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| A chimeric mouse with pups |
Identical twins arise when a zygote (a fertilized egg) begins
to divide. About five days after fertilization, it reaches the blastula stage and is implanted in the uterus,
but for unclear reasons it can be broken into two separate parts, which will
result in two independent embryos that may or not share the same placenta,
although they usually have a different amniotic sac. The two siblings who are
born share the same genetic endowment (the same DNA), except for possible
post-separation mutations.
In contrast, two non-identical twins arise
when two distinct eggs are fertilized, each by one spermatozoid, forming two
different blastulas, each of which is implanted in the uterus through a
placenta of its own. The two brothers will have different genetic endowments,
similar to those of two non-twin brothers, because they come from different
gametes.
But there is a third possibility: a chimera arises when two blastulas that would
normally give rise to two non-identical twins merge before being implanted in
the uterus and give rise to a single embryo and, consequently, to a single
individual possessing, in different cells, two different genetic endowments.
Thus, it may happen that a chimeric individual has (for example) the liver with
a genetic endowment and the kidneys with another. Typically, chimeras are
difficult to detect, unless (for example) just one of the blastulas would have given
rise to an albino, in which case the resulting chimeric individual may have
unequally pigmented skin. Even in this case, the cause could be different. It
could also happen (although it is very rare) that one of the two zygotes is
male (with X and Y chromosomes) and the other female (with XX chromosomes), in
which case part of the cells of the chimera would be male and another part
female.
