Achondroplasia (cont.)
How is achondroplasia inherited?
Achondroplasia is inherited as an autosomal dominant trait
whereby only
a single copy of the abnormal gene is required to cause
achondroplasia.
The gene for achondroplasia is fully penetrant, meaning that
everyone who
possesses it has achondroplasia. No one with the gene escapes
achondroplasia. However, there is some variation in expression
of the
gene, meaning that children with achondroplasia are not carbon
copies of
each other, although they may look alike to the untutored eye.
In only about an eighth of cases is the gene inherited from a
parent who
has achondroplasia. Conversely, about seven-eighths of cases
are due to a
new mutation (a new change in the gene). This means that most
cases of
achondroplasia occur sporadically (out of the blue) and are the
result of
a new mutation in a sperm or ovum of one of the normal-
appearing parents.
The chance of a new mutation rises with the age of the father.
As early as
1912 it was noted that sporadic (new) cases were more often
last-born than
first-born children. This fits with the fact that the chance of
an
achondroplastic birth has been shown to increase with paternal
age (age of
the father).
What if someone with achondroplasia has children?
Although most children with achondroplasia do not have an
achondroplastic parent but have a new mutant gene for
achondroplasia, they
can still transmit the gene to their children. No matter
whether a person
with achondroplasia carries a new or an "old" gene,
the risk for
passing that gene down to a child is 50%. It is 50% with each
pregnancy,
irrespective of the outcome of prior pregnancies. It is like
flipping a
coin each time.
What if two people with achondroplasia have
children?
People with achondroplasia sometimes have children together.
If so, each
parent has a 50:50 chance of passing on the gene. Thus, with
each
conception, there is a 25% chance for an average-size child, a
50% chance
for a child (like them) with achondroplasia and a 25% chance
for a
conception with two achondroplasia genes. The combined presence
of two
genes for achondroplasia (called homozygous achondroplasia)
causes a
grievous skeletal disorder that leads to early death from
breathing
failure due to constriction by a tiny chest cage and neurologic
problems
from hydrocephalus.
Homozygous achondroplasia, although fatal, has led to insights
into
other medical conditions. For example, similarities were
noticed between
homozygous achondroplasia and a condition called thanatophoric
dwarfism.
It was proposed that the two disorders might be due to
different mutations
in the same gene and this has proven to be the case. Another
skeletal
disorder called hypochondroplasia has also turned out to be due
to yet
another mutation (change) in the same gene. Achondroplasia,
thanatophoric
dwarfism, and hypochondroplasia are thus due to mutational
changes at the
same gene locus.
What gene causes achondroplasia?
The locus of the gene for achondroplasia (which is also the
locus of the
genes for thanatophoric dwarfism and hypochondroplasia) has
been mapped.
Linkage studies were done of families with achondroplasia
tracing the
transmission of DNA markers whose chromosomal locations were
known. These
studies revealed that the gene for achondroplasia was on the
short (p) arm
of chromosome 4 in chromosome band 4p16.3.
Next: What is the molecular story with achondroplasia? »
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