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| A gene that makes an eye: how changes in the genomic sequence of a gene alter the visible phenotype of an organism |
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Author:
Alexander Picker
Description:
One of the best ways to find out what a gene does in an organism is to see what happens to the organism when that gene is mutated or missing.
The change, the deletion or the insertion in the nucleotide sequence of the gene will disrupt its function and will result in a particular appearance of the organism, the phenotype. Working back from the phenotype, it is possible to determine the genotype, letís say, the form of the gene responsible for that characteristic.
In fish, one way to generate mutations on a large scale is 'chemical mutagenesis'. By this method, the mutations are randomly distributed over the genome, and the modified gene resulting in a specific phenotype needs to be identified afterwards by means of genetics and molecular biology techniques. This can be a very tedious process at times, but once the molecular basis of the morphological defect is determined (i.e. a given phenotype is found to correspond to a specific mutation in a gene, which therefore encodes a dysfunctional protein), it becomes extremely easy to identify carriers of the genetic mutation. This process is called 'genotyping'.
To understand this process, we carried out the genotyping of a known mutant, the eyeless mutant of the Japanese medaka fish. Medaka fish, or Oryzias latipes, is a close relative of the zebrafish. Embryos carrying the eyeless mutation are simple to recognise, as they have no eyes. Embryos were sorted under a microscope into normal and eyeless mutant phenotypes.
We then extracted genomic DNA from both types of embryos and by means of PCR we checked for the presence or absence in it of the eyeless mutation.
Objective:
Introduce students to:
the relationship between phenotype and genotype
the experimental use of genetic mutants
Material:
Background:
The eyeless mutation [PDF]
Tutorial:
Polymerase chain reaction [PDF]
Procedure
and protocol [PDF]
Glossary [PDF] |
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