Osteogenesis Imperfecta.
Last updated Thursday, December 23, 2004
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Causes and effects
Causes
This brief section on heredity may answer some of your questions about what causes osteogenesis imperfecta.
We encourage you to discuss your concerns with a genetic counselor or with a knowledgeable physician. Contact the March of Dimes for a referral to a genetic counseling center in your area.
Remember, parents have no control over which genes they pass on to
their children and should not feel at fault if one or more of their
children has OI.
To understand OI, it's important to know how genes influence a
person's biological makeup. Genes are chemical units of hereditary
material that direct the development and function of every cell in the
body. Genes determine which features or traits children will inherit
from their parents. Genes affect growth, development, and appearance.
They also influence one's tendency to get sick or resist disease.
Chromosomes are threadlike parts of cells that carry the genes.
Every cell in our body contains 23 pairs of chromosomes, with the
exception of reproductive cells (sperm and egg). Reproductive cells
each contain 23 unpaired chromosomes. At conception, each parent
provides a reproductive cell with one set of chromosomes. The two
reproductive cells combine to form a single cell with a full set of
chromosomes and corresponding genes.
Most of the body's 50,000 or so genes function normally. Sometimes,
however, a gene becomes changed from its original form. This is called
a "mutation." If the mutation causes a collagen gene to function
improperly, problems with collagen production and structure occur.
This, in turn, can cause problems with bone growth or structure and
result in OI.
OI usually is inherited in a family in a pattern called autosomal
dominant inheritance. Another way OI can be inherited is by autosomal
recessive inheritance, which is very rare. OI also can occur for the
first time in a child of unaffected parents as a result of a new or
"spontaneous" autosomal dominant mutation. (Note that in all types of
0I there is an equal likelihood that boys and girls will be affected.)
Autosomal dominant inheritance is a disorder that affects family
members in every generation. This is the most common inheritance
pattern. The dominant altered gene overrides the effects of the normal
gene. Even when the altered gene is paired with a normal one, the child
will have OI. With autosomal dominant inheritance, there is a
50-percent chance with each pregnancy that an affected parent will have
a child with OI. However, both parents may contribute the normal gene,
resulting in a 50-percent chance that their child will not inherit OI.
If both parents have an autosomal dominant form of OI, there is a
75-percent chance they will has a child with OI.
Autosomal recessive inheritance is a pattern in which the disorder
is inherited only if both genes of the pair are altered C genes. A
child can be affected only if he or she inherits an altered OI gene
from each parent. Parents who have one recessive gene are "carriers" (a
person who carries the altered OI gene but does not have symptoms of
OI). With each pregnancy there is a 25-percent chance that the child
will inherit the disorder, a 50-percent chance that the child will be
an unaffected carrier like the parents, and a 25-percent chance that
the child will inherit the normal gene from both parents and be
unaffected. This form of OI is very rare.
New or spontaneous autosomal dominant mutations make it possible for
a child to have OI even though neither parent has an altered collagen
gene. The affected person then has the same chance to have an affected
child as in autosomal dominant inheritance.
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