Genes are made of DNA, which determines what an organism is like, its appearance, how it survives, and how it behaves in its environment. All living beings have genes, which can also affect a person’s health throughout their life.

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Different organisms have different numbers of genes. The human genome, which is all human genes together, contains about 30,000 genes.

All humans are 99.9% identical across their human genome, with only minor genetic differences leading to visible physical differences.

These differences, which may be inherited or result in the interaction between a person’s genes and the environment, can also explain why some people develop certain diseases and others do not.

A geneticist is a person who studies genes and how to target them to improve aspects of life. Genetic engineering can provide a range of benefits for people, including preventing diseases in humans.

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Genes are strands of DNA that contain the biological instructions for life. Each gene contains sequences that determine physical and biological traits.
Medical illustration by Bailey Mariner

Genes are composed of a substance called deoxyribonucleic acid, or DNA. DNA contains the biological instructions that allow for the development, growth, and reproduction of life.

Chromosomes, which are located in each cell’s nucleus, contain genes. Each gene contains sequences of DNA that are instructions for making specific proteins.

These proteins lead to the expression of specific physical characteristics like hair color, height, and eye color. They can also determine a person’s risk of having or developing certain genetic disorders.

DNA passes from adult organisms to their offspring during reproduction. This means that people inherit their gene-containing chromosomes from their parents.

Chromosomes come in pairs. Humans have 46 chromosomes. A person inherits one set of 23 chromosomes from their mother and another set of 23 chromosomes from their father.

Learn more about the structure of DNA.

A gene consists of a long combination of four different nucleotide bases, which are the basic building blocks of DNA. There are many possible combinations.

The four types of bases found in nucleotides are:

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

A gene consists of two strands wound around each other. Each strand holds together via bonds between the bases. Different bases pair together as follows:

  • adenine pairs with thymine
  • cytosine pairs with guanine

The sequence of these bases determines what instructions exist in a strand of DNA. For example, the sequence ATCGTT might instruct for blue eyes, while ATCGCT might instruct for brown eyes.

Human chromosomes range in size from about 50 million-300 million base pairs. The entire human genome contains about 3 billion bases and about 20,000 genes on 23 pairs of chromosomes in humans.

Genes affect hundreds of internal and external factors, such as whether a person will get a particular eye color or what diseases they may develop.

Changes in genes can also lead to incorrectly formed proteins that cannot perform their functions. These are called gene mutations and may lead to genetic disorders.

Some diseases such as sickle-cell anemia and Huntington’s disease occur due to gene mutations. Because genes pass down from parents to children, some diseases tend to run in families.

Gene mutations can also occur due to exposure to toxins in the environment, like cigarette smoke.

Diploid is a term referring to an organism with cells that contain two complete sets of chromosomes. Humans are diploids. Most of the body’s cells contain 23 chromosome pairs.

One set of chromosome pairs comes from the female parent, and the other set comes from the male parent.

Two of the chromosomes called the X and Y chromosomes to determine an embryo’s sex as male or female.

  • Females have two X chromosomes
  • Males have one X and one Y chromosome

The female parent gives an X chromosome to the child. The male parent may contribute an X or a Y chromosome. The remaining 22 chromosomes are called autosomal chromosomes. Scientists refer to them as chromosomes 1 through 22.

The Human Genome Project (HGP) is a major scientific research project started in 1990. The goal of the project was to identify and map the entire human genome.

By doing this, the scientists hoped to find powerful tools to understand the genetic factors in human disease and to open the door for new strategies for diagnosis, treatment, and prevention.

Scientists completed the HGP in 2003. All the data generated is available for free access on the internet. Apart from humans, the HGP also looked at other organisms and animals, such as the fruit fly and E. coli.

Scientists have discovered more than three billion nucleotide combinations, or combinations of ACGT, in the human genome. They also discovered more than 1,800 genes that can cause disease.

The human genome project also allowed scientists to make around 2,000 genetic tests that help doctors diagnose genetic disorders. This is called genetic testing and can determine if a person has a gene suspected of causing an inherited disease.

Genetic testing can also look for changes, or mutations, in a person’s DNA that may put a person at risk for a specific disease. The results can help healthcare professionals diagnose conditions.

Doctors may use genetic testing for various reasons, including:

  • to identify genetic diseases in unborn babies
  • to screen newborn babies for certain treatable conditions
  • to lower the risk of genetic diseases in embryos created using assisted reproductive technology
  • to find out if a person carries a gene for a disease that could be passed on to a child
  • to see if a person is at an increased risk of developing a certain disease
  • to help a doctor in deciding the best medication and dosage for a person (pharmacogenomic testing)

Scientists created a catalog of common genetic variations or haplotypes in the human genome in 2005. It is called the Haplotype map, or “HapMap.” This data has helped to speed up the search for the genes involved in common human diseases.

