612
Chirality in Biology
different from the original. Faces on some
statues are actually mirror symmetric and
have been described as having an ‘‘eerie
beauty.’’
The internal arrangement of body or-
gans, however, lacks symmetry. As the
embryo develops, the heart, spleen, and
stomach normally assume the left side of
the chest cavity, while the major lobe of
the liver and the gall bladder locate on the
right side of the abdomen. The left lung
usually has two lobes, the right, three. In
the condition of
situs inversus
,the
rei
sa
transposition of the viscera, the liver being
on the left, the heart on the right. This
condition occurs with a frequency of 1 per
10 000 births. It seems surprising that in-
dividuals with
situs inversus
generally are
right-handed. Of 160 cases of
situs inversus
in which handedness was known, only 11
were left-handed.
A rarer situation is that of individuals
who are much more symmetrical than
normal. In ‘‘right isomerism,’’ both lungs
have three lobes, the heart has two right
atrial appendages, the liver is central, and
the spleen is absent. For a male patient,
the testicles are of the same length (the
right testicle is usually longer). In ‘‘left
isomerism,’’ the lungs are two-lobed, the
heart has two left atrial appendages, the
liver is central, and the individual pos-
sesses multiple spleens. Individuals with
either isomerism have serious problems
with heart function.
Mutant mice showing
situs inversus
are
known. The
iv
mutation (
inversus viscerum
)
gives about 50% of cases of
situs inversus
.
The different
inv
mutation (abbreviation of
‘‘inversion of embryonic turning’’) yields
100% inversion; these mice die early
in life.
Of the primary axes of the embryo (an-
terioposterior, dorsoventral, and left/right
asymmetry), much attention has been fo-
cused on the left/right asymmetry axis in
recent years. Answers are being sought,
primarily at the genetic level, to questions
about what determines the asymmetry and
how does it develop. Several genes or gene
clusters have been identi±ed, but there still
is relatively little information about the
gene products. The subject is too complex
for discussion here.
There are many exceptions to overall
bilateral symmetry; only a few examples
can be given. Lobsters have one small limb
for clutching and a large one for tearing.
In 50% of the population, the small limb
is on the left and in the other 50% it is on
the right. Similarly, in male ±ddler crabs
the large claw or pincer is to the left in
50% of the population and to the right
in the other 50%. In male narwhals, a
large ivory tusk (up to 10 feet long) pierces
the upper lip on the left side of the body
and shows a striated spiral pattern from
right to left. In birds, the New Zealand
wry-billed plover has a beak that is always
bent to the right-hand side of the bird.
Similarly, a breed of European ±nches,
termed
crossbills
, has an upper mandible
pointing to the bird’s right and the lower
to the left. This crossed arrangement may
facilitate removal of a seed from a pinecone
directly with the mouth. North American
crossbills have the opposite asymmetry.
Clearly, the arrangement provides some
advantage, whatever direction it takes.
Chirality is well illustrated in mollusk
shells. In most cases, the helical direction
is right-handed, but some whelks (e.g.
the
Florida
lightning
whelk)
are
left-
handed. Rare examples of ‘‘sinistral,’’ left-
handed shells, for normally right-handed
types, are prized by collectors. In India,
the left-handed form of the chank shell
(
Turbinella pyrum
) is considered sacred
by Hindus. Very ancient statues portray
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