Among the various markers used to study
genetic diversity in individuals and populations, the most
interesting are those DNA sequences that are inherited from
only one parent: mitochondrial DNA (mtDNA), which is inherited
from the mother, and the Y chromosome, which is inherited
from the father. These genetic markers are very useful, because
their DNA sequences are not subject to meiotic recombination. This makes it easier to identify a population’s
paternal and maternal ancestors.
In contrast, each of us inherits two copies of the FOXP2 gene:
one from our mother, and one from our father, located on each
of the two chromosomes in pair 7. Both copies, it appears,
must be intact for our language functions to be normal.
GENES THAT ARE ESSENTIAL FOR SPEECH
The
identification of the FOXP2 gene on human chromosome
7 established the first scientific connection between heredity
and language. Subsequent research provided more information
about the protein produced by this gene. This protein is
what is known as a transcription factor, meaning that it
can bind directly to DNA
molecules to regulate the expression of other genes—including
genes possibly involved in the development of the brain
areas associated with language. In other words, if
you think of all the genes that contribute to language
as forming a tree, then you can think of the FOXP2 gene
as the trunk of that tree.
Average activation recorded by
fMRI during a word-generation task in normal subjects and
in subjects with specific language impairments
It is no surprise, then, that when one of a person’s
two copies of this gene is defective, that person will
suffer from specific
language impairments (SLI). Moreover, brain-imaging
studies have shown that the brains of people who had SLI
did in fact display many abnormalities in areas of key
importance for language. In particular, in these subjects,
both caudate
nuclei and Broca’s area were smaller, and the
left caudate nucleus was hyperactive during tasks involving
oral expression.
This atrophy of the caudate nucleus, a structure involved in motor
control, might have something to do with the great difficulties
that people with the mutant
version of the FOXP2 gene have in moving their lips and tongue.
Some researchers therefore believe that it is the human version
of the FOXP2 gene that enables the fine mouth and facial movements
that make articulate language possible.
Some researchers have even asserted that in the final analysis, FOXP2 may not
be a “language gene”, but rather a gene in which defects can cause
motor disorders serious enough to prevent the use of language. Other explanations
that avoided directly linking FOXP2 with language included the suggestion that
defects in this gene might diminish intellectual faculties to such an extent
that language became impossible. But subsequent experiments have shown that the
links between FOXP2 and language do in fact appear to be very close.
Thus, though SLI may tend to strike those family members who score lowest on
IQ tests, some people with this condition achieve normal IQ scores, while others
may even achieve higher scores than their family members who do not have it.
People with SLI also have trouble in identifying some elementary sounds of language,
as well as in understanding sentences, using grammar, and so on. The observed
atrophy in Broca’s area may also affect their language abilities, because
this area contains mirror
neurons that some authors consider very important for language development,
and because the destruction of this area is known to cause many different kinds
of aphasias.
