Biology Reference
In-Depth Information
8.5.8 Truncated gene copies
In some cases, only a portion of a gene may be involved in these intra-chromoso-
mal duplication events [e.g. the polycystic kidney disease 1 (
PKD1
; 16p13.3)
gene; Hughes
et al
., 1995]. This often results, as in the
PKD1
example cited, in the
generation of a linked pseudogene representing a partial copy of the parental
source gene (for other examples, see Chapter 6, section 6.1.1). However, truncated
copies need not always be inactive. Thus, the melanin-concentrating hormone
(
PMCH
; 12q23-q24) gene has become partially duplicated during primate evolu-
tion to generate two truncated copies (
PMCHL1
and
PMCHL2
) which have been
translocated to chromosome 5p14 and 5q12-q13 respectively (Viale
et al
., 1998).
These variant gene copies possess open reading frames, are expressed in a distinc-
tive tissue-specific fashion and, in the authors' opinion, may represent 'genes in
search of a function'.
8.5.9 Duplicational polymorphisms
Some gene duplications occur as polymorphic variants in the human population,
for example the red (
RCP
; Xq28) and green (
GCP
; Xq28) visual pigment (
Figure
9.6
; Neitz
et al
., 1995; Neitz and Neitz 1995) gene. Human males with trichro-
matic vision typically possess one
RCP
gene, one, two, or more
GCP
genes and an
RCP
/
GCP
hybrid gene (
Figure 9.6
; Drummond-Borg
et al
., 1989; Nathans
et al
.,
1986a; 1986b; Neitz and Neitz, 1995; Neitz
et al
., 1995). However, some males
with apparently normal color vision can possess 4
RCP
genes and 6 or 7
GCP
genes (Neitz and Neitz, 1995). This notwithstanding, only one
GCP
gene is nor-
mally expressed, probably as a result of the activity of a locus control region
upstream of the
RCP
gene (Winderickx
et al
., 1992).
Other examples of duplicational polymorphisms include the
1-globin (
HBA1
,
16p13.3; Lie-Injo
et al
., 1981),
-globin (
HBZ
, 16p13.3; Winichagoon
et al
., 1982),
G
-globin (
HBG2
; 11p15; Thein
et al
., 1984), haptoglobin (
HP
; 16q22.1; Maeda
et al
., 1986) and proline-rich protein (
PRB1, PRB2, PRB3, PRB4
; 12p13.2;
Lyons
et al
., 1988) genes. Duplicational polymorphisms may even manifest as
gene cluster copy number variation, for example that involving the
CYP21
,
Figure 8.10.
Common haplotypes of genes present on human chromosome 6p21.3 (after
Figueroa, 1997).
CYP21
Open square.
CYP21P
Solid square.
C4B
Stippled square.
C4A
Hatched square.
