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Bardet-Biedl
Syndrome is a complex human trait caused by altered genes.
Six genes for BBS have been mapped to various human
chromosomes, and at least a quarter of families cannot be assigned to
none of them, suggesting either that there is at least one more site
of a BBS gene not yet discovered, or that there is another much more
complex mechanism for the inheritance of BBS.
The gene responsible for Laurence-Moon Syndrome is unknown at
this time.
We know from research in families with each of these syndromes
is inherited in an autosomal recessive manner.
To explain this, we first have to explain something about our
cells.
Every
human being consists of living “building blocks”: cells. Cells are so small that they are only visible under a
microscope. There are
different kinds of cells, serving different functions.
For example, millions of heart muscle cells make sure that the
heart can do its intended work. Nerve
cells guide the movement of our limbs and make sure that we can feel.
Bone cells produce bone; skin cells form our skin, etc.
Almost every living cell (except red blood cells) has a
nucleus. This nucleus
contains long threads of a substance called DNA.
Part of this DNA contains the genetic information that keeps
each cell functioning. These
pieces of DNA contains individual units of instructions that we call
“genes”. Genes are
grouped in “packets” called chromosomes.
Chromosomes are recognizable through their length and the band
pattern they produce after treatment according to certain color
staining techniques. Each
chromosome was given its own number.
Each nucleus normally contains 46 chromosomes.
These 46 chromosomes form 23 pairs: one pair of sex-determining
chromosomes and 22 pairs (numbered 1-22) so-called autosomes that
contain general body characteristics and instructions.
Parents
pass their genetic identity to their children through their
chromosomes. Each of a
father’s sperm contains half of his genetic identity (23
chromosomes, one of each pair) and each of the mother’s ova (eggs)
contains half of her genetic information (23 chromsomes).
When a sperm and an egg merge, the resultant conception has a
full complement of genetic information and 46 chromosomes again, two
representatives of each of the 23 pairs.
The new combination of the two sets determines the genetic
make-up of the child.
Our
body is so complex that occasionally things can go wrong with the
forming of a new human life. Many
of these “errors” have no noticeable consequences.
Every human being has a few of them.
However, some of these errors can have far-reaching effects.
For example, one gene of a pair contains an error (also called
a mutation) (gene a) while the other gene of this pair is normal (gene
A). Usually this does not
present a problem. Apparently
the normal copy of the gene (A) is stronger than the one with the
error (a). Someone with
this kind of genetic alteration, which does not affect him/her at all,
is called a carrier of an autosomal recessive gene.
Only
when both copies of a pair of genes contain an error (combination aa),
and there is no normal copy, does the combination yield a recognizable
disorder. To achieve this
combination aa, both the
mother and the father must be carriers and thus pass on the two
altered copies of the gene so that the child has no normal copy and is
affected.
Both
Bardet-Biedl and Laurence-Moon Syndromes follow this pattern of
autosomal recessive inheritance.
Each parent of a child with either of these disorders is
therefore, by definition, a carrier of the trait leading to this
syndrome. With each
subsequent pregnancy they have a 25% (one in 4) chance of having
another child with the same condition.
At
the moment it is not possible to identify the carriers of these
syndromes. This is
especially important for siblings of people with Bardet-Biedl or
Laurence-Moon syndrome. A sibling without noticeable signs of the syndrome must
necessarily not have the combination aa (because then he/she would
have the syndrome); this sibling therefore has two chances out of
three to be a carrier (Aa), or has one chance out of three to be
genetically normal-normal (AA) and thus not a carrier.
Assuming that that unaffected sibling matures and chooses a
partner who is not a sibling of a person with the same syndrome, the
partner’s chances to be a carrier are estimated to be small
(possibly 1 in 200 or 250). After
all, each of these is a rare disorder.
Therefore the chance that the sibling would have children with
this syndrome is approximately 2/3 x ˝ x 1/200 x ˝, or about one
chance in 1200.
Reliable
prenatal diagnosis is not yet possible.
Neither syndrome is detectable (as yet) through amniocentesis
or chorionic villous sampling (since only three genes for BBS have
been found as of this writing and commercial testing is not
available).
This will change once all the genes for the Bardet-Biedl
Syndromes or Laurence-Moon Syndromes are found.
Research is being done to find the genetic causes of these
disorders.
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