Along with our height, build, eye and hair color, and other physical traits, the genes that were passed from our parents to us and that we may pass to our children can also determine aspects of our health.
Genes are made up of DNA and contain information that determines specific traits. We have two copies of each gene—one inherited from our father and one from our mother. Inherited mutations (abnormalities) in genes can cause certain health conditions or increase the likelihood that we will develop particular diseases or disorders. These are known as hereditary illnesses.
Colleen Caleshu, MS, CGC, a genetic counselor at the Stanford Center for Inherited Cardiovascular Disease, describes hereditary illness as “any health condition that we can inherit and that other people who share our genes [other family members such as siblings and children] may also be at risk to have.” These conditions, she says, can be entirely determined by genes or can be the result of a combination of genetic susceptibility and environmental or lifestyle factors that trigger the disease or disorder. (Conditions that run in families can be complex, involving multiple genes and other factors. For the sake of clarity, this article focuses on inherited mutations in single genes.)
How Genetic Inheritance Works
There are several ways in which a genetic abnormality is passed from generation to generation. These patterns of inheritance determine the likelihood that a child will inherit the genetic abnormality or condition, and they explain the path a mutation takes from one family member to the next. Different types of inheritance include autosomal dominant, autosomal recessive, X-linked, and mitochondrial inheritance as well as chromosomal abnormalities.
Autosomal Dominant Inheritance
With autosomal dominant inheritance, only one copy of the abnormal gene must be passed down to a child for the disorder to appear. Even though we get most of our genes in two copies (one from our mother and one from our father), only one mutated copy is needed for a child to inherit an autosomal dominant disorder. “An autosomal dominant disorder is a condition in which having one nonfunctional gene, or a mistake in the gene, is enough to give you the disorder,” explains Ada Hamosh, MD, MPH, a professor in the department of pediatrics at the Institute of Genetic Medicine at Johns Hopkins School of Medicine.
A parent with an autosomal dominant disorder has a 50/50 chance of passing the disorder to each of his or her children. An example of an autosomal dominant disorder is Marfan syndrome, a connective tissue disorder that affects the skeletal system, cardiovascular system, eyes, and skin. People with this disorder tend to be tall and have long legs, arms, and fingers.
Different types of cancers can also have a genetic link, such as breast and colon cancer, and specifically be the result of an autosomal dominant mutation. “A good example would be breast cancer,” explains Allison W. Kurian, MD, MSc, a medical oncologist and assistant professor at Stanford Medicine. “A small minority—but an important subset—of breast cancer cases are caused by inheritance of basically a single gene mutation,” she says. Specifically, these include mutations in the BRCA1 and BRCA2 genes. “Women who inherit those,” says Dr. Kurian, “have a very high risk of developing breast cancer.”
Autosomal Recessive Inheritance
“An autosomal recessive condition,” explains Dr. Hamosh, “is a condition in which both parents are carriers.” As carriers they don’t have the condition, but they carry the genetic trait because they have one copy of that particular gene that works. “But,” she says, “if you have two copies of the gene not working, then you have a problem.” The so-called problem—an autosomal recessive condition—arises if two carriers have a child together and each passes on to their child a gene that doesn’t work.
If two carriers of the same autosomal recessive condition have a child together, there’s a 25 percent chance that the child will have the condition and a 50 percent chance that the child will be an unaffected carrier (meaning that he or she will inherit one copy of the abnormal gene). This risk applies to each pregnancy. Examples of autosomal recessive conditions include cystic fibrosis, a disease that causes mucus to build up in the lungs, digestive tract, and other areas of the body, and sickle cell disease (or sickle cell anemia), a condition in which red blood cells form an abnormal sickle or crescent shape instead of a normal disc shape.
In X-linked inherited conditions, the nonfunctioning gene is located on the X chromosome. Because women have two copies of the X chromosome and men have one X and one Y chromosome, X-linked conditions are more likely in men. “In general, conditions that are X-linked manifest either only in boys or far more severely in boys because they don’t have a second X chromosome to protect them from the gene not working on the one X chromosome they have,” Dr. Hamosh explains.
