Yes, diseases can be inherited through genes. Thousands of conditions pass from parent to child through DNA, ranging from single-gene disorders like cystic fibrosis to complex diseases like heart disease and diabetes where multiple genes interact with lifestyle and environment. An estimated 3.5% to 5.9% of the global population has a rare disease, and many of these are genetic conditions diagnosed in early childhood.
How a disease is inherited, and how likely you are to develop it, depends on the type of genetic change involved and whether one or both parents carry it.
Single-Gene Disorders
The most straightforward form of genetic inheritance involves a change in just one gene. These are sometimes called Mendelian disorders because they follow predictable patterns first described by Gregor Mendel. The pattern depends on where the gene sits in your DNA and whether one or two copies need to be altered for the disease to appear.
Autosomal dominant: Only one altered copy of the gene, from either parent, is enough to cause the disorder. If one parent carries the variant, each child has a 50% chance of inheriting it. Huntington’s disease is a well-known example.
Autosomal recessive: Both copies of the gene must be altered for the disease to develop. A person with just one altered copy is a “carrier” who typically has no symptoms but can pass the variant to their children. If both parents are carriers, each child has a 25% chance of being affected. Cystic fibrosis and sickle cell disease follow this pattern.
X-linked: Some conditions are tied to genes on the X chromosome. Because males have only one X chromosome, a single altered copy is enough to cause symptoms. Females, who have two X chromosomes, often have a working copy to compensate, which is why X-linked conditions like hemophilia and Duchenne muscular dystrophy primarily affect boys.
How Cystic Fibrosis Illustrates Recessive Inheritance
Cystic fibrosis is one of the most common inherited diseases and a clear example of how recessive inheritance works in practice. It’s caused by changes in a gene that controls the flow of salt and water across cell membranes. When both copies of that gene are faulty, cells lining the lungs, pancreas, and other organs produce abnormally thick, sticky mucus.
In the lungs, this mucus clogs airways, leading to chronic bacterial infections, coughing, wheezing, and over time, permanent scarring. In the pancreas, mucus buildup impairs the organ’s ability to produce digestive enzymes and insulin, which can cause malnutrition, poor growth, and digestive problems starting in infancy. Some affected babies develop intestinal blockages shortly after birth.
A person carrying just one copy of the altered gene has no symptoms and may never know they’re a carrier unless they get tested. Two carriers have a one-in-four chance with each pregnancy of having a child with the full condition.
Complex Diseases: Genes Plus Environment
Most common diseases don’t follow a single-gene pattern. Heart disease, type 2 diabetes, and many mental health conditions are polygenic, meaning dozens or even hundreds of gene variants each contribute a small amount of risk. On their own, none of these variants would cause disease. Combined with each other and with environmental factors like diet, physical activity, stress, and exposure to toxins, they shift your overall probability.
This is why these conditions tend to run in families without following a clean inheritance pattern. You might share your parent’s genetic predisposition to heart disease, but whether that predisposition becomes actual disease depends heavily on how you live. Some gene variants even enhance disease risk only under certain conditions. A disease-causing variant in one gene can be suppressed by protective variants elsewhere in the genome, or a person’s overall health can influence how strongly a genetic predisposition expresses itself.
Inherited Cancer Risk
Cancer offers one of the clearest examples of how inherited genes raise risk without guaranteeing disease. Changes in the BRCA1 and BRCA2 genes are among the most studied. More than 60% of women who inherit a harmful change in either gene will develop breast cancer during their lifetime, compared to about 13% of women in the general population. For ovarian cancer, the numbers are even more striking: 39% to 58% lifetime risk with a BRCA1 change, versus roughly 1.1% in the general population.
But notice that not everyone who carries these variants develops cancer. This phenomenon is called reduced (or incomplete) penetrance: having the genetic variant doesn’t always produce the disease. The gap between carrying the gene and developing the condition likely comes down to a combination of other genetic factors, environment, and lifestyle, many of which researchers are still working to identify.
Mitochondrial Inheritance
Not all inherited DNA sits in the cell’s nucleus. Mitochondria, the structures that generate energy inside your cells, carry their own small set of genes. This DNA is inherited almost exclusively from mothers. During fertilization, the mitochondria contributed by the sperm are nearly always destroyed, so a father’s mitochondrial DNA doesn’t pass to his children.
This means mitochondrial diseases, which typically affect energy-demanding organs like the brain, muscles, and heart, follow a maternal line. A mother with a mitochondrial DNA change will pass it to all of her children, but only her daughters can pass it on to the next generation.
Epigenetic Inheritance
Your DNA sequence isn’t the only thing that can be inherited. Chemical tags that sit on top of DNA, controlling which genes are turned on or off, can also be passed between generations. These are called epigenetic changes. They don’t alter the genetic code itself, but they affect how genes behave.
One key mechanism involves small chemical groups (methyl groups) attaching to DNA and silencing specific genes. Environmental factors like diet, stress, and toxic exposures can add or remove these tags during a person’s lifetime. In some cases, those modifications persist through reproduction. About 100 known genes are already subject to a process called imprinting, where one parent’s copy is naturally silenced through epigenetic marks, leaving only the other parent’s copy active. This means disruptions to the silenced or active copy can cause disease even though the underlying DNA sequence is normal.
Epigenetics helps explain why identical twins, who share the same DNA, can develop different diseases as they age. It also explains how a parent’s environment, what they ate, what they were exposed to, can influence their children’s health risks without changing a single letter of genetic code.
Genetic Testing and Carrier Screening
If you’re wondering whether you carry genes for an inherited disease, several types of testing are available. Carrier screening can be done before or during pregnancy to check whether you or your partner carry variants for conditions like cystic fibrosis or sickle cell disease. This is a simple blood draw from the mother (or both parents).
During pregnancy, screening and diagnostic tests serve different purposes. Screening tests estimate the probability that a fetus has a genetic condition. These include first-trimester blood tests combined with ultrasound measurements, second-trimester blood panels, and cell-free DNA testing, which analyzes fragments of fetal DNA circulating in the mother’s blood. None of these require entering the uterus.
Diagnostic tests like amniocentesis and chorionic villus sampling are more definitive but more invasive. They confirm whether a specific genetic condition is actually present. These are typically offered when screening results suggest elevated risk or when there’s a known family history of a genetic disorder.
Outside of pregnancy, broader genetic testing through clinical sequencing can sometimes reveal inherited disease risks unrelated to the original reason for testing. Guidelines from the American College of Medical Genetics and Genomics recommend reporting certain high-impact findings when they’re discovered, particularly for conditions where early knowledge could lead to prevention or better management.
Why Carrying a Gene Doesn’t Always Mean Getting Sick
One of the most important things to understand about genetic inheritance is that carrying a disease-related gene variant and developing the disease are not the same thing. Penetrance varies widely across conditions. Huntington’s disease has nearly complete penetrance: if you inherit the variant, you will almost certainly develop the condition. BRCA-related cancers, by contrast, affect a large majority of carriers but not all of them.
For polygenic conditions like type 2 diabetes, the relationship is even looser. You can carry dozens of risk-associated variants and never develop the disease if other factors work in your favor. Conversely, someone with fewer genetic risk factors can develop the condition if environmental and lifestyle factors push hard enough in that direction. Genes load the odds, but for most conditions, they don’t write the final outcome on their own.