Inheritance of Autosomoal Dominant Genetic Diseases
These genetic diseases are diseases caused by an error in a single DNA gene.
Autosomal means the errors occurs on chromosome 1..22 rather than on the 23rd sex-linked X chromosome.
There are also X-linked dominant genetic diseases affecting the 23rd X chromosome.
Some examples of autosomal dominant diseases
and achondroplasia (dwarfism).
See autosomal dominant diseases and autosomal dominant for a full list.
A genetic trait is often said to be dominant or recessive.
A dominant trait is more likely to cause disease, because only one of the two copies
of each gene needs to be damaged.
In dominant genetic diseases, the "bad" gene overcomes the "good" gene and disease occurs,
whereas in recessive diseases the good gene is an adequate backup and recessive diseases
do not occur unless both copies are damaged.
But it is not always so black and white.
One gene does not always win or lose,
and there is a whole spectrum of levels of dominance,
depending on how much damage a bad gene does and how adequately
the second good gene can compensate for the failing bad gene.
Generally speaking, a dominant disease affects a gene for a structural protein,
causing malformed proteins that cause disease even though the other half of
the produced proteins
For comparison, recessive genes often affect genes producing hormones or enzymes,
because the backup good gene can often still produce adequate quantities of the hormone,
and only if both are wrong is there a disease.
Inheritance patterns for autosomal dominance:
This refers to diseases where the error is in one
of the autosome chromosomes,
and the bad gene dominates.
Some features of autosomal dominant
genetic diseases are:
- No carriers:
Everyone who has the genetic error gets the disease,
because the bad gene is dominant.
There is no such thing as a carrier for a dominant disease.
A few dominant genetic diseases like Huntington's disease
only cause symptoms later in life, so that people cannot know
that they have the disease in early life,
but this is not the same as being a carrier: these people
actually have the disease.
- Usually inherited:
For a person to have the disease,
one of the parents must have had the disease.
A child with the disease cannot be born to parents
without the disease, except very
rarely due to random genetic mutations.
- Parent-to-child transmission:
For one parent with disease,
there is usually a 50% chance of passing the disease onto children.
Male and female children are equally at risk.
- Both affected parents to child transmission:
Usually 75% chance.
If both parents have the disease,
they each have one bad gene and one good gene.
The child has a 75% chance of disease (50% chance of dominant disease,
25% chance of double-dominance)
and 25% chance of being disease-free (and also not being a carrier
as there are no carriers for dominant diseases).
- Double-dominant parent to child transmission:
If one parent has double-dominant disease (see discussion of double dominance below),
even if the other parent is not affected,
the chance is 100% chance of
having children with the disease.
Males and female children have the same chance.
- Undiseased parents to child transmission:
People without the disease cannot give the disease
to children, because they do not have a bad gene.
There are no carriers for dominant genetic diseases
so it is unlikely to be affected without knowing it (though
some dominant genetic diseases like Huntington's disease
only cause symptoms later in life).
If neither parent has the genetic disease,
the risk for a child is almost nil.
Extremely rarely, a random genetic mutation may give
rise to the disease,
which is presumably how the diseases occurred in the first place
through the history of the human race.
- Other children:
If a couple has a child with this dominant disease,
what are the odds for another child.
Usually a child getting the disease means that one of the
parents have the disease.
This makes the chance for a second child the same
as parent-to-child transmission rates as discussed already,
In the very rare case of a diseased child born to parents
without the dominant disease, this is probably a rare
acquired genetic mutation,
and the chance of a repeat is low (but detailed genetic testing
needs to be done to check the parents).
- Vertical inheritance:
Every generation is affected, called a "vertical" pattern,
as seen on a family tree.
By comparison, recessive diseases tend to have a
horizontal pattern with alternating generations affected.
- Gender bias:
Male or females get the disease equally,
because an autosomal error is unrelated to the sex chromosomes.
Even though only one damaged gene copy causes disease,
people can sometimes get two bad gene copies.
This is called "double dominance" and usually causes very severe disease
or even spontaneous abortion or stillbirth.
Double dominance is usually only possible for a child born
to parents that both have the same dominant genetic disease,
and the child must have inherited one copy of
the bad gene
from each parents, about a 25% chance.
A double-dominant disease is like a recessive disease,
except that carriers of it have the ordinary single-dominant condition.
For example, the genetic dominant condition
of achondroplasia (dwarfism)
causes reduced growth
especially in long bones, and also larger head with
a prominent forehead and a depressed nose.
These people are
not mentally handicapped
and have a normal lifespan.
However, the double-dominant achondroplasia condition is not viable
and the fetus dies.
A genetic disease that occurs when neither parent has any genetic defect
is called a sporadic genetic disease.
These cases arise via random genetic mutations in the DNA.
A sporadic genetic mutation is more likely
for a dominant disease than for a recessive genetic disease
because a dominant disease requires only a single random DNA mutation.