Chimaera child

-- THE chimaera - in Greek mythology -
is a beast with the head of a lion, the
body of a goat and the tail of a snake.
Biologists use the word to refer to something no less exotic: an organism whose
tissues have two or more genetically different origins.

Theoretically, chimaeras could arise
when the egg nucleus - which has half
the normal complement of genes of the
adult (n instead of 2n) - divides once
before fertilisation. Of the two daughter
nuclei, one might get fertilised by a
sperm (resulting in a 2n nucleus, with
an equal genetic representation from
both parents) and the other might
divide once again (resulting in a 2n
nucleus where all genes come from the
mother).

This is what may have happened in
the case of a three and a half-year old
boy (identified only as ED), as reported
in Nature Genetics (Vol 11, 1995) by L
Strain and her colleagues from the
University of Edinburgh, UK. A
blood test of the boy had revealed
that he had two x chromosomes: the
blood was characteristic of a female, not
a male (who has one x and one Y
chromosome).

Research showed that while the skin
cells Of FD were male and carried genes
from both parents as expected, the
white blood cells or leukocytes possessed only maternal genes. Also, the
maternal genes were present in two
identical copies even when the mother
herself had two different copies. This
suggests that a single (n) set of the
maternal genome must have duplicated
prior to the differentiation of white
blood cells.

The Findings, although limited
owing to the lack of information on the
genetic make-up of all of ED'S tissues,
excite interest on more than one
ground. Firstly, the child is the first
human chimaera reported. Secondly, he
is normal, except for a left-right facial
asymmetry and a small testes; he is also reported to have exhibited aggressive
behaviour in early childhood. ED, who is
left-handed, raises the prospect that
parthenogenesis (being born from a single parent - most often only the mother) in mammals may not be as unlikely
as was thought, The reason embryologists consider parthenogenesis impossible in mammals is because of the phenomenon of genetic imprinting -
which means that the same gene in your
body can behave differently depending
on whether you got it from your father
or your mother.

As a result of imprinting, we need
one copy each of a paternal and
maternal gene. if we possessed two
identical genes that are also similarly
imprinted, as would happen if we had a
father but no mother (or vice versa),
development would be haphazard;
abundant evidence from mice testifies
to this. But results of the tests on ED
show that in terms of his blood, his
genes lack a father. Could it be that
nature allows mammals to get away
with a little bit of parthenogenesis?