Biology Reference
In-Depth Information
diploid paternal embryos, it is the other way around (reviewed by Monk, 1988).
Clearly, maternal and paternal chromosomes must differ epigenetically and in
such a way that different developmental programmes are followed.
Only some 100-200 genes in mammalian genomes are imprinted and these are
often clustered (Barlow, 1995). Human examples include the
INS
,
H19
(D11S878E) and
IGF2
(11p15.5),
SNRPN
(15q11-q12),
WT1
(11p13),
IGF2R
(6q25-q27), and
XIST
(Xq13.2) genes. The
XIST
gene is essential for X-inactiva-
tion (a special case of imprinting) and is expressed exclusively from the inactive X
chromosome (Jamieson
et al
., 1996; Kay 1998). It gives rise to a non-coding mRNA
product that controls the production of an inactivation signal which spreads along
the chromosome, silencing all but a handful of genes (Heard
et al
., 1997).
A common function of imprinted genes is in the control of embryonic growth
with paternally expressed genes (e.g.
IGF2
) tending to enhance growth rates and
maternally expressed genes (e.g.
H19
,
IGF2R
) reducing them. This dichotomy
has led to the proposal of the
genetic conflict hypothesis
(Haig and Trivers 1995)
which attempts to explain the evolution of imprinting in terms of a conflict of
interest between the maternal and paternal genes of an individual. This theory
might predict antagonistic coevolution between maternally and paternally
derived genes for the control of fetal growth. However, contrary to the expecta-
tions of the conflict hypothesis, the rate of evolution of imprinted genes is not sig-
nificantly different from that exhibited by non-imprinted genes encoding
receptors (McVean and Hurst, 1997).
It has been suggested that the imprinted expression of genes is usually con-
served between human and mouse (Barlow, 1995) but whilst the maternal-specific
expression of the
Igf2r
gene is seen in mouse (Barlow
et al
., 1991), imprinting of
the
IGF2R
gene is only apparent in a minority of the human population (Xu
et al
.,
1993; Ogawa
et al
., 1993). Such an 'imprinting polymorphism' has also been noted
for the
WT1
gene in human populations (Nishiwaki
et al
., 1997).
Imprinted genes may have fewer and smaller introns than nonimprinted genes
(Hurst
et al
., 1996; Haig, 1996). Whether this is an adaptation to allow these genes
to be transcribed rapidly or whether it is merely a property of the chromosomal
region in question is however unknown.
It is at present unclear if imprinting is confined to mammals. Parent-of-origin-
specific effects have been claimed in the zebrafish,
Danio rerio
(McGowan and
Martin, 1997). If confirmed, this would be reconcilable with the observed survival
of androgenetic zebrafish only if the expression of the imprinted genes were not
required for development.
1.1.4 Repetitive sequence elements
There is repetition everywhere, and nothing is found only once in the world.
J. G. v. Goethe
Repetitive DNA comprises the bulk (>90%) of the human genome. A large num-
ber of different types of repetitive sequence element are found in the human
genome (reviewed by Jelinek and Schmid, 1982; Vogt, 1990) and their analysis
may go some way toward explaining the patterns of chromosome bands noted
