Thesis: With
the increase in the cases of chimeraism, there
is also an increase in criminal cases as the suspect
is able to more around freely without being convicted
owing to his/her blood sample not matching with
the DNA samples found at the venue of the crime.
Chimera according to the genome glossary is
an organism which possesses cells or tissues with
a different genotype. This can be caused due to
mutated cells of the host organism or cells from
a different organism or species.
According to Greek mythology, the original "Chimera"
(pronounced as kih-MEE-ra, and the adjective form
"chimerical" pronounced as kih-MER-ih-cal)
was a fire-breathing female monster with the head
of a lion, the body of a goat and the hindquarters
of a dragon. The terrifying Chimera may have merely
been a product of a substandard family environment
-- her father was the giant Typhon, her mother
the half-serpent Echidna. Her siblings were the
Cerebrus (the three-headed hound who eventually
found work guarding the gates of Hell), Hydra
(a nine-headed aquatic monster) and Orthrus (the
runt of the litter, a prosaic two-headed dog).
As per the mythology, Chimera pounced around making
everyone wretched until one day a man named Bellerophon,
riding on the winged horse Pegasus slays her.
[Author unknown, Chimera, 2004]
Analysis: Though believed to
be a mythological epic, it was found that cases
of Chimera were actually prevalent in humans when
certain intriguing criminal cases erupted. One
such case was when a woman has been raped by a
man and the women claimed a particular person
to be the culprit. However, the crime justice
authorities had found it difficult to convict
the suspect. It so happened that the DNA taken
from his blood sample did not match the DNA of
the suspect. However, extensive investigation
and the victim’s statements revealed it
to be same person being responsible for the crime.
When another series of DNA tests were taken from
the suspect, this time from his hair sample it
was found that the DNA sample perfectly matched.
This revealed a breathtaking phenomenon among
human genome which has resulted in various problems
in the criminal justice system.
Those people who have two types of DNA in their
body are called chimeras after the mythological
creature with a head of lion, body of a goat and
a tail of a serpent. These people are sometimes
also referred to as mosaics. It has found that
these people have two different types of DNA in
various parts of their body. Though a rare occurrence,
this could have been due to various reasons. [Vladar,
2004]
One of the common reasons is that chimeras are
formed if developing fraternal twin embryos join
together to become one embryo. This is something
which is completely opposite of identical twins
who are formed when a single embryo splits into
two. This occurrence takes place in the very early
stages of the embryo development i.e. when it
is still in the form of unspecialized cells, so
when it develops the baby that is born is healthy
but with two types of DNAs. It is also found that
fraternal twins don’t have same set of DNA,
however, they the two fraternal twin embryos fuse
together it results in chimera.
Another reason for chimeras to form is when developing
fraternal twin share the same blood supply. This
usually happens when the twins with different
DNA share a placenta in the mother’s womb
and cells from their blood mix together. This
results in the twins being chimeras based on only
their blood as other cells are not affected by
the blood supply. Chimeras are also formed through
some mistakes or mutations taking place in the
division of the cells in the development embryo.
This mainly takes place when the cells split into
two so that they are able to be formed into more
of themselves for the growth of the embryo. It
also results in the the doubling up of the DNA
by the cells which gets divided between the two
new halves. During this process, if something
goes wrong, then some new cells result in having
different DNA. If this happens in the early stages
of the development, then the tissues which are
developed from the cells will have different genotype.
It is difficult to gauge the exact number of
chimeras in the world. We are able to find out
whether a person is a chimera or not only when
his / her DNA is analysed. This causes varied
problems with regards to solving of crime cases
and also results in complication with regards
to donation of organs. In recent years, exciting
results have been found out that a section of
genetically incompatible cells may contribute
to conditions like infertility, autism and Alzheimer's
disease. [Migliore, L et al, 1999]
Chimeras, however, should not be confused with
hybrids which are organisms created from two gametes
of different species resulting in a single zygote.
All the future cells in the hybrid are formed
from this single zygote like in the case of a
mule which is a hybrid of a sperm from a donkey
and the egg of a horse.
With the increase in the cases of chimeras and
mosaics owing to various reasons, it has been
found out that there might be an increase in the
complications which might occur while administering
drugs. This becomes difficult due to people’s
individual genetic constitutions. Two genetically
different tissues in one body might produce an
unpredictable response to a drug, speculates Roland
Wolf, who studies pharmacogenetics at the University
of Dundee, UK. "It's completely unknown.
[Author unknown, Genetic Chimeras, human genetics,
2004]
Human chimeraism came into being with advent
of blood typing and it was found that some people
had more than two types of blood group. Most of
the cases of chimeraism are the blood chimeraism
i.e. non-identical twins sharing a blood supply
in the womb. Twin embryos often share a blood
supply in the placenta, allowing blood stem cells
to pass from one embryo and settle in the bone
marrow of the other, seeding a lasting source
of blood. As a result, almost 8% of non-identical
twin pairs have chimeric blood. [van Dijk, B A;
Boomsma, D I and de Man, A J M, 1996]
Apart from those who have chimera due to different
blood types, there are even larger numbers of
people who have this phenomenon due to carrying
smaller numbers of different blood cells that
may have passed through the mother and the fetus
across the placenta or persist from a blood transfusion.