In recent years, geneticists have found another layer of heritable genetic data that is not held in the genome but in the “epigenome,” a group of chemical compounds that can tell the genome what to do.

DNA holds the instructions for building proteins in the body. These proteins are responsible for a number of functions in a cell.

The epigenome is made up of chemical compounds and proteins that can attach to DNA and direct a variety of actions. These actions include turning genes on and off. This can control the production of proteins in particular cells and alter gene expression.

Gene switches can turn genes on and off at different times and for different lengths of time. The differences among cells are determined by how and when different sets of genes are turned on or off in various cells.

Gene marking

When epigenomic compounds attach themselves to DNA in the cell and modify the function, it means they have “marked” the genome.

The marks do not change the sequence of the DNA, but they do change the way cells use the DNA’s instructions.

The marks can pass from cell to cell as they divide, and they can even pass from one generation to the next.

Specialized cells can control many functions in the body. For example, specialized cells in red blood cells make proteins that carry oxygen from air to the rest of the body. The epigenome controls many of these changes within the genome.

The chemical tags on the DNA and histones – which are proteins that support the structure of a chromosome – can rearrange as the specialized cells and the epigenome change throughout a person’s lifetime.

Lifestyle and environmental factors such as smoking, diet, and infectious diseases can bring about changes in the epigenome. They can expose a person to pressures that prompt chemical responses.

These responses can lead to direct changes in the epigenome, and some of these changes can be damaging. Some human diseases happen due to malfunctions in the proteins that “read” and “write” epigenomic marks.

For example, cancer can result from changes in the genome, the epigenome, or both. Changes in the epigenome can switch on or off the genes that are involved in cell growth or the immune response. These changes can cause uncontrolled growth, a feature of cancer, or a failure of the immune system to destroy tumors.

Researchers in epigenomics are focused on trying to chart the locations and understand the functions of all the chemical tags that mark the genome. This information may lead to a better understanding of the human body and knowledge of ways to improve human health.

Gene therapy

Gene therapy is a medical technique that uses sections of DNA to treat or prevent a disease or medical disorder. Genes are inserted into a patient’s cells and tissues to treat a disease.

Gene therapy often works by adding copies of a broken gene or by replacing a defective or missing gene with a healthy version of that gene.

Gene therapy is still in its early stages. However, scientists are using it to treat inherited diseases like hemophilia and sickle cell disease, as well as acquired disorders like leukemia.

Gene testing to predict cancer

Another use of genetic information is to help predict who is likely to develop a disease like early-onset Alzheimer’s disease and breast cancer.

For example, females with the BRCA1 gene have a significantly higher chance of developing breast cancer. A female can have a test to find out whether she carries that gene.

BRCA1 carriers have a 50% chance of passing the anomaly to each of their children.

Genetic tests for personalized therapy

Genetic testing can help doctors determine which specific medicines are best for certain patients depending on their genetic makeup. This is called pharmacogenetic testing.

It looks for changes in genes that can help determine what medications or doses are appropriate for a patient. Doctors can use it to predict if a person will have a serious side effect from a medication.

The following are answers to common questions about genes.

What are genes vs. DNA?

A gene is a basic unit of inheritance passed on from male and female parents to their children. Genes contain DNA, which is made up of sequences that determine the physical and biological traits of each person.

Why are genes so important?

Genes are the building blocks of life. They contain information for making specific molecules and proteins that allow human cells to function and that control how the body grows and operates. They also lead to the expression of particular physical characteristics and traits like hair or eye color.

What are examples of things determined by genes?

Genes are made up of DNA, which contains instructions to produce molecules called proteins. These proteins are responsible for characteristics including eye color, blood type, and height. Genes can also determine a person’s risk of having or developing certain diseases like breast cancer or sickle cell anemia.

How many genes are in a human?

The Human Genome Project has determined that humans have an estimated 30,000 genes.

Genes are a set of instructions passed down from parents to offspring. They contain the information that determines a person’s specific physical and biological traits, like hair color, eye color, and blood type.

Most genes code for specific proteins which have different functions throughout the body and allow humans to live, grow, and reproduce.

Genes are made of sections of DNA. DNA is made of chemical building blocks called nucleotides. A gene consists of four different nucleotide bases, which can be sequenced in different ways.

Different sequences of bases determine different instructions, which account for various physical traits, like having blue eyes or brown eyes.

Changes in genes can lead to incorrectly formed proteins that can’t function correctly. These are called gene mutations and may lead to genetic disorders.

Researchers are studying genetic testing to be able to identify changes in a person’s DNA that may show if they are at risk for developing a disease or passing down a disease to their offspring.