancy