Genes are specific segments of DNA that influence certain aspects of growth and health. Collectively, scientists refer to all of an organism’s genes as a genome.

DNA is the genetic material or code that tells cells in the body how to replicate themselves. Almost every cell in an organism’s body contains a complete copy of its genome, packaged into chromosomes. Chromosomes are thread-like structures consisting of DNA and protein that sit inside cells.

This article will explain what DNA, genes, and genomes are. It will also look at the differences between a gene and a genome and answer some frequently asked questions.

A diagram showing DNA, genes, and chromosomes.Share on Pinterest
Medical illustration by Bailey Mariner

Genes consist of DNA, which is the genetic material that tells cells how to reproduce. Strands of DNA look like a twisted ladder, which scientists call a double helix.

DNA consists of four chemicals, which are known as bases. They are:

  • adenine (A)
  • cytosine (C)
  • guanine (G)
  • thymine (T)

The order of these chemical bases in a person’s DNA determines how their cells grow and develop.

A gene is a segment of DNA that contains sequences of many bases, varying in size from a few hundred to 2 million. Each gene affects a specific aspect of health. For example, some genes contain instructions on how to make specific proteins.

Parents pass on their genes to their biological children. As a result, each person has two copies of each human gene — one from each parent. In total, the human body has between 20,000 and 25,000 genes.

The word “genome” refers to all the genetic material in an organism. The human genome consists of around 3 billion DNA base pairs.

Almost every cell in the body contains a complete copy of the organism’s genome, tightly packaged inside its chromosomes. Chromosomes are present in the nucleus of every cell.

Most of the human genome is the same in all people. However, about 0.001% of the genetic material will be different from person to person. This figure is even smaller in people who are related to each other.

Below are some of the differences between genes and genomes:

GeneHuman genome
Functioncontains the information a cell needs to carry out a specific function, such as how to make a particular proteincontains the entirety of the person’s genetic material
LengthA few hundred to 2 million DNA base pairs3 billion DNA bases
Field of studygeneticsgenomics

Genes influence health in two main ways:

Genetic disorders

Some variations in genes directly result in health conditions. These conditions are known as genetic disorders. Genetic disorders can be:

  • Autosomal dominant: This means that inheriting just one copy of a gene can cause the disorder. If one biological parent carries the gene, the child has a 50% chance of developing the associated condition. Examples of dominant genetic conditions include Huntington’s disease, Marfan syndrome, and achondroplasia.
  • Autosomal recessive: This means that a person must inherit two copies of the gene, one from each biological parent, to cause the disease. If both parents each have a copy of the gene, the child has a 25% chance of developing the condition. Examples of recessive genetic disorders include cystic fibrosis and sickle cell disease.

In addition to inheriting genetic disorders from parents, people can also spontaneously get them if the gene mutates during fetal development or later in life. This can result in a person having a genetic variation that their parents do not have themselves.

Learn more about autosomal inheritance.

Risk factors

In other cases, genes are merely risk factors for a condition. This means they can raise or lower the risk of someone getting a disease but do not directly cause it.

For example, some people carry genes that raise the risk of certain cancers, diabetes, or heart disease. Doctors call this a nonmodifiable risk factor because a person cannot change their genes.

However, having genes that raise the risk of a condition does not guarantee that a person will get it. Different genes, and different combinations of genes, pose different levels of risk. A person’s environment, diet, and lifestyle also influence the risk.

Working on modifiable risk factors, which are things a person can control, can help offset the elevated risk of developing a condition.

Other terms doctors use when talking about genes include:

  • Genetics: This is the study of genes and how parents pass them on to their children.
  • Genomics: This is the study of the genome. It includes the analysis of all of a person’s genes and how they interact with each other and the environment.
  • Genotype: Genotype refers to a person’s unique genome, which people can find out by undertaking personal genome sequencing. Genotype influences things such as eye color or the risk of certain diseases.
  • RNA: RNA stands for ribonucleic acid. Similarly to DNA, it is a genetic material, but it only has one strand, or a single helix. RNA helps the cells in the body understand, interpret, and use DNA coding.

A gene is a specific segment of DNA that tells cells how to function. A genome is the entirety of the genetic material inside an organism. The human genome consists of between 20,000 and 25,000 genes.

Most of the human genome is the same from person to person, but variations in genes can influence someone’s health, appearance, and risk of developing certain diseases. There are also some conditions that occur as a direct result of specific genetic variants, such as sickle cell disease.