Biology Reference
In-Depth Information
5. Candidate Regions
Occasionally, an individual with a single gene disorder is also found to have
a structural chromosomal abnormality or rearrangement such as a dele-
tion, inversion or translocation (Giersch and Morton, Chapter 3). In such
individuals, the structural chromosomal abnormality or rearrangement is
thought to have caused either the physical loss (deletion) of the gene or
disrupted the gene, both of which could lead to a loss of function. The cyto-
genetic location of the structural abnormality or rearrangement constitutes
a candidate region for the location of that gene, which can be confirmed by
linkage studies. The report of an individual with typical features of the auto-
somal dominant disorder Waardenburg syndrome type I (WS1), in which
sensorineural hearing impairment can occur, with a
de novo
chromosomal
inversion on chromosome 2 at 2q35-q37.3 (Ishikirikyama et al. 1989), sug-
gested that the gene for WS1 was located in that region. Subsequent linkage
analysis confirmed this suggestion (Foy et al. 1990).
The mouse is the mammal that offers the best understanding of the gen-
etics and biology of human inherited disorders and diseases. Comparison
of the genetic maps of the mouse and humans have established conserved-
linkage groups (Copeland et al. 1993; Dietrich et al. 1995). This has allowed
a comparative map of mouse and man to be established, enabling identifi-
cation of potential mouse models for the study of inherited human disease
(Brown 1994). Over 100 mutations that affect the development or function
of the mouse inner ear have been identified. In many cases, mouse-human
homology has facilitated identification of genes for hearing impairment
(Steel, Chapter 8).
6. Physical Mapping
Once linkage analysis assigns a gene to an appropriately small interval of
a specific region of a chromosome, the next stage is to carry out physical
mapping of the region of interest to refine the localization of the gene. This
is done by identifying overlapping clones from a library with large human
genomic DNA inserts to form what is known as a contig. In the first
instance, this is often done with yeast artificial chromosomes (YACs) and
once the interval of interest is narrowed down, with bacterial artificial chro-
mosomes (BACs) and/or bacteriophage P1 artificial chromosomes (PACs).
Up to two Mb of DNA can be cloned into YACS, whereas BACs typically
contain DNA inserts up to 150 kilobases in size.
6.1 Contigs
Originally the first stage of physical mapping was to screen YAC libraries
(Albertson et al. 1990; Anand et al. 1990) to identify by PCR those clones
