Genes & Traits

All humans are basically the same — that is, we are all members of the same species. Yet we are also each unique, with different traits that allow us to stand out as individuals. Our similarities and differences are due to similarities and differences in our genetic "instructions." Our own set of genetic instructions, our “genes,” determines our particular traits, inherited from our parents.

A gene is the basic physical unit of inheritance. Genes are passed from parents to offspring and contain the information needed to specify traits, like height and eye color, but also susceptibility to getting a disease, and how a person’s body handles a disease. Genes are arranged, one after another, on long DNA molecules called chromosomes, only a portion of which corresponds to a single gene. Humans have approximately 20,000 genes arranged on their chromosomes.

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DNA (Deoxyribose Nucleic Acid)

DNA is the chemical name for the molecule that carries genetic instructions in all living things. Part of the DNA molecule consists of a collection of molecular structures called nucleic acid base pairs, where adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G).

DNA's Chemical Bases

Source: Adapted and republished with permission from Coriell Institute for Medical Research, DNA, Genes, and SNPs, 2016 ( This material is protected by copyright law.

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  • Genetic code and proteins: The coded genetic information hard-wired into DNA is used by the body to create proteins. Proteins are the fundamental components of the body that determine how everything is organized and acts.
  • DNA as the instruction manual: The DNA code provides the instructions for how, when and how many proteins to make in the human body. Since proteins are the fundamental building blocks of life, DNA acts as an instruction manual for a human body. We are just beginning to understand what all of the instructions mean, but with our growing knowledge, doctors can refer to our DNA manual to help analyze problems when something is wrong.

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The genome is the entire set of genetic instructions found in every cell in the human body. The human genome consists of 23 pairs of chromosomes, found in the nucleus, as well as a small chromosome found in a part of cells called mitochondria. Each set of 23 chromosomes contains approximately 3.1 billion bases of DNA.

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Human Genome Project

The Human Genome Project was an international project, completed in April 2003, that mapped and sequenced the entire three billion base pairs of DNA of the human genome, the molecular instruction book of human life.