Genes are hereditary information units that organisms pass down to new generations. Genes contain coded information for the production of proteins that enable cells to function. An organism’s entire collection of genes is called its genome.

The human genome, for example, contains somewhere between 20,000 and 25,000 genes. We resemble the people in our family because our genes are more similar to their genes than they are to those of strangers.

On the other hand, your genes are more similar to the genes of all other humans than they are to the genes of a different kind of organism, such as a rabbit. That being said, humans resemble chimpanzees because we share nearly 99 percent of the same genes.

Heredity is a concept that humans have been aware of for thousands of years as we’ve cultivated crops and selectively bred animals.

However, the first formal genetic study was undertaken by a monk named Gregor Mendel in the middle of the 19th century—Mendel bred peas and noticed he could cross-pollinate them in certain ways to get green or yellow seeds.

Nowadays, the field of genetics is breaking new ground searching for new ways to treat disease or develop crops more resistant to insects or drought.

While genomes may differ greatly, the genes themselves are all built in the same way. A gene is a section of a long molecule called deoxyribonucleic acid (DNA), or—in some viruses—a similar molecule called ribonucleic acid (RNA).

DNA and RNA are made of building blocks called nucleotides. Each nucleotide is constructed around one of the four different subunits, called bases. These bases are known as guanine, cytosine, adenine, and thymine (in RNA thymine is replaced with uracil).

A gene carries information in the sequence of its nucleotides, just as a sentence carries information in the sequence of its letters. Although most genes are made of DNA, the genes use RNA to make the proteins that are coded for in the DNA.

Genes are located on threadlike structures called chromosomes in the cell nucleus (a chromosome itself is a long molecule of DNA with proteins attached). Each cell of an organism carries at least one chromosome. Many organisms, including humans, have numerous chromosomes in their cells.

To make a protein from a gene in DNA, a cell first builds a strand of RNA that copies the information from the DNA molecule. This process is called the transcription of a gene.

The cell then uses the message in the RNA strand to build a protein molecule. This process is called the translation of a gene.

Only about one percent of our DNA is in protein-coding genes. The other 99 percent is noncoding DNA. Geneticists do not know the purpose of all this noncoding DNA, but they believe at least some of it is necessary for the proper functioning of cells, especially in controlling gene functions.

Genetic disorders occur when a mutation (a harmful change to a gene, also known as a pathogenic variant) affects our genes or when we have the wrong amount of genetic material.

We receive half our genes from each biological parent and may inherit a gene mutation from one of them or both. Sometimes genes change due to issues within the DNA (mutations). This can raise our risk of having a genetic disorder. Some cause symptoms at birth, while others develop over time.

Genetic disorders can be:

• Chromosomal: This type affects the structures that hold your genes/DNA within each cell (chromosomes). With these conditions, people are missing or have duplicated chromosome material;
• Complex (multifactorial): These disorders stem from a combination of gene mutations and other factors. They include chemical exposure, diet, certain medications, and tobacco or alcohol use;
• Single-gene (monogenic): This group of conditions occurs from a single gene mutation.

There is often little we can do to prevent a genetic disorder. But genetic counseling and testing can help us learn more about our risks.

If you have a family history of a genetic disorder, you may wish to consider genetic counseling to see if testing is appropriate for you. Lab tests can typically show whether you have gene mutations responsible for that condition. In many cases, carrying the mutation does not always mean you’ll end up with it.