Chromosomal abnormalities can sometimes provide an alternative method of localizing a disease gene, in place of linkage analysis.
Normal sporadic, like many severe dominants, chromosome aberrations may provide the only method of arriving at a candidate gene.
Balanced abnormalities (translocations or inversion) are particular useful
Submicroscopic deletions and cryptic translocations are at least as valuable as visible chromosome abnormalities
Patients with a balanced chromosomal abnormality and an unexplained phenotype are interesting
A balanced translocation or inversion, with nothing extra or missing, would not be expected to have any phenotypic effect on the carrier.
Three possible explanation of balanced chromosomal abnormalities is phenotypically abnormal
The finding is coincidental
The rearrangement is not in fact balanced—there is an unnoticed loss or gain of material
One of the chromosome breakpoints causes the disease
A chromosomal break can cause a loss- of- function phenotype if it disrupts the coding sequence of a gene, or separates it from a nearby regulatory region.
It could cause a gain of function
By splicing exons of two genes together to create a novel chimeric gene (this is rare in inherited disease but common in tumorigenesis)
The breakpoint provides provides a valuable clue to the exact physical location of the disease gene.
The prcise position of the breakpoint is most easily defined by using FISH
Example of the power of this approach is the identification of the Sotos syndrome gene
The positional clue is not infallible: sometimes breakpoints can alter expression of a gene located hundreds of kilobases away by affecting the structure of large-scale chromatin domains.
Even if translocation breakpoint disrupts a gene, we have lost function of only one or two copies of the gene.
There will be no phenotypic effect unless a 50% reduction in the level of the product causes problems