What Are Genes, DNA, and Chromosomes?

Understanding Genetics as It Relates to the Human Body

Genetics is a branch of science that studies genes and hereditary (inheritance) patterns. This includes the study of chromosomes (the structure within cells that contain genes) and DNA (the building blocks of genes).

By understanding how genes work and influence human health, scientists are increasingly able to identify the causes of illnesses, predict the risk of certain diseases, and develop drugs able to target certain specific cells, including cancer.

Scientist holding DNA gel in front of samples for testing in laboratory
Andrew Brookes / Getty Images

What Is a Genome?

The best way to understand genetics is to break down an organism's genome. The genome is the complete set of genetic instructions of an organism. It contains all of the information needed to build that organism and allow it to grow and develop. It determines the physical characteristics of an organism and how certain cells, tissues, and organs work.

The genome is comprised of three parts:

  • DNA: a molecule that provides the genetic instructions of an organism
  • Genes: a collection of DNA that is encoded to produce specific proteins
  • Chromosomes, structures within cells that contain genes

Almost every living organism has a genome, including bacteria, plants, birds, fish, and mammals. Some species contain more genes than others, The human genome, for example, is made up of about 20,000 to 25,000 genes. The genome of bacteria consists of one strand of DNA.

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.

What Is DNA?

In the simplest terms, DNA (short for deoxyribonucleic acid) are the building blocks of our genes.

Within DNA is a unique chemical code that guides an individual's 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 (G)

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 with sugar and phosphate molecules to form what ultimately looks like a spiraling ladder, known as a double helix.

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

Human DNA consists of around 3 million 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 distinct portion of a cell’s DNA that is encoded for a specific purpose. Some genes act on instructions to produce proteins, the building blocks of tissues and a source of the body's energy. Different proteins play different roles in the structure, function, and regulation of the body's tissues and organs.

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

Genes are the basic physical unit of inheritance. How they are encoded ultimately determines your physical traits and how your body works. Every person has two copies of each gene, one inherited from each parent.

Different versions of a gene are referred to as alleles. The alleles you inherit from your parents may determine if you have brown eyes or blue eyes. Other alleles are directly associated with diseases like cystic fibrosis or Huntington’s disease, while others still can increase the risk of certain diseases such as breast cancer.

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

What Is a Chromosome?

Genes are packaged into bundles called chromosomes. Humans have 23 pairs of chromosomes for a total of 46 individual chromosomes. Chromosomes are contained within the nucleus of nearly every cell of the body.

One pair of chromosomes, called the X and Y chromosomes, determines whether you are 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 allele—one from each parent—for a trait to develop (such as green eyes or cystic fibrosis).

What Is Genetic Variation?

Some people think of genetic variations as mutations, but that's only part of the picture. There are actually three types of genetic variation:

  • Genetic mutation is a change in the sequence of DNA. This can occur due to copying errors made during cell division, exposure to radiation, exposure to chemicals called mutagens, or infections by certain viruses. Disorders like sickle cell disease, Tay-Sachs disease, and phenylketonuria are all due to specific genetic mutations.
  • Genetic recombination is a process in which pieces of DNA are broken, recombined, and repaired to produce a new combination of alleles. Down syndrome is one such example of gene recombination.
  • Genetic migration is an evolutionary process in which the addition or loss of people in a population changes the gene pool, making certain traits less common or more common. A hypothetical 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.

A Word From Verywell

Although genetics may be difficult to comprehend, it 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 so you can avoid them.

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.

By grasping the general concepts of genetics, you can better appreciate their applications in medicine and their implications on your health.

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