It has also been analysed that the presence of
some foreign white blood cells might be the cause
for autoimmune diseases wherein the immune system
affects the body’s own tissues. [Nelson,
J L, 2002]
Whereas the rarest cases of true chimeraism is
when many tissues of the body are affected and
are caused when non-identical twin embryos join
shortly after fertilization. According to clinical
geneticist David Bonthron, who led the team at
the University of Edinburgh, UK, that reported
on the British hermaphrodite boy, "If you've
got two embryos there's the chance of two becoming
one.” This phenomenon can result in series
of other actions like in 1995, Bonthron described
another boy who was partly parthenogenetic which
meant the cells from his blood and some other
tissues contained none of his father's chromosomes.
However, he had a duplicated set of one half of
his mother's chromosomes. Although it is not unknown
for an egg to start developing without being fertilized,
fully parthenogenetic human embryos cannot develop
to term. Bonthron, who is currently working at
the University of Leeds, UK, thinks that the partially
parthenogenetic boy got his odd genetic constitution
due to an egg that suddenly divided into two cells,
one of which was fertilized. The second cell then
copied its maternal chromosomes, allowing the
resulting chimera to become a viable embryo. [Strain,
L; Warner, J P; Johnston, T and Bonthron, D T,
1995]
True chimeras are usually identified when they
possess male and female cells resulting in the
formation of hermaphrodites or any related problem
in the person’s sexual organs and their
chromosomal sex is identified by their blood test.
So the real number of chimeras and mosaics are
much more than those that have been identified
till date. Unless and until a chimera comes forward
to get his blood sampled or has been convicted
for a crime, it wouldn’t be possible to
know whether he actually is a chimera. This has
resulted in further complication in solving some
crime justice cases.
It has been also analyzed that with the increase
in in-vitro fertilization (IVF), there has an
increase in the cases of chimeras. To avoid infertility
among women, IVF has been quite common in the
present day scenario. In this process, two or
more embryos are placed in a woman’s uterus
so there is an increased success rate. This results
in 25% more cases of twin pregnancies than expected.
With the increased number of twins, there is likelihood
of more cases of chimeras. This was proved by
Bonthron who found out that the British hermaphrodite
boy who was a chimera was formed by IVF. Strain,
L; Dean, J C S; Hamilton, M P R and Bonthron,
D T – 1998]
According to various tests undertaken along with
ultrasound examinations performed on the mother
who was carrying the single fetus, the baby that
was born had a normal right testis and a semi-developed
left testis. An operation was conducted when the
child was 15 months old and the surgeons removed
"an abnormal gonad and vas deferens."
Pathology studies later exposed that these were
actually an ovary with a fallopian tube attached.
Studies revealed that the child's chromosomes
had both female (XX) and male (XY) cell lines.
This was explained to be that the child was an
exceptional case of a true chimera which was a
mixture of two embryos from separately fertilized
eggs. The other likelihood was that two different
sperm had unusually developed into a single mature
egg before cell division had taken place. To distinguish
between these possibilities, the researchers analyzed
DNA near the centromeres of the child's chromosomes,
which are involved in cell division. [Author unknown,
DNA puzzle of mother 'made of two women', 2004]
Apart from this, the study conducted also revealed
that chimerism can occur naturally, resulting
in children with vague genitalia, however published
data on the phenomenon is very restricted and
many XX/XY children may go unnoticed. Even so,
the fact that chimerism occurred in a child born
following in vitro fertilization should be taken
seriously. [Author unknown, 2004 In Vitro Fertilization
Results In Chimera, 2004]
Conclusion: Considering these above points, it
is becoming more and more difficult for the crime
justice system to analyze the crime due to the
increase in the cases of chimeraism and mosaicism
which in turn is increasing with the increase
in in-vitro fertilization. The criminals are able
to move about freely as their blood samples usually
differ from their other DNA samples taken from
different parts of the body. It is also becoming
difficult for the crime justice system as well
as the medical system to analyze the exact number
of chimeras as it is very difficult with the increase
in mutations.
References :
Author unknown, Chimera, website visited on 3rd
December, 2004 http://www.word-detective.com/091801.html
Vladar Eszter. Ask a Geneticist, website visited
on 3rd December, 2004 http://www.thetech.org/genetics/ask.php?id=23
Migliore, L et al - "Preferential Occurrence
of Chromosome 21 Malsegregation in Peripheral
Blood Lymphocytes of Alzheimer Disease Patients",
Cytogenetics and Cell Genetics, 1999, 87, 41 -
46,
Author unknown, Genetic Chimeras, human genetics,
website visited on 3rd December, 2004, http://www.nature.com/nsu/020429/020429-13.html
van Dijk, B A; Boomsma, D I and de Man, A J M
- "Blood Group Chimærism in Human Multiple
Births Is Not Rare", American Journal of
Medical Genetics, 1996: 61, 264 - 268.
Nelson, J L - "Microchimærism: Incidental
Byproduct of Pregnancy Or Active Participant in
Human Health?", Trends in Molecular Medicine,
2002: 8, 109 – 113.
Strain, L; Warner, J P; Johnston, T and Bonthron,
D T in Nature Genetics, 1995:11, 164 – 169.
Strain, L; Dean, J C S; Hamilton, M P R and Bonthron,
D T - "A True Hermaphrodite Chimera Resulting
from Embryo Amalgamation after in vitro Fertilisation",
New England Journal of Medicine, 1998: 338, 166
– 169.
Author unknown, DNA puzzle of mother 'made of
two women' website visited on 3rd December, 2004,
Author unknown, 2004 In Vitro Fertilization Results
In Chimera, SOURCE: The New England Journal of
Medicine (1998;338:166-169, 194-195), website
visited on 3rd December, 2004
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