Genetic Screening and Recessive Inheritance
~ Cystic Fibrosis ~ Sickle Cell Disease and Thalassemia ~ Spinal Muscular Atrophy ~
Tay-Sachs Disease ~ Jewish Panel ~ Bloom Syndrome ~ Canavan Disease ~ Familial Dysautonomia ~ Fanconi Anemia Group C ~ Gaucher Disease ~
~ Niemann-Pick Disease Type A/B ~ Mucolipidosis Type IV ~ Other Carrier Traits ~
Genes produce the instructions for our bodies’ growth and function. Our genes come in pairs; one of each pair is inherited from each of our biological parents. Recessive genetic diseases are those that result when both copies of a gene that a person has inherited have been altered or changed in a way to make the genes non-working. Everyone carries a few of these altered genes, and each ethnic group has certain genes that are more likely to be non-working.
A child who inherits an altered or non-working gene from only one biological parent will be an unaffected carrier for the condition. Carriers function normally, usually exhibiting no symptoms of the disease, but they can pass the carrier gene on to their own offspring.
When both biological parents are carriers for the same recessive condition, each pregnancy has
exactly the same risks (see diagram below):
- There is a 25% chance of inheriting the non-working gene from both biological parents and being affected with the disease.
- There is a 50% chance of inheriting one non-working gene and being an unaffected carrier.
- There is a 25% chance of inheriting no non-working genes (unaffected and non-carrier).
When only one biological parent is a carrier, the child has no chance of being affected with the disease,
but there is a 50% chance of being a (unaffected) carrier like the parent.
If a TSBC program donor is found to be a carrier for any of the following recessive diseases, he is not accepted and we do not release any of his semen samples. We recommend that Directed Donors and Known Donors be screened for these diseases, as applicable. If a Directed Donor or Known Donor is found to be a carrier, we would recommend that his recipient be tested to determine her own carrier status. We will release semen samples from a Directed Donor or Known Donor who is a carrier provided his recipient has been informed of the potential risk.
Cystic fibrosis (CF) is a debilitating disease affecting the lungs and pancreas, and usually results in chronic respiratory infections and shortened life expectancy. CF primarily affects people of Northern European ancestry. About 1 in 25 people of Northern European descent are carriers, while the odds of members of other ethnic groups being carriers are considerably lower, but unknown.
At TSBC, we screen all our program donors for cystic fibrosis, regardless of their ethnic heritage, and we recommend that all Directed Donors or Known Donors and/or their recipients be screened as well.
Hemoglobin is the red compound in blood that helps transport oxygen in our cells. Hemoglobin disorders can affect the function or the amount of hemoglobin made in the body.
Sickle cell anemia is an inherited hemoglobin disorder that can lead to blocked blood flow, susceptibility to infection and stroke, and organ damage. It is found almost exclusively in individuals of African descent and affects about one in 400 African-Americans. About 1 in every 12 individuals of African descent is a carrier for sickle cell disease.
The thalassemias are caused by an inadequate quantity of normal hemoglobin produced. Symptoms range in severity from mild anemia to the need for frequent blood transfusions and possible heart failure. Thalassemias are more commonly carried by individuals of African, Asian, Southeast Asian, and Mediterranean descent. At TSBC, we screen all our program donors for sickle cell disease and thalassemia regardless of their ethnic heritage—the hemoglobin electrophoresis test screens for both traits.
Spinal muscular atrophy (SMA) is a severe neuromuscular disease characterized by degeneration of motor neurons in the spinal cord which results in progressive muscle weakness and paralysis. The incidence in the general population is about 1 in 10,000 live births. There are a few different types of SMA, the most common and most severe form is known as SMA Type 1 (also known as Werdnig-Hoffman disease). SMA Type 1 is usually fatal in infancy or early childhood. The loss of motor neurons leads to weakness and wasting of muscles used for activities such as crawling, walking, sitting up, controlling head movement and eventually breathing and swallowing. The condition affects both males and females and can be identified in any ethnic group. The carrier rate for mutations in the gene for SMA (SMN1) has been estimated at 1 in 40 to 1 in 70 individuals.
In addition to the testing described above, program donors with any known Ashkenazi Jewish ancestry are also screened for the following conditions. This includes all donor applicants who have at least one grandparent with Jewish ancestry.
Children with Bloom syndrome are affected with growth retardation, abnormalities in skin pigmentation, immunodeficiency, a predisposition to cancer, and chromosomal instability. Death usually occurs in the teens or twenties, most often caused by cancer.
Canavan disease (aspartoacylase deficiency) is a progressive neurologic disease characterized by increased head circumference, decreasing muscle tone and motor activity, progressive loss of visual responsiveness, and mental retardation. Death usually occurs by age four.
Familial dysautonomia is characterized by abnormal functioning of the sensory and autonomic nervous systems. This causes decreased sensitivity to pain, abnormal regulation of body temperature, paroxysmal hypertension, and gastrointestinal abnormalities.
Fanconi anemia is characterized by deficient bone development and bone marrow function. This can lead to pancytopenia (a shortage of all blood cells), anemia, leukemia, and malformations of the limbs, kidneys, and heart. The disorder may be mild or severe.
Gaucher disease is a lysosomal glycolipid storage disorder caused by an enzymatic deficiency (acid beta galactosidase deficiency). Individuals may have an enlarged liver and spleen, thrombocytopenia, anemia, bone pain, bone lesions, and fractures. Life expectancy depends on severity of the symptoms.
This lysosomal storage disorder is characterized by diminished acid sphingomyelinase activity. Type A is usually fatal within 2 to 3 years. These children fail to thrive, have an enlarged liver and spleen, and exhibit progressive mental and physical degeneration.
Mucolipidosis IV is characterized by severe intellectual and motor delay by the end of the first year of life, and progressive visual impairment during the first decade as a result of retinal degeneration and corneal clouding. Mucolipidosis type IV is estimated to occur in 1 in 40,000 people. About 70 percent of affected individuals have Ashkenazi Jewish ancestry.
Tay-Sachs disease is a disorder in which a deficiency in the amount of an important enzyme (hexosaminidase-A) results in the progressive destruction of brain and nerve cells, leading to neurological damage and death before the age of five. While it is quite rare in the general population, it occurs in about one in 3,600 infants of Ashkenazi (Eastern European) Jewish, Cajun, or French Canadian ancestry. About 1 in 30 individuals of Ashkenazi Jewish descent is a carrier for Tay-Sachs disease, and the carrier rate is similar in the French Canadian and Cajun population. At TSBC, we screen all our program donors of Jewish, Cajun, or French Canadian descent for Tay-Sachs.
What if I know I am a carrier for a genetic condition?
Sometimes a recipient asks for additional screening for a donor after she has learned that she is a carrier for a condition through her own testing. We respond to registered recipient requests for additional donor carrier testing on a case-by-case basis. The decision can depend on a number of issues.
- Is an effective screening test available?
- Has the recipient received genetic counseling regarding her result?
- Is the donor available to be tested?
- Does the donor agree to be screened?
If additional screening can be performed for the donor, the recipient pays for any additional lab tests and all administrative fees for this service.
If you would like more specific information regarding testing for a specific donor, you may register as a recipient and once you have selected a donor, we can explore testing options for him.
You may also set up a genetic counseling appointment with our genetic counselor through the website. Specific information about your individual situation is best addressed in this way. Please read more information about these appointments, including how to schedule.
You may want to consider obtaining genetic counseling services for yourself through your own provider to ensure that you are aware of your own family history issues (if any), and to learn the risks, benefits and limitations of genetic testing for your pregnancy.