How DNA, Genes and Heredity Work: The Basics of Inheritance Explained

Genetics is the study of genes and heredity — how traits are passed from parents to children through DNA, according to the U.S. National Institute of General Medical Sciences (NIGMS). DNA, or deoxyribonucleic acid, is the hereditary material found in nearly every cell of the human body, according to MedlinePlus Genetics, a resource maintained by the U.S. National Library of Medicine.

What DNA actually looks like

DNA is built from four chemical bases — adenine (A), thymine (T), guanine (G) and cytosine (C) — that pair up (A with T, C with G) to form rungs on a twisted, ladder-like structure called a double helix, MedlinePlus explains. Human DNA contains about 3 billion of these base pairs, and more than 99 percent of them are identical in every person; the remaining fraction accounts for individual differences, MedlinePlus states.

Most DNA sits inside the cell nucleus, where it is called nuclear DNA. A small amount also exists inside mitochondria, the cell structures that convert food into usable energy; this is known as mitochondrial DNA, or mtDNA, according to MedlinePlus.

Genes: the units of heredity

A gene is described by MedlinePlus as "the basic physical and functional unit of heredity." Genes are made of DNA and typically carry instructions for building proteins that the body needs to function; some genes instead regulate other genes. Human genes range from a few hundred DNA base pairs to more than 2 million base pairs in length, according to MedlinePlus. Estimates of the total protein-coding gene count have varied: the Human Genome Project (1990–2003) initially estimated 20,000 to 25,000 such genes, while more recent estimates put the figure at around 19,900, per MedlinePlus, and NIGMS cites approximately 20,000.

Genes make up only about 1 percent of human DNA, according to NIGMS; the rest consists of other sequences, including regions that regulate when and how genes are switched on or off.

Chromosomes and how traits pass between generations

Genes are carried on chromosomes, and people typically have 23 pairs of them, per NIGMS. One chromosome in each pair comes from the mother and one from the father, which means a person usually inherits two copies of every gene — one from each parent, MedlinePlus notes. Because chromosome pairs carry matching genes that can differ slightly, this is the basic mechanism behind why children resemble, but are not identical to, their parents.

A gene variant is called an allele. Less than 1 percent of genes vary between individuals, but these variants are responsible for differences in physical traits such as eye or hair color, and in some cases for inherited disease risk, according to MedlinePlus and NIGMS.

Dominant, recessive and other inheritance patterns

When a gene has two different alleles, how a trait appears depends on the inheritance pattern. In autosomal dominant inheritance, a single altered copy from one parent is enough for the trait or condition to appear; a person with such a variant has a 50 percent chance of passing it to each child, MedlinePlus states, citing standard risk-assessment figures used in clinical genetics.

In autosomal recessive inheritance, a trait only appears when both gene copies carry the variant. People with just one copy are carriers and are usually unaffected. If both parents are carriers, MedlinePlus states there is a 25 percent chance per pregnancy that a child will be affected, a 50 percent chance the child will be an unaffected carrier, and a 25 percent chance the child will neither be affected nor a carrier.

X-linked inheritance depends on the sex chromosomes. According to MedlinePlus, an affected father passes an X-linked variant to all of his daughters but none of his sons, because sons receive his Y chromosome instead. Y-linked conditions, by contrast, can only affect males, and an affected father passes the condition to all of his sons.

Researchers at the University of Utah's Genetic Science Learning Center caution that "there is no universal mechanism by which dominant and recessive alleles act," and that the two terms describe outcomes rather than one fixed biological process. They point to sickle-cell disease as an example: two copies of the sickle-cell allele cause the disease, but carriers with one copy have some sickled cells and, under certain conditions, gain resistance to malaria — a pattern that does not fit a simple dominant/recessive label.

Beyond simple patterns

Not every trait follows a clean dominant or recessive rule. NIGMS notes that many conditions involve multiple genes and environmental factors, making inheritance harder to predict than for single-gene traits. MedlinePlus adds that for such conditions, family history and genetic test results can sometimes refine — but not fully determine — the chance that a trait will be passed on.

Epigenetics adds another layer: chemical tags attached to DNA or to the proteins DNA wraps around can switch genes on or off without changing the underlying DNA sequence itself. NIGMS cites smoking as an example of an environmental factor that can trigger such changes.