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Nearly all of the animal cloning experiments to
date have suffered high rates of fetal and
neonatal mortality in the resulting offspring. All
sorts of things go wrong, according to George
Seidel, a cloning researcher at Colorado State
University. For example cloned cattle and sheep
are often born dangerously large. "Normally
you might expect a 100-pound birth-weight in a
calf, but with a clone, you might get 160
pounds," said Seidel (i.e. between one and a
half to twice the size and weight). Genes which
control the "switching on" and
"switching off" of growth may not work
correctly in clones.
Oversized fetuses don’t have room to wriggle
around in the uterus and can be born with limb
deformities. They may have to be delivered by
caesarian section or may die in utero. Seidel
points out that ‘Sometimes the kidneys aren’t
right, they’re just plain put together wrong -
or the heart is, or the lungs, or the immune
system.’ Currently, nobody really knows why, but
the abnormality rate is around 30 per cent in
cloned animals. The normal rate of congenital
defects (i.e. non-cloned offspring) is probably
less than 5 per cent, which is possibly why most
species use sexual reproduction - it has a form of
error correction mechanism.
Among the defects found in cloned animals are
developmental abnormalities where a vital organ is
only partially formed or is formed largely out of
undifferentiated cells. The organ cannot function
once the offspring is born; it does not contain
the appropriate cells. A normal fertilized egg
contains instructions to tell tissues when to make
specialized cells and also when stop developing.
In cloned animals this can go wrong - blood
vessels may be four or five times too large and
the heart cannot cope with pumping blood around
these enormous vessels; the liver or lungs might
be little more than tumors of non-specialized
cells which cannot act as liver cells.
It appears
that genes which were active in a fertilized egg
and then switched off later in life are not
"switched on again" if a clone is
attempted using a cell from an adult animal. This
may be why there has been greater success with
creating clones from embryos - the crucial
"switching on" and "switching
off" genes are still active in embryos.
In 2002, American
scientists identified a single gene which could
explain why most attempts to clone mammals end in
failure. It is believed that cloning failures
happen because genes from a tissue cell are not
reprogrammed into being able to produce a new
individual. For cloning to succeed, tissue cell
nuclei (which would simply produce more tissue
cells) must be reprogrammed in to behaving as
embryo cell nuclei (able to produce a whole
individual). It appears that gene-reprogramming
often fails to occur adequately during cloning
attempts. As a result, only about 1% of
manipulated eggs lead to a live animal. Of those
clones which are born live, many have
abnormalities.
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