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, and the risk
for passing that gene down to a child is 50% in each pregnancy.
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.
Next: What gene causes achondroplasia? »
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