Enlarged illustration of the double helix of DNA

How genes work

What are genes?
What do genes do?
How do genes work?
What is a genetic disease?

Knowing the basics of genetics can help you understand the role genes play in inherited diseases.

What are genes?

Genes are the basic unit of heredity. They are passed down from parents to children, 1 from each biological parent. So each person has 2 copies of each of the more than 20,000 genes in the human body. They can determine many things—from what a person will look like to whether they are at risk of having certain diseases.

Learn about the connection between genes and inherited disease

Genes are segments of DNA (deoxyribonucleic acid) that contain instructions for building proteins and other molecules that help the body work. DNA is shaped like a corkscrew-twisted ladder, called a double helix. The rungs are made up of 4 building blocks that form a sequence that tells the cells how to build proteins.

Changes in genes can prevent the gene from doing its job the way it normally would. Some differences in DNA, for example, can lead to incorrectly formed proteins that can't perform their functions.

What do genes do?

Genes serve as an instruction booklet for cells so they can do all that is needed for a person to develop, survive, and reproduce. For the instructions to be used by cells, a gene's DNA sequences must be converted into messages. The messages tell the body how to produce proteins, the complex molecules that do most of the work in our bodies.

For the genetic code to be read by the cells, it must undergo a 2-step conversion process

The double-stranded, double helix of DNA is transcribed into single-stranded RNA to be transcribed. RNA is translated into amino acids. It the precise order of these amino acids that leads the body to produce certain proteins.
STEP 1

Transcription

DNA is transcribed into a molecule called messenger ribonucleic acid (mRNA).

STEP 2

Translation

mRNA links amino acids together in a precise order to produce a specific protein. This is important because there are about 20 amino acids, which can be arranged to form many different proteins. Proteins have different functions in different parts of the body. Some proteins have a role in building specific tissues, while other proteins are involved in the body's functions that sustain life.

How do genes work?

A gene's instructions are written in a sequence made up of 4 nucleic acids, which serve as the building blocks of DNA: adenine (A), cytosine (C), guanine (G), and thymine (T). These nucleic acids form groups of 3, called codons, each of which corresponds to a specific amino acid and determines the composition of a protein. Scientists use the first letter of each nucleic acid to depict genetic code.

Potential variants leading to a gene's message not working properly

TCA changes to TAA. Then TCA is deleted. Next TCC is inserted between CAG and TCA. 3 new CAGs are inserted into in a series TCA changes to TAA. Then TCA is deleted. Next TCC is inserted between CAG and TCA. 3 new CAGs are inserted into in a series

Genetic variations

Occasionally, there is a "typo" when the genetic code deviates from its typical sequence. This kind of change from the typical sequence is called a gene variant.

For example, a gene variant may cause a cell to build a protein that works poorly or doesn't work at all. Sometimes, the error means that no protein is made. In some cases, cells can catch and correct these errors.

Not all variants are harmful. Some are beneficial and may lead to changes such as dimples, temperament, athletic performance, etc. Some variants create diversity needed in nature and may help organisms evolve and adapt to the environment.

What is a genetic disease?

Genetic diseases are caused—in whole or in part—by 1 or more gene variants. Many genetic diseases are present at birth, but others may not show symptoms until later in life. However, symptoms of single-gene diseases often occur in childhood and can cause severe illness, or even death. Diseases that involve multiple genes may be more likely to appear in adulthood or later in life.

How does a gene cause disease?

Single gene variant as a cause of genetic disease is depicted as a dot on a chromosome icon
Multiple gene variants as a cause of genetic disease
Variations to chromosomes as a cause of genetic disease depicted as four dots in a circle icon
Type A single gene variant Multiple gene variants Variations to the structures that carry genes
Type of disease Monogenic diseases Polygenic diseases, complex or multifactorial disorders Chromosomal diseases
Inheritance
  • Can be passed down to a child from parents

  • Can happen for the first time in the sperm or egg so that the child will have the variation, but the parents will not
  • Diseases often cluster in families, but there is not a clear-cut pattern of inheritance

  • Lifestyle factors, like exercise, diet, and exposure to pollution, can contribute to the development of these diseases
  • Most diseases caused by chromosomes are not passed from one generation to the next, although it is possible to inherit some types of chromosomal abnormalities

  • They occur as random events during the formation of reproductive cells
Examples of diseases in this category* Sickle cell disease, cystic fibrosis, myotonic dystrophy, Pompe disease, Duchenne muscular dystrophy, spinal muscular atrophy, X-linked myotubular myopathy (XLMTM) Some forms of cancer, type 2 diabetes mellitus, heart disease Down syndrome, Turner syndrome

These are examples and not a comprehensive/exhaustive list. Not all of these diseases can be treated with gene therapy modalities.

Single gene variant as a cause of genetic disease is depicted as a dot on a chromosome icon
Type A single gene variant
Type of disease Monogenic disease
Inheritance
  • Can be passed down to a child from parents

  • Can happen for the first time in the sperm or egg so that the child will have the variation, but the parents will not
Examples of diseases in this category* Sickle cell disease, cystic fibrosis, myotonic dystrophy, Pompe disease, Duchenne muscular dystrophy, spinal muscular atrophy, X-linked myotubular myopathy (XLMTM)
Multiple gene variants as a cause of genetic disease
Type Multiple gene variants
Type of disease Polygenic diseases, complex or multifactorial disorders
Inheritance
  • Diseases often cluster in families, but there is not a clear-cut pattern of inheritance

  • Lifestyle factors, like exercise, diet, and exposure to pollution, can contribute to the development of these disease
Examples of diseases in this category* Some forms of cancer, type 2 diabetes mellitus, heart disease
Variations to chromosomes as a cause of genetic disease depicted as four dots in a circle icon
Type Variations to the structures that carry genes
Critical name Chromosomal diseases
Inheritance
  • Most diseases caused by chromosomes are not passed from one generation to the next, although it is possible to inherit some types of chromosomal abnormalities

  • They occur as random events during the formation of reproductive cells
Examples of diseases in this category* Down syndrome, Turner syndrome

These are examples and not a comprehensive/exhaustive list. Not all of these diseases can be treated with gene therapy modalities.

Learn more

This is not a comprehensive list of resources, and it is provided for reference only.

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