Why do you look more like your relatives than like anyone else? Why do kids often grow up to look like their parents? Why do “identical” twins look almost exactly the same? The answer to all of these questions has to do with genes.

Genes are units of hereditary information, which 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 contains about 20,000 genes that code for proteins. Only about 1% percent of our DNA is in protein-coding genes. The other 99% of our DNA percent is noncoding DNA. Most of the human genome is made of genes that do not contain instructions for making proteins, called noncoding DNA. Geneticists are still working to understand the purpose of all this noncoding DNA, but they know some of it helps control which protein-coding genes cells use.

You resemble the people in your family because your genes are more similar to their genes than they are to the genes of others. 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 is why you look nothing like a rabbit. But human beings resemble chimpanzees more than other nonhuman species, because humans’ and chimpanzees’ protein-coding DNA are similar. The more similar an organisms' genomes are, the more closely related those organisms are.

Genomes may differ greatly, but genes are all constructed in the same way. A gene is a section of a long molecule called deoxyribonucleic acid (DNA), or—in some viruses—a similar, simpler molecule called ribonucleic acid (RNA). DNA and RNA are made of building blocks called nucleotides. Each nucleotide is built around one of four different nitrogen-containing subunits, called bases. These bases are 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, all organisms also have RNA.

To make a protein from a gene in DNA, a cell first builds a strand of RNA by copying the information from the DNA molecule. This process is called transcription. The cell then uses the message in the RNA strand to build a protein molecule. This process is called translation.

When a cell produces a protein from an encoded gene, it is called gene expression. Not all genes are expressed, or turned on, in every cell. Instead, a cell expresses only the genes it needs at a given time. This process, called gene regulation, is how organisms can have different types of cells throughout the body. Gene regulation also allows cells to react to changes in their environments.

Epigenetics refers to gene regulation that does not involve altering the DNA sequence. (The alteration of the DNA sequence is a process known as mutation.) Instead, other factors act upon the genes, causing them to turn on or off. An example of how this happens includes a process known as methylation. During methylation, a methyl group—one carbon atom bonded to three hydrogen atoms—binds to a section of DNA. Methylation can block transcription, turning off that gene. Environmental factors, such as diet or exposure to pollutants, can cause epigenetic changes. Even though they do not change the sequence of the DNA, epigenetic changes to a genome can be passed down to future generations. But these changes are also reversible. Some epigenetic changes can lead to negative impacts on health, including the development of cancers.

DNA is a long molecule with many genes, so it forms a threadlike structure called a chromosome to fit in the cell nucleus. Proteins called histones act like a spool for the DNA. The threadlike DNA molecule wraps around the histones so the chromosome can fit inside the nucleus. Each cell of an organism carries at least one chromosome. Many organisms, including humans, have numerous chromosomes in their cells.

In learning about genes, DNA and chromosomes, scientists have created new technologies to help humans, other species and the environment. In medicine, our increased knowledge of genetics has paved the way for people to learn whether some of their genes are more likely to lead to a certain disease. Genetic counseling, or informing people about health risks related to their genes, allows people to understand potential health risks and make informed decisions about their health.

Scientists have also developed gene therapies that target specific genes to treat genetic disorders. For example, CRISPR-Cas9, a molecular gene-editing tool, can precisely edit a patient’s genome and reengineer a detrimental gene into a harmless form. However, scientists are still working to understand the long-term impacts and feasibility of gene editing.

Genetics has also impacted wildlife conservation. Studying the genetics of a wildlife population can inform scientists about the overall health of the population. Scientists can even collect genetic information from wildlife without handling the animals directly. Environmental DNA (eDNA) is DNA that an organism sheds into its environment through skin cells that come off the body or through excretions such as waste. With eDNA, scientists can identify species, including endangered or invasive species, and individual organisms from water or soil samples.

Genetic technology has also significantly impacted agriculture. Some agricultural operations use genetically modified organisms (GMOs), including food crops. The altered genomes allow the plants to yield more food, grow in harsher conditions or increase in nutritional content. This technology has allowed lower-income nations to have greater food security. Despite the relative safety of GMO technology, it has drawn public scrutiny. This pressure has led to increased study and monitoring on health and environmental impacts and labeling GMO foods. But there is no evidence that eating such foods is harmful.