Genetic Problems
Errors in the genetic code or "gene recipe" can happen in a variety of ways. Sometimes
information is missing from the code, other times codes have too much information,
or have information that's in the wrong order.
These errors can be big (for example, if a recipe is missing many ingredients —
or all of them) or small (if just one ingredient is missing). But regardless of whether
the error is big or small, the outcome can be significant and cause a person to have
a disability or at risk of a shortened life span.
Abnormal Numbers of Chromosomes
When a mistake occurs as a cell is dividing, it can cause an error in the number
of chromosomes a person has. The developing embryo then grows from cells that have
either too many chromosomes or not enough.
In trisomy, for example, there are three copies of one particular
chromosome instead of the normal two (one from each parent). Trisomy
21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome)
are examples of this type of genetic problem.
Trisomy 18 affects 1 out of every 7,500 births. Children with this syndrome have
a low birth weight and a small head, mouth, and jaw. Their hands typically form clenched
fists with fingers that overlap. They also might have birth defects involving the
hips and feet, heart and kidney problems, and intellectual disability (also called
mental retardation). Only about 5% of these children are expected to live longer than
1 year.
Trisomy 13 affects 1 out of every 15,000 to 25,000 births. Children with this condition
often have cleft lip and palate, extra fingers or toes, foot
abnormalities, and many different structural abnormalities of the skull and face.
This condition also can cause birth defects of the ribs, heart, abdominal organs,
and sex organs. Long-term survival is unlikely but possible.
In monosomy, another form of numerical error, one member of a
chromosome pair is missing. So there are too few chromosomes rather than too many.
A baby with a missing autosome has little chance of survival. However, a baby with
a missing sex chromosome can survive in certain cases. For example, girls with Turner
syndrome — who are born with just one X chromosome — can live normal,
productive lives as long as they receive medical care for any health problems associated
with their condition.
Deletions, Translocations, and Inversions
Sometimes it's not the number of chromosomes that's the problem, but that the chromosomes
have something wrong with them, like an extra or missing part. When a part is missing,
it's called a deletion (if it's visible under a microscope) and a
microdeletion (if it's too tiny to be visible). Microdeletions are
so small that they may involve only a few genes on a chromosome.
Some genetic disorders caused by deletions and microdeletions include Wolf-Hirschhorn
syndrome (affects chromosome 4), Cri-du-chat syndrome (chromosome 5), DiGeorge syndrome
(chromosome 22), and Williams syndrome (chromosome 7).
In translocations (which affect about 1 in every 400 newborns),
bits of chromosomes shift from one chromosome to another. Most translocations are
"balanced," which means there is no gain or loss of genetic material. But some are
"unbalanced," which means there may be too much genetic material in some places and
not enough in others. With inversions (which affect about 1 in every
100 newborns), small parts of the DNA code seem to be snipped out, flipped over, and
reinserted. Translocations may be either inherited from a parent or happen spontaneously
in a child's own chromosomes.
Both balanced translocations and inversions typically cause no malformations or
developmental problems in the kids who have them. However, those with either translocations
or inversions who wish to become parents may have an increased risk of miscarriage
or chromosome abnormalities in their own children. Unbalanced translocations or inversions
are associated with developmental and/or physical abnormalities.