Duchenne muscular dystrophy, which is the most common form of muscular dystrophy, is an example of an X-linked condition. People with Duchenne muscular dystrophy develop muscle weakness that gets worse very quickly.
Our mitochondria (cellular structures, or organelles, that primarily produce energy for the cell) come from our mother’s egg, or, as Dr. Hamosh explains, “the egg that made us.” This egg contains about 5,000 mitochondria and, she says, “there are certain mitochondrial diseases where every single mitochondrion in that egg has the same mutation”—which is referred to as homoplasmy. It’s also possible that only a portion of the mitochondria have the mutation, which is known as heteroplasmy. When the egg divides to make multiple cells (daughter cells), the daughter cells will have a greater or lesser degree of homoplasmy or heteroplasmy, depending on how the mitochondria are dispersed among them.
Dr. Hamosh says that mitochondrial diseases tend to affect tissues that consume a lot of energy, such as muscles, the brain, and the liver, and complications can include vision and hearing loss as well as stroke, diabetes, and other conditions. One particular type of mitochondrial disorder is triggered by exposure to a class of antibiotics called aminoglycosides and can result in hearing loss.
Chromosomal abnormalities can occur when there is an extra whole chromosome or there are missing or extra pieces of a chromosome, which can be an inherited trait or occur for the first time in an individual. They can occur when there’s an error in cell division. “In general, big pieces [of missing chromosome] are new in a patient and not inherited from either parent,” Dr. Hamosh explains, whereas “many smaller [missing pieces] may be inherited from a parent.”
Down syndrome is an example of a condition caused by a whole extra chromosome—a person with this has 47 chromosomes instead of the usual 46—and it affects physical, mental, and social development. (Though Down syndrome can be inherited, most cases occur at random.) Examples of conditions associated with missing pieces of chromosomes include Williams syndrome, a developmental disorder that affects many parts of the body, and 22q11.2 deletion syndrome, a condition with widely varying symptoms that can include heart defects and a cleft palate.
Coping with Genetic Risk
For many people who have a known genetic condition or may be at risk, a genetic counselor can help them navigate the many complex issues surrounding their health; this may begin with genetic counseling and, if determined appropriate, genetic testing. According to genetic counselor Colleen Caleshu, genetic testing may be appropriate “anytime that there’s a hereditary condition in an individual or in their family member”—so long as there’s a test available for the condition in question. Testing may also be considered when a patient has a health event or family history that suggests a hereditary link.
Dr. Kurian encourages patients who feel there may be a genetic link to an illness they have or that runs in their family to discuss the issue with their doctors. “I think it’s always reasonable for people to ask their healthcare providers if there could be a genetic risk and [if this] should be evaluated,” she explains.
Dr. Kurian also emphasizes the importance of family history when determining if there’s a genetic component to a diagnosis and if genetic counseling and testing are warranted, particularly in cancer genetics. “For cancer it’s very much driven by a family history of a specific pattern of cancers,” she says, adding that early-onset disease can also be a reason to look into a genetic cause. “One thing that’s really helpful is if a patient can assemble as much information as possible about the family history,” she says. “Having an accurate, multigeneration family history can help us think about what [type of] testing is going to be most helpful.”
Caleshu says that genetic counseling is done in conjunction with genetic testing to help patients understand what to expect from genetic testing and how to handle the resulting information. In some cases, patients may determine not to go forward with genetic testing; she explains that there are circumstances in which other medical evaluations may be more appropriate than genetic testing, such as conditions that are “partly genetic, partly lifestyle, partly environment”—diabetes and coronary artery disease, for example. In these cases, blood tests, imaging, or a thorough family history may answer questions and guide health decisions more effectively than a genetic test.
