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(a)
preGH
preCSH
(b)
preGH
preCSH
preGH
preCSH
(c)
GH1
CSHP1
GH2
CSH2
GH1
CSHP1
GH2
CSH2
(d)
GH1
CSHP1
CSH1
GH2
CSH2
Figure 4.14.
Evolution of the human growth hormone gene cluster by serial unequal
crossing over events (after Phillips, 1995). (a) Duplication to yield pre-growth hormone
(
GH
) and pre-chorionic somatomammotropin (
CSH
) genes. (b) Unequal crossing over
between pre-
GH
and pre-
CSH
genes to yield (c) the
GH1
,
GH2,
and
CSH2
genes plus a
CSHP1
locus which was to become inactivated. (d) Subsequently, the
CSH1
gene emerged
by duplication of the
CSHP1
gene.
1997). Both the
GYPB
and
GYPE
genes lack exons 6 and 7 present in the
GYPA
gene that encode the cytoplasmic domain.
The ancestral gene from which the
GYPA
gene was derived is thought to have
been inactivated but its remnants may still be present on chromosome 4, just
downstream of the
GPA
gene (Onda
et al
., 1993, 1994). The ancestor of the human
GYPB
and
GYPE
genes underwent a rearrangement in its 3
region by recombi-
nation between
Alu
sequences and in so doing, acquired 3
sequence from another
gene. This unequal crossing over event served to generate short last exons in both
GYPB
and
GYPE
genes; that of
GYPB
comprises a single amino acid whilst that
of
GYPE
encodes an untranslated region.
At least one
GYPA
-like gene occurs in all hominoid primates whereas one
GYPB
-like gene is present in man, both species of chimpanzee and the gorilla but
not orangutan and gibbon (Rearden
et al
., 1993). The
GYPE
gene is present in all
hominoid primates that possess a
GYPB
gene, but is polymorphically
present/absent in gorillas (Rearden
et al
., 1993). Thus duplication of the
GYPB
/
GYPE
progenitor sequence to yield the
GYPB
and
GYPE
genes must have
occurred prior to the divergence of the human-gorilla-chimpanzee clade ~10 Myrs
ago. Subsequently, the
GYPE
gene has acquired DNA sequence from the
GYPA
gene by gene conversion (Kudo and Fukuda, 1994; Rearden
et al
., 1993).
Homeobox genes.
Homeobox (
HOX
) genes represent a major class of transcription
factors whose origin preceded the radiation of the triploblastic metazoa (Finnerty
