Preconception Carrier Screening for Genetic Diseases
Genetic preconception carrier screening at a glance
- Someone with a genetic mutation may not know he or she is a potential carrier of an inherited disease.
- A simple blood, cheek swab or saliva test can screen against one or more of hundreds of inherited diseases.
- Ethnicity and family history are the largest indicators of someone possibly carrying a genetic disease.
- The odds of passing on an inherited condition depend on several genetic factors.
What is genetic preconception carrier screening?
Genetic preconception carrier screening is how scientists can detect genetic mutations associated with hundreds of genetic diseases. All it takes is a simple blood, cheek swab or saliva sample, which is then sent to a lab for analysis. The process typically takes two to three weeks.
As explained in the Genetics Primer on this website, genes are made up of deoxyribonucleic acid (DNA). DNA is important because it instructs the body how to grow and develop.
Genes control specific bodily functions and attributes, such as hair and eye color or insulin production. Genes are arranged on strings called chromosomes. Everyone has 23 pairs of chromosomes, with each parent contributing one chromosome per pair. Each chromosome has hundreds of genes, and each of us possesses from 20,000 to 25,000 genes total.
Sometimes a gene can change, causing it to not function properly. This is called a mutation.
A Mendelian inheritance is the transmission of a mutation to children. In simple terms, these are called autosomal dominant, autosomal recessive and X-linked disorders.
What are the odds of passing on a genetic disease?
The answer to this question depends on how the condition is inherited. There are several inheritance factors.
- Autosomal dominant inheritance means that a person affected by a genetic disorder has a 50-50 chance of passing that mutated gene to each child. Autosomal refers to the 22 of the 23 pairs of chromosomes that are not sex chromosomes (there are two types of sex chromosomes, X and Y). Dominant genes are those that trump the other kinds of genes, known as recessive genes, which are weaker because they actually do nothing.
For example, people with brown eyes have a dominant gene for that eye color. Blue-eyed people lack the gene causing the brown coloring, and their eyes are blue simply because an eye with no pigment is blue.
- Autosomal recessive inheritance means that two people carrying a genetic disease but unaffected by it have a 25-percent chance with each pregnancy of passing the disorder to their children.
Correspondingly, there is a 50-percent chance of giving birth to a child unaffected by the disorder but who will still be a carrier. And there is a 25-percent chance of having a child who does not have the disorder and is not a carrier.
- X-linked dominant inheritance occurs when either a man or a woman affected by a certain genetic disease linked to the X chromosome passes on that chromosome to his or her daughters, who then inherit the disease.
If the man is the only disease carrier, his sons are unaffected, because men have one X chromosome and one Y chromosome, whereas women have two X chromosomes. Thus, males receive the Y chromosome only from their fathers. However, with each pregnancy, a female carrier has a 50-percent chance of affecting a daughter or son.
- X-linked recessive inheritance also means there are different chances of men and women passing on a disease related to the X chromosome. As in an X-linked dominant genetic disorder, the sons of a male carrier will not inherit it, while his daughters will carry one copy of the mutated gene.
For female carriers of X-linked recessive mutations, mothers have a 50-50 chance of having sons who are affected and an even chance of having daughters who carry one copy of the mutated gene.
- Codominant inheritance refers to each parent contributing a different version of a particular gene, which both influence the resulting genetic trait. The odds of developing a genetic condition through this kind of inheritance depend on which versions of the gene are passed on.
- Mitochondrial inheritance refers to mutations in mitochondrial DNA. Mitochondria are energy-producing centers inside cells, with each containing a small amount of DNA.
Mutations in mitochondrial DNA can affect both males and females, but only females can pass on the mutations. A woman with a mitochondrial DNA-related disorder will pass on the mutation to all her sons and daughters. However, a man with the same kind of disorder will not pass on the mutation to any of his children.
Why is genetic preconception carrier screening encouraged?
Humans with a genetic mutation may enjoy perfect health, because they are carriers of a recessive or an X-linked disorder and not a dominant disorder. However, if the woman is a carrier and her husband is also a carrier, the couple could have a child with a lethal or serious genetic disorder.
Anyone may carry a genetic disease and not know it because they have no symptoms. Ethnicity and family history are factors in genetic diseases. However, some people may be carriers regardless of these factors.
What are common genetic diseases?
- Tay-Sachs disease is a deadly disorder of the central nervous system. Although rare, it progressively destroys cells in the brain and spinal column. Symptoms do not usually appear until an infant is three to six months old but progress rapidly. Most Tay-Sachs sufferers live only into early childhood.
- Sickle-cell anemia is an unusually painful blood disorder occurring when red blood cells are unable to carry sufficient oxygen throughout the body. It can compromise the immune system and cause anemia, bone pain, delayed puberty, and shortness of breath, among other complications. It is most common in people of African, Caribbean, Middle Eastern, Mediterranean, South American and Central American heritage.
- Cystic fibrosis causes life-threatening lung and digestive problems. It mostly affects Caucasians of Northern European backgrounds, notably Ashkenazi Jews.
- Huntington’s disease leads to degenerative nerve cells in the brain and nervous system. Symptoms usually begin in a person’s 30s, causing memory, speech and cognitive problems, as well as difficulty swallowing, walking and balancing.
- Fragile X syndrome causes developmental problems including mental retardation and learning disabilities. It is not linked to any specific ethnicity and can appear in almost any family.
- Thalassemia is a blood disorder most commonly found in people of African, Southern Asian and Mediterranean ethnicities. It can lead to liver, bone-growth, spleen and heart problems. In extreme cases, babies born with the disorder do not survive.
- Duchenne muscular dystrophy causes fatigue and muscle weakness, typically starting before age 6 in the legs and progressing to the upper body. Most sufferers are wheelchair bound by age 12. Boys are mostly affected with other symptoms, such as back and chest deformities, heart and respiratory disorders.
Who should get genetic testing?
Ethnicity and family health history are the two most important factors in determining whether genetic testing is right for you. However, Advagenix recommends that all couples attempting to have a healthy baby, undergo preconception screening.
If someone in your family has inherited a genetic disorder, perhaps the best step is to consult with a genetics professional, such as a medical geneticist or genetic counsellor. Your family doctor may be able to refer you to one. Also, the National Society of Genetics Counselors offers a directory of genetics counselors in the United States and Canada.
A genetic counselor is a master’s level healthcare specialist who specializes in genetics, or is a certified healthcare professional with genetics experience, such as a medical geneticist. Consultations usually take place in the professional’s office or a medical center; in other cases, a consultation may occur by videoconference or phone.
Reasons to consider genetic testing and counseling
- You or someone in your family, including your child, has suffered a genetic disorder.
- You have had two or more miscarriages or a baby who did not survive.
- You are planning a pregnancy after age 35.
- You have had ultrasound or screening tests suggesting a potential problem.
- You are a member of any of the following ethnic groups at increased risk of inherited disease:
- Eastern European Jews and Caucasians, who are at risk for conditions such as cystic fibrosis, Canavan, Tay-Sachs and familial dysautonomia
- African Americans, Greeks, Turks, Italians and people of other Mediterranean lineage are at risk for sickle cell anemia and thalassemia.
- Southeastern Asians, at risk for thalassemia.
- French Canadians, at risk of Tay-Sachs and cystic fibrosis.