Biology Reference
In-Depth Information
9
Gross gene
rearrangements
Cases are known of the doubling of the entire chromosome outfit, the dou-
bling of single chromosomes, and of parts of chromosomes; in other cases a
part of a chromosome appears to be translocated from its habitual site and
attached to some other chromosome. The grosser forms of mutation may
indeed play a special evolutionary role in supplying a mechanism of repro-
ductive incompatibility.
R.A. Fisher (1930) The Genetical Theory of Natural Selection .
Gross gene rearrangements involving the inversion, translocation or fusion of
DNA sequences are relatively rare in inherited disease although much more com-
mon in cancer. Clearly, such mutations can lead to dramatic changes in gene
structure, function and/or expression. At first glance, it may therefore seem some-
what surprising that there are numerous examples of such mutations that have
occurred during evolution. However, genes do not necessarily always change by a
slow incremental process of single base-pair substitution and sudden gross
changes have also played an important role in fashioning our genes.
9.1 Inversions
Most examples of inversions that have occurred during mammalian evolution
have involved large portions of chromosomes and are usually pericentric in that the
breakpoints are located on opposite chromosomal arms with the inversion span-
ning the centromere. This contrasts with paracentric inversions in which the
inversion involves breaks on the same chromosomal arm, with a segment from
only one chromosome arm being inverted.
9.1.1 Pericentric inversions
A good example of a pericentric inversion that has occurred during evolution is
that of two clusters of zinc finger protein genes on human chromosome 10
(Tunnacliffe et al ., 1993). Cluster A, comprising ZNF11A , ZNF25 , ZNF33A, and
ZNF37A , is located on chromosome 10p11.2 whilst cluster B, which is a partial
 
 
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