Re: How are genes identifies during chromosome mapping?

Sharyn,

Genes can be identified several ways.  Now that much of the human genome is
being sequenced, it is possible to search for homology between a human gene
and a gene from a different organism such as a mouse.  If we know what a
certain gene does in a mouse and we find a gene with a similar sequence in
humans, then we might be able to find what the human gene does based on how
the gene functions in a different organism.

The genes for more complex diseases such as cystic fibrosis (which has
already been discovered), retinoblastoma, breast cancer, psoriasis are
being sought in other ways though.  People with diseases have a mutation in
their DNA sequence. This may be a base pair change such as insertion or
deletion of a base pair.  They could also have chromosome rearrangements. 
The area of a mutation could be where the gene is.

Patients with these diseases are found and their DNA is collected. 
Researchers spend months, even years looking for mutations on DNA
fragments.  The more patients collected, the more likely you can find a
common mutation that affects most of the individuals.  The DNA of an
affected patient is compared to relatives of the patient who don't have the
disease.  Eventually, if the researcher gets lucky, he will find the right
DNA fragment which has a mutation.

Here is a more detailed explanation of how that happens:
Statistical analyses can  be used to link potential candidate genes with
genetic markers - recognizable variations in DNA.  This can give you a good
idea of what chromosome the gene is on.

After scientists discover what chromosome a gene is on, they could do
chromosome walking.  Chromosome walking means looking at overlapping parts
of DNA fragments and checking to see if that gene segment is inherited with
the disease.  This means that family members with the disease will have
that segment but family members without the disease won't have that
particular segment.  Chromosome walking is very slow and could possibly
take 20 or more years because you might be very far away from the gene -
100,000+ bases away or even 1 million bases away.  Chromosome jumping
though is much faster.  Then you can hop over a large area of sequence.

DNA segments found are then matched to mouse genes and other organisms.  If
other organisms have these genes, then these genes must be important
because they are evolutionarily conserved.  Evolution has maintained these
genes because they have important functions.

Then scientists start sequencing in that area in search of the gene.  They
look for mutations.  They look at protein products and consider whether a
problem with this protein could cause the problem associated with the
disease.

All of this would take many, many years and many people collaborating to
find genes.

The U.S. Department of Energy has this web site about molecular genetics: http://www.ornl.gov/hgmis/publicat/primer/intro.html

I found this web site explained the information I gave in more scientific
terms.

Bonnie