Before a patient begins genetic testing, Caleshu says that there are several important things to consider. “I think it’s really important that someone understands what a genetic test can and cannot tell them,” she says, explaining that it’s important to be realistic and know that a test result might not deliver absolute answers. “They’re often not as simple as positive or negative,” Caleshu says, and she suggests that you consider the impact that various results could have on your medical and personal choices as well as those of your family.
Managing Your Risk
Because not all genetic abnormalities inevitably cause illness, screening and prevention can be effective, especially for those hereditary illnesses that are the result of a genetic predisposition combined with environmental or lifestyle triggers. For example, a person at risk of an inherited cardiovascular condition could trigger that illness by smoking. By testing for genetic mutations, it’s possible to establish protective measures with the goal of preventing the onset of illness. As well, a positive test for a genetic abnormality does not mean you will definitely develop the related condition; a genetic counselor can help determine the chances that you’ll develop that condition.
Also understand that a negative test for a genetic mutation doesn’t mean you can’t get the related condition. It only means you don’t have the very high risk associated with a mutation. For example, a woman who does not carry her family’s BRCA mutation can still get breast cancer—she just doesn’t have the same high risk that family members who do carry the mutation have.
“The whole goal of genetic testing, when we know about a genetic mutation that conveys a high risk of disease, is to be able to fit our preventive management to the level of risk,” Dr. Kurian says. With an early warning that someone is at high risk for a particular disease, she explains that doctors and genetic counselors can “tailor our screening and prevention strategies accordingly.”
For example, whereas a woman at average risk of breast cancer will begin screening mammography around age 40, a woman with a known genetic risk for the disease might adopt an escalated approach. In this case, Dr. Kurian says, “We would add to her screening with a more sensitive test—so not just mammogram but also an MRI of the breast.” Other preventive strategies might also be considered, she says; possibilities include medication to reduce risk, as well as preventive surgery, such as removal of the breasts before cancer develops.
Caring for Yourself and Your Family
A genetic counselor may be your best ally as you face a hereditary illness. Caleshu recommends finding a counselor who has expertise in the particular condition that runs in your family. In addition to guiding you through the genetic-testing process by determining what tests may be done, who in the family may be tested, and who’s at risk, he or she can help you with some other significant issues surrounding genetic risk. Some areas in which you may want guidance include how to talk to family members about risk or a condition, changes to make in lifestyle or medical care (such as screening measures), and how to learn more about your risk or condition.
Coping with hereditary illness can also present emotional challenges, as intense personal questions—such as whether or not to have children—may be raised; a genetic counselor can help you handle your emotional response and find appropriate additional support if needed. He or she may connect you with local, in-person support groups as well as national advocacy organizations. FORCE (Facing Our Risk of Cancer Empowered, facingourrisk.org), an organization devoted to hereditary breast and ovarian cancer, and the Genetic Alliance (geneticalliance.org), an advocacy group that includes more than 1,000 disease-specific organizations, are examples of resources that a genetic counselor may recommend.
A Call to Action
If you have or are at risk of hereditary illness, it may appear like there’s nothing you can do about the situation. After all, you’re born with it, right? Not necessarily—the fact is that with education, support, and the expert guidance of physicians and genetic counselors, there’s a lot you can do to manage your condition and, in some cases, protect yourself from developing a disease or disorder. In this way a family history of a disease or known genetic risk is a call to action, and the resources are in place to help you face the challenge and manage your risk. _
Know Your Rights
Genetic Information Nondiscrimination Act of 2008
Under the Genetic Information Nondiscrimination Act (GINA), which took effect in November 2009, you are protected from discrimination from employers and insurers based on genetic information. In other words, if you are found to have a genetic risk of a health condition, it’s unlawful for you to be treated unfairly by your employer or health insurance provider as a result of these findings.
Though GINA does provide some protection against discrimination, it’s also useful to know that there are areas in which the statute isn’t effective. Protection does not cover, for example, life insurance, disability insurance, or long-term care insurance or require that any particular test or treatment is covered. As well, employment protection under GINA may not take effect for employers with fewer than 15 employees.