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Tetragametic chimerism is a condition which
occurs through the fertilization of two ova by two sperm,
followed by the fusion of the zygotes and the development of an
organism with intermingled cell lines. This phenomenon happens
at a very early stage of development, often during the
blastocyst phase. A tetragametic chimera is named such because
it is formed from four gametes: two eggs and two sperm. For
instance, a chimera can be formed from the merger of two
nonidentical twins in a very early (zygote or blastocyst) phase.
As such, they can be male, female, or hermaphroditic. |
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Chimera
Organism
As the chimera organism develops, it may come to
possess organs that have different sets of
chromosomes. For instance, a chimera may have a
heart composed of cells with one set of
chromosomes and have a brain composed of cells
with a second set of chromosomes. This type of
chimerism has occurred in humans and, at one time,
was thought to be extremely rare. However, newer
evidence seems to indicate that it may not be so
rare as once considered. Since phenotypic traits
of chimerism are often quite subtle, the majority
of chimeras may go through life not realizing
their condition. |
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Chimerism
Chimerism can be diagnosed by the finding of two
populations of red cells or, if the zygotes are of
opposite sex, ambiguous genitalia and
hermaphroditism alone or in combination.
Sometimes, affected persons may also have patchy
skin, hair, or eye pigmentation (heterochromia).
If the blastocysts are of the same sex, the
condition can only be detected through DNA
testing. If the blastocysts are of the opposite
sex, genitals of both sexes may be formed, thereby
exhibiting hermaphroditism. In 2003, New Scientist
reported that there were approximately 30-40
documented human cases in the literature. |
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Chimeras Detection
Naturally occurring chimeras are rarely identified
unless their offspring have abnormalities such as
male/female or hermaphrodite characteristics or
skin discolouring (visible Blaschko's lines).
However, chimerism can be detected in DNA testing.
A landmark chimera case, the Lydia Fairchild case
of 2002, was brought to court after DNA testing
showed that her children could not be hers, since
their DNA did not match. She was charged with
fraud, but the charges against her were dismissed
when it became clear that she was a chimera, with
the matching DNA being found in her cervical
tissue. |
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