Biology Reference
In-Depth Information
-globin gene cluster (16p13.3) therefore comprises the embryonically expressed
(
HBZ
) gene, the post-natally expressed
2 (
HBA2
) and
1 (
HBA1
) genes, the
-globin (
HBQ1
) gene (probably a transcribed pseudogene) and three conven-
tional pseudogenes (
HBZP
,
HBAP1
,
HBAP2
) (
Figure 4.12
). The genes in the
-
globin gene cluster are therefore arranged in the order of their activation during
ontogeny. The human
HBA1
and
HBA2
genes have remained virtually identical
to each other as a consequence of crossing over and gene conversion (Bailey
et al
.,
1992; Hess
et al
., 1984; Michelson and Orkin 1983).
The ancestral
-globin gene is thought to have duplicated about 200 Myrs ago to
yield a
/
ancestral gene and a
/
ancestral gene (Hardies
et al
., 1984;
Figure 4.11
).
The
- and
-globin genes diverged from the
/
ancestral gene about 40 Myrs ago.
,
2
,
G
γ
A
γ
2
1
1
40
50-60
100
100
260
200
400
500
Figure 4.11.
Evolution of the vertebrate globin genes (redrawn from Efstratiadis
et al
.,
1980 and Higgs
et al
., 1989). Numbers denote approximate estimated times of divergence
in Myrs before present.
0
10
20
30
40
50
60
70 kb
HS 40
2
1
2
1
2
1
α
-Globin
cluster
LCR
HS4
HS3
HS2
G
γ
A
γ
ψ β
β
-Globin
cluster
LCR
Figure 4.12.
Organization of the human α - and β -globin clusters. Arrows denote the
direction of transcription. Solid boxes denote genes, empty boxes pseudogenes and the
hatched box the θ -globin (
HBQ1
) gene which is transcribed but probably not expressed.
The relative positions of the locus control regions (LCRs) are denoted together with the
constituent DNase I hypersensitive sites (HS).
