- People who grow up left-handed have a different, more
flexible brain structure than those born to take life by the right hand,
say researchers at the University of California, Los Angeles, who used
twins to study heredity.
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- The reason is that right-handers have genes that force
their brains into a slightly more one- sided structure, according to research
published yesterday in the Proceedings of the National Academy of Sciences.
Left-handers appear to be missing those genes.
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- "There really is a difference in brains that results
in a more symmetric brain in left-handers, where the two sides are more
equal," said UCLA neurogeneticist Daniel Geschwind, who led the research
team. "There is more flexibility, and that is under genetic control."
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- In the effort to understand how the brain shapes the
mind, researchers have been striving to document the way genes and environment
affect intelligence and mental abilities. The human insistence on preferring
one hand over the other poses a particularly nagging question that touches
on both anatomy and behavior.
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- "There is clearly something fundamental here we
need to comprehend if we are to understand what makes us uniquely human,"
Geschwind said.
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- Of all the primates, only human beings display such a
strong predisposition to right-handedness. Right-handers make up about
90 percent of the population. The left and right halves of the brain are
different in both their anatomy and their functions, related in part to
hand preference.
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- But until now, no one could document the connection.
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- The UCLA study is the strongest evidence yet that heredity
shapes the brains of left-handed and right-handed people differently, Dartmouth
neuroscientist Michael Gazzaniga said.
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- The UCLA researchers conducted brain scans on 72 pairs
of male identical twins between 75 and 85 years old.
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- Identical twins, who share the same genes, offer a unique
lens through which to study the relative effects of heredity on human nature.
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- Right- and left-handedness is partially determined by
genetics. If a person inherits the gene for right-handedness, that person
will be right-handed. People who do not have that gene, however, can be
either left- or right-handed. There is no specific gene for left-handedness.
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- Right-handers typically have a larger left brain hemisphere,
where their language abilities are concentrated.
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- Conversely, left-handers have more balanced brains, with
both sides relatively symmetrical. The language abilities of left-handers
more often are concentrated on the right side.
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- If identical twins carry the gene for hand preference,
both must be right-handed. If they lack the gene, one twin can develop
right-handed while the other develops left- handed.
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- The researchers found that the brains of identical right-handed
twins were very similar in size and structure. But when a left-hander was
part of the twin set, the brains were different. The conclusion, researchers
said, is that the absence of the gene for hand preference allows the brain
to develop differently as the individual grows up.
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- A similar pattern did not appear in 67 sets of fraternal
twins used as a control group.
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- http://seattletimes.nwsource.com/html/healthscience/134415110_lefty05.html
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