Genes are segments of DNA (deoxyribonucleic acid) that are located inside every human cell. The DNA inside each cell is tightly coiled in structures called chromosomes.

Each chromosome contains a single thread of DNA with many different genes. The genes provide instructions for different traits, such as eye or hair color, or male or female sex.

Chromosomes come in pairs. Humans have 46 chromosomes, in 23 pairs. People inherit chromosomes from their parents. A child gets one of each pair of chromosomes from their mother and one of each pair of chromosomes from their father.

The term “genetic inheritance” is the passing down of DNA from parents to children.

What is a genetic disorder?

Genetic disorders, such as Down syndrome or cystic fibrosis, occur when:

  • there’s a change (mutation) in a gene on a chromosome
  • a chromosome is missing a part (called a deletion)
  • when genes move from one chromosome to another (called a translocation)
  • when a cell has extra chromosomes or missing chromosomes

A worldwide gene research project, called the Human Genome Project, is creating a map of all human genes and their location on chromosomes. Doctors hope to use this map to find and treat genetic disorders.

Genes are involved in almost every human trait and disease. They influence how your body responds to:

  • certain health conditions, such as infections
  • medications
  • treatments for health conditions
  • certain behaviors, such as smoking or alcohol use

The more we understand how genes affect our health or are linked to disease, the earlier doctors can respond to diseases and provide more effective targeted treatments.