August 18, 2026
What are genes? #
Every cell in our bodies (except red blood cells) carries two full copies of the human genome, which together provide an instruction manual for how to build and run the human body. The genome includes sections that encode the body’s protein building blocks, which in turn function as cellular machinery and messenger molecules. These protein-encoding sections are called “genes.” The genome also includes regions with instructions that tell the cell when to read out and build each protein, and when not to. These are called “regulatory regions.”
Each person carries one copy of their genome that they inherited from their mother, and one copy that they inherited from their father. While these usually are faithful copies, sometimes copy errors arise, leading to changes in the genome’s sequence. When those errors occur at important locations, like within a section that encodes a protein or important instructions, this may cause changes in how the body or brain functions. Depending on what the alteration is and where it occurs, some changes will have only minor effects, while others can cause severe symptoms.
Genetic variants #
Changes that cause minor effects are very common in the population. These are often called “variants.” Gene variants are passed down from parent to child. Sometimes, the combined action of many gene variants can be important for understanding complex health conditions.
Changes with major effects can cause what we call “rare genetic diseases.” While some of these can be passed down in families, many arise due to a copy or division error in the creation of an embryo. This type of genetic change is called “de novo”, meaning “from the beginning.” De novo genetic changes are not found in parents or siblings, only in the individual.
Other changes with major effects may be passed down in families, but only cause problems when the variant is inherited from both parents; this type of variant is known as a recessive gene. A few of these, like the variant that causes sickle cell disease, are relatively common in certain populations. Asymptomatic people with a single copy of the variant are called “carriers,” and may pass on the gene to their children.