Chirality in Biology
613
the god, Vishnu, as holding a left-handed
chank shell
In tobacco plants, leaves alternate along
the stem in either a left-handed or a right-
handed spiral. In large populations, the
left : right ratio is 1 : 1. Vining plants coil
upward in a left- or right-handed spiral
characteristic of the species; honeysuckle
is right-handed and bindweed is left-
handed.
Even microorganisms can show chi-
ral growth patterns. In
B. subtilis
,o
n
e
structural possibility consists of Flaments
of many individual cells with a double-
stranded helical arrangement. The helix
chirality and degree of twist are influenced
by growth conditions. There is a range
from ‘‘maximum tight left-handedness’’
to ‘‘maximum tight right-handedness.’’
Some mutants have only one helical chi-
ral sense. In a truly bizarre case, one
fungal species of the genus
Laboulbenia
parasitizes beetles. It occurs only on a sin-
gle joint of the left hind leg; no other part,
including the right leg, is involved.
7.2
Behavioral Asymmetries
That the two parts of the brain have differ-
ent functions became apparent from stud-
ies of brain-damaged individuals. Early
work was carried out in ±rance by Marc
Dax,
with
more
extensive
studies
by
Pierre Paul Broca (1824–1880), a surgeon.
Broca determined that language facility
and action are usually associated with
the left hemisphere of the brain – in his
words (1865), ‘‘On parle avec l’h´emisph`ere
gauche.’’ In the condition of Broca’s apha-
sia (or motor aphasia), the power of
expression by writing, speaking, or signs
is lost. In 1995, another form of mental
performance, perfect pitch, was attributed
to the left brain. Still more recently, left-
hemisphere cerebral specialization was
reported for the babbling of babies, ei-
ther those acquiring English or those
acquiring ±rench. The right hemisphere
is involved with processes of perception.
Hence, right-brain damage involves not
only sight but also other senses such as
touch, sound, and taste. Much has been
written on the left-brain/right-brain sit-
uation and specialized texts should be
consulted for further details. The situa-
tion is complex and there are exceptions to
the generalizations just noted.
Perhaps
even
more
complex
is
the
subject of handedness; again, much has
been, and continues to be, written on
this subject. In the Western nations, a
little over 10% of the population is left-
handed (deFned as those using the left
hand for at least half of their tasks).
Left-handedness is more common among
women than men, and individuals can be
inconsistent in their hand usage. Similar
preferences exist for the use of feet, eyes,
and ears. There has been much speculation
concerning the origin of handedness. A
genetic component is probably involved
and left-handedness in humans (but not in
animals) runs in families. Studies of twins
have posed some problems for genetic
hypotheses. ±or instance, with identical
twins, about one in Fve pairs shows
nonidentical handedness, one twin being
right-handed, the other, left-handed.
There are two genetic models, one pro-
posed by Annett, the other by McManus.
In brief, Annett postulates a gene,
RS
+
,
that displaces the chance distribution to
the right (the right-shift model), whereas
McManus postulates two alleles,
D(Dex
-
tral)
and
C(Chance)
, at a single autosomal
locus. The
DD
genotype produces exclu-
sively right-handed individuals, while the
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