Biology Reference
In-Depth Information
FIGURE 5.3. Infant with Down syndrome. Notice low-set, posteriorly rotated ears,
flat facial profile and protruding tongue (Jones 1997, with permission of W.B.
Saunders Co.).
It is detected in approximately 95% of CML patients. The translocation
creates a fusion transcript between the
BCR
(break point cluster region)
gene on chromosome 22 and the
ABL
(Abelson) gene on chromosome 9.
The fusion protein is thought to create an aberrantly regulated kinase that
activates a number of signal transduction proteins, leading to dysregulated
cellular proliferation.
Inversions are due to breakage and reunion within the same chromo-
some. Inversions are of two types: pericentric, which involves a break in
each arm with the centromere in between, and paracentric, in which both
breaks are within the same chromosomal arm. A number of inversions, such
as a small pericentric inversion involving chromosome 9, appear to be clin-
ically insignificant, and are recognized as a chromosome polymorphism in
humans. About one percent of the human population carries a chromoso-
mal inversion without phenotypic consequence (Therman and Susman
1993). However, inversions can cause disease when a breakpoint occurs in
a functional gene. Difficulty can also arise during meiosis when an inverted
chromosome attempts to pair with its normal homolog. Depending on the
size of the inverted region, faulty pairing and missegregation can occur,
resulting in deleted or duplicated chromosomal segments.
2.1.3 Deletion Syndromes
Chromosomal deletions are usually associated with a constellation of
clinical findings. A number of well known chromosomal deletion and
