What Are Genes, DNA, and Chromosomes?

Genetics is the study of heredity, meaning the traits that we inherit from our parents, they inherited from their parents, and so on. These traits are controlled by coded information found in every cell of the body.

This code is written in DNA, genes, and chromosomes. Together, these units make up the complete set of genetic instructions for every individual—referred to as a genome—including our sex, appearance, and medical conditions we may be at risk of. No two people have the same genome.

This article offers a basic explanation of genetics, including what genes, DNA, and chromosomes are. It also looks at errors in genetic coding that may place a person at risk of genetic diseases or birth defects.

What Is a Genome?

In the simplest terms, a genome is the complete set of genetic instructions that determine the traits (characteristics and conditions) of an organism. It is made up of DNA, genes, and chromosomes.

DNA is a molecule in cells that carries the genetic information. It is made up of building blocks. The genetic coding of our traits is based on how these building blocks are arranged.

Genes are segments of DNA that determine our traits. Every human has between 20,000 and 25,000 different genes, half of which are inherited from our biological mothers and the other half from our biological fathers.

Chromosomes are long, bundled strands of DNA, each of which contains many genes. In total, there are two sets of 23 chromosomes in a cell. Each set is inherited from our biological parents.

Your genome determines how your body will develop before birth. It directs how you will grow, look, and age. And, it will determine how cells, tissues, and organs of the body work (including times when they may not work as they should).

While the genome of each species is distinct, every organism within that species has its own unique genome. This is why no two people are exactly alike, even twins.

What Is DNA?

In the simplest terms, DNA (deoxyribonucleic acid) is the building blocks of your genes.

Within DNA is a unique chemical code that guides your growth, development, and function. The code is determined by the arrangement of four chemical compounds known as nucleotide bases.

The four bases are:

• Adenine (A)
• Cytosine (C)
• Guanine (G)
• Thymine (T)

The bases pair up with each other—A with T and C with G—to form units known as base pairs. The pairs are then attached to form what ultimately looks like a spiraling ladder, known as a double helix.

The specific order, or sequence, of bases determines which instructions are given for building and maintaining an organism.

Human DNA consists of around 3 billion of these bases, 99% of which are exactly the same for all humans. The remaining 1% is what differentiates one human from the next.

Nearly every cell in a person’s body has the same DNA.

What Is a Gene?

A gene is a unit of DNA that is encoded for a specific purpose.

Some genes provide instructions to produce particular proteins. Proteins are molecules that not only make up tissues like muscles and skin but also play many critical roles in the structure and function of the body.

Genes are encoded to produce RNA (ribonucleic acid), a molecule that converts the information stored in DNA to make the protein.

How genes are encoded will ultimately determine how you look and how your body works. Every person has two copies of each gene, one inherited from each parent.

Different versions of a gene are known as alleles. The alleles you inherit from your parents may determine, for example, if you have brown eyes or blue eyes. Other alleles may result in congenital (inherited) disorders like cystic fibrosis or Huntington’s disease, Other alleles may not cause disease but can increase your risk of getting things like cancer.

Genes only make up between 1% and 5% of the human genome. The rest is made up of non-coded DNA that doesn't produce protein but helps regulate how genes function.

What Is a Chromosome?

Genes are packaged into bundles known as chromosomes. Humans have 23 pairs of chromosomes for a total of 46 individual chromosomes. Chromosomes are contained within the control center (nucleus) of nearly every cell of the body.

One pair of chromosomes, called the sex chromosomes, determines whether you are born male or female. Females have a pair of XX chromosomes, while males have a pair of XY chromosomes.

The other 22 pairs, called autosomal chromosomes, determine the rest of your body’s makeup. Certain genes within these chromosomes may either be dominant or recessive.

By definition:

• Autosomal dominant means that you need only one copy of an allele from one parent for a trait to develop (such as brown eyes or Huntington's disease).
• Autosomal recessive means that you need two copies of the allele—one from each parent—for a trait to develop (such as blue eyes or cystic fibrosis).

What Is Genetic Variation?

Genes are prone to coding errors. Many errors won't make any significant difference in the structure or function of a person's body, but some can.

Some genetic variations will directly cause a defect or disease, some of which may be apparent at birth and others of which may only be seen later in life. Other variations can lead to changes in the gene pool that will affect inheritance patterns in later generations.

There are three common types of genetic variation:

Genetic Mutations

A genetic mutation is a change in the sequence of DNA. This is often due to copying errors that occur when a cell divides. It can also be caused by an infection, chemicals, or radiation that damages the structure of genes.

Genetic disorders like sickle cell disease, Tay-Sachs disease, and phenylketonuria are all caused by the mutation of a single gene. Radiation-induced cancer is caused by genetic changes caused by excessive exposure to medical or occupational radiation.

Genetic Recombination

Genetic recombination is a process in which pieces of DNA are broken, recombined, and repaired to produce a new allele. Also referred to as "genetic reshuffling," recombination occurs randomly in nature as a normal event during cell division. The new allele is then passed from parents to offspring.

Down syndrome is one such example of genetic recombination.

Genetic Migration

Genetic migration is an evolutionary process in which the addition or loss of people in a population changes the gene pool, making certain traits either less common or more common.

A theoretical example is the loss of red-haired people from Scotland, which over time may result in fewer and fewer Scottish children being born with red hair.

Summary

DNA is the building blocks of genes that contain the coded instruction for building and maintaining a body. Genes are a portion of DNA that are tasked with making specific proteins that play a critical role in the structure and function of the body. Chromosomes are structures containing many genes each. They are passed from parents to offspring and determine an individual's unique traits.

Together, DNA, genes, and chromosomes make up each organism's genome. Every organism—and every individual—has a unique genome.

A Word From Verywell

Genetics increasingly informs the way in which diseases are diagnosed, treated, or prevented. Many of the tools used in medicine today were the result of a greater understanding of DNA, genes, chromosomes, and the human genome as a whole.

Today, genetic research has led to the development of targeted drugs that can treat cancer with less damage to non-cancerous cells. Genetic tests are available to predict your likelihood of certain diseases.

Genetic engineering has even allowed scientists to mass-produce human insulin in bacteria and create RNA vaccines like some of those used to treat COVID-19.