A number of genetic disorders occur more frequently
in certain ethnic populations. In the Ashkenazi
Jewish population (those of Eastern European descent), it has been
estimated that one in four individuals is a carrier of one of several
genetic conditions. These diseases include Tay-Sachs
Disease, Canavan, Niemann-Pick, Gaucher, Familial Dysautonomia,
Bloom Syndrome, Fanconi anemia, Cystic Fibrosis and Mucolipidosis IV. Some
of these diseases may be severe and may result in the early death of
a child. Carrier screening is available for all of these diseases with
a simple blood test.
In the nucleus of every cell in the body there are
46 chromosomes. Each chromosome is a package that holds many genes. Our
genes contain DNA, the set of instructions that makes up who we are. All chromosomes
(and the genes that are on those chromosomes) come in pairs. We receive
one member of each pair of chromosomes from our mother and the other
member of the pair from our father. Sometimes there is a change in a
gene (called a mutation) that causes the gene to malfunction.
All of the above-mentioned conditions are inherited
in an autosomal recessive manner. This means that an affected person
has a change in both genes of the pair of genes, one change inherited from each
parent. Neither gene in the pair is working properly, which causes the
symptoms of the disease.
A carrier is someone who has a change in only one gene of the pair of genes. Carriers
are healthy individuals who are only at risk for passing the gene change
on to their children. Most often these diseases occur in families
with no prior history of the disease.
Tay-Sachs Disease
A condition
where children develop normally until about four to six months of age. It
is at this time that the central nervous system begins to degenerate. Individuals
with Tay-Sachs Disease lack an enzyme called
hexosaminidase (Hex A). The child loses all motor skills and becomes
blind, deaf and unresponsive. Death usually occurs by the age of four. The
carrier rate in the Ashkenazi Jewish population is approximately 1 in
25. More rare than the infantile type is Late Onset Tay-Sachs Disease, where the progression of symptoms
is slower and milder.
Canavan Disease
Very similar to Tay-Sachs
Disease, with normal development until age two to four months,
followed by progressive loss of previously attained skills. Most individuals
with Canavan Disease die by the age of five. An estimated 1 in 40 Ashkenazi
Jews is a carrier for this disease.
Niemann-Pick Disease – Type A
A disease in which a harmful
amount of a fatty substance accumulates in different parts of the body. Failure
to thrive and a progressive neurodegenerative course lead to death by
three years of age. The carrier rate in the Ashkenazi Jewish population
is approximately 1 in 90.
Gaucher Disease – Type 1
(Pronounced go-shay) is a variable condition,
both in age of onset and in progression of symptoms. A painful, enlarged
and overactive spleen, with anemia and low white blood cell count are
usually the initial features of Gaucher Disease. Bone deterioration is
a major cause of discomfort and disability. Approximately 1 in 14 Ashkenazi
Jews is a carrier of this condition. Treatment is available.
Familial Dysautonomia
A disease that causes the autonomic
and sensory nervous systems to malfunction. This affects the regulation
of body temperature, blood pressure, stress response, normal swallowing
and digestion. An estimated 1 in 30 Ashkenazi Jews is a carrier of FD.
Bloom Syndrome
Characterized by short stature, sun-sensitive
facial skin lesions, an increased susceptibility to infections and a
higher incidence of leukemia and certain cancers. The carrier rate is
about 1 in 100 in the Ashkenazi Jewish population.
Fanconi anemia – Type C
A disease associated with short stature,
bone marrow failure and a predisposition to leukemia and other cancers. Some
children may have learning difficulties or mental retardation. Approximately
1 in 89 Ashkenazi Jews is a carrier for this condition.
Mucolipidosis IV
Caused by the accumulation of certain
harmful substances throughout the body. Individuals with ML IV experience
a range of levels of motor and mental retardation, with developmental
delays often manifesting themselves as early as the first year of life. Other
symptoms can be related to the eyes, such as corneal clouding, pseudostrabismus and retinal degeneration.
Cystic Fibrosis
A multi-system disorder that causes the
body to produce a thick mucus. The mucus accumulates
primarily in the lungs and the digestive tract, resulting in chronic
lung infections and poor growth. CF does not affect intelligence. The
carrier rate for CF among all Caucasian individuals is approximately
1 in 25. The CF carrier test has a detection rate of 97% in the Ashkenazi
Jewish population.
If two carriers of the same disorder have children,
there is a 25% chance of having an affected child, a 50% chance of having
a child who is a carrier like themselves, and
a 25% chance of having a child who is neither affected nor a carrier. If
an individual is found to be a carrier, genetic counseling is available
at many clinics throughout the country to discuss the implications of
this finding. If partners are found to be carriers of the same disorder(s),
a genetic counselor can provide information and support, which may be
helpful in making important family planning decisions.
The results of these tests are highly accurate. However,
there is a slight possibility that someone who tests negative for being
a carrier could still be a carrier. There may be rare mutations that
DNA testing may not pick up.
