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The drawback is that an offspring whose cells
contain only half the normal complement of
chromosomes would probably be sterile because it
would be unable to form egg cells. This is because
it only has one set of chromosomes, not the normal
two sets - the chromosome set cannot be split any
further. Either the unfertilized egg must be
induced to make a second set of chromosomes (which
would be identical to the original set) or the egg
must be fused with a second egg cell. If the
chromosomes spontaneously doubled in the
unfertilized egg, the individual would be
homozygous (true-breeding) for every gene and
trait. If it went on to produce offspring by the
same method, those offspring would be identical to
it. Barring any spontaneous mutation, they would
be true clones. This is parthenogenesis (virgin
birth) and is seen in aphids.
Fusing 2 eggs from the same donor would provide
2 sets of chromosomes, with a little genetic
variation since each egg gets a slightly different
mix of genes when it is formed. The offspring
would be female, but not identical to the mother.
This sounds straightforward but there is a
problem. In humans, it appears that a few genes
need to be inherited from a male parent even
though they are not on the Y chromosome. If those
genes are inherited from the female parent (the
"wrong" parent), abnormalities
apparently occur. So far, no-one knows whether
this is true for cats as well as for humans, or
whether it is true for embryos created by
egg-fusion where both parents are female.
Cloning
and the Cat Fancy
Can the cat fancy keep up with reproductive
technology and what impact do these techniques
have on pedigree cats? Cloning and artificial
insemination carry plenty of implications for the
cat fancy. For example, a show-stopping neuter
could be cloned and the unneutered clone used for
breeding. Or an excellent stud cat might be cloned
(or sperm might be banked) and its clones or
stored sperm might be used in several breeding
lines, passing its genes on to far more offspring
than that single stud cat could have fathered in
its lifetime. A cat rendered infertile by an
accident could be cloned and the clone (or banked
sperm) could pass the genes on.
Cells or banked sperm from a deceased cat could
be used many generations after the cat has died.
Genes which have accidentally been bred out could
be re-introduced. Breeds which produce small
litters could be propagated more quickly using
clones, IVF and surrogate mothers. Suddenly there
is no apparent limit to inter-generational
matings, back-crossing and litter sizes (where
litters are spread across several surrogate
mothers)!
Because the DNA of the clone is identical to
the DNA of the donor but in a younger body, this
can lead to more inter-generational matings - both
planned and accidental (if record-keeping is
poor). A female who has offspring fathered by a
clone could, in essence, be mating with someone
from a previous generation. If clone embryos are
frozen before re-implantation, she could be mating
with a cat which died years or decades previously.
Back-crossing is already used in selective
breeding of animals (see The Pros
and Cons of Inbreeding), cloning increases the
number of generations a breeder can back-cross to.
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