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Thousands of genes are found on
thread-like structures, called chromosomes,
which are located in the center or "nucleus" of
each cell. Genes and chromosomes exist in pairs. At the
time of conception, each parent contributes one gene and
one chromosome from each of his or her gene and
chromosome pairs. The genes and chromosomes from each
parent combine into new pairs which determine the traits
a child inherits.
| The human genome comes in 23 pieces, packaged
in proteins to form chromosomes. Our cells have
two copies of each chromosome--one from each
parent--making 46 in all. The 23rd pair consists
of the sex chromosomes. |
Most often, genes produce
normal traits. Sometimes, however, a gene becomes altered
from its original form. A gene that is thus changed is
called a "mutant" gene.
Although researchers know that mutations occur, the
reasons for their occurrence is not completely
understood. Some mutations are not harmful, but most
often a mutant gene functions improperly and causes a
disorder, malfunction, or malformation within the body.
Mutations can occur in any generation. If a spontaneous
mutation occurs, it is then possible for a child
to be the first family member to get a genetic disorder
such as Marfan syndrome, even though both parents are
normal.
| A mutation is a misspelling in the code for a
protein that can be passed from one generation to
the next--an A instead of a G, say. The result
can be a disease, such as cystic fibrosis. |
There are
present multiple variations of a gene (called alleles)
at the same genetic location.. For most locations, each
individual possesses two alleles, one derived from each
parent. If the two alleles are identical, the individual
is said to be homozygous; if they are different, the
individual is heterozygous.
In addition to mutations, the
"inheritance pattern" or the way in which
genetic traits or disorders are expressed also varies.
The Marfan syndrome follows a pattern of inheritance
called "autosomal
dominant inheritance."
"Autosomal" means the inheritance is linked to
any chromosome other than those which determine the sex
of the child. "Dominant" means the effects of
the Marfan gene dominate or override the effects of the
normal gene in the pair. Therefore, if one parent
contributes the Marfan gene and one parent contributes a
normal gene, the child will inherit the Marfan disorder.
There is a 50% chance during each pregnancy that the
affected parent will pass on the Marfan gene and the
child will get the Marfan syndrome. There is, of course,
also a 50% chance that the affected parent will pass on
his or her normal gene, in which case the child will not
get the disorder.
It is important to remember that
parents have no control over which genes they pass on to
their children, and, therefore, should not feel at fault
if one of their children inherits a disorder such as the
Marfan syndrome.
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