How to Speak Genetics: A Glossary

Ever get confused about what's what when it comes to genetics? We've defined the main terms.


Nucleotide Commonly called "the building blocks of DNA," nucleotides consist of a nitrogen-containing chemical base either adenine, guanine, thymine or cytosine attached to a sugar and a phosphate group. One nucleotide's sugar group will bind to another nucleotide's phosphate group, creating a strand of DNA.

Base Pair A pair of nucleotides on opposite strands joined by a hydrogen bond. Adenine pairs up with thymine; guanine pairs with cytosine. Through their bonds, base pairs join complementary or "mirror" strands of DNA.

Deoxyribonucleic acid (DNA) A long molecule found in the nucleus of a cell and shaped like a double helix. DNA, which is built of sequences of nucleotides, contains genetic information that determines the traits of an organism and passes from parent to offspring. If DNA is a ladder, the sugars and phosphate groups join together in sequence to act as the railing, and the base pairs act as "rungs."

Gene A segment of DNA considered a "unit of heredity." The sequence of nucleotides in the gene acts like a code, instructing cells regarding how and when to build a particular protein, which will then influence other actions. Some genes correspond directly to an actual trait of an organism; in other cases, several genes work together in concert to produce observable traits.

Intron A DNA sequence in a gene that does not code for anything, sometimes referred to as "junk DNA." Introns are interspersed with exons.

Exon A DNA sequence in a gene that codes for a gene product, such as a protein.

Chromosome A threadlike structure in the cell nucleus composed of a long stretch of DNA (in humans, hundreds of millions of base pairs in length) wrapped around a protein scaffold. Chromosomes are the structures by which DNA is physically transmitted from one generation to the next. Humans have 23 pairs of them.

Allele A variation of a gene located at a specific location in a chromosome. Organisms have two alleles for each trait one inherited from each parent. The two alleles may be the same or different. If they are different, the dominant allele will often take precedence over the recessive one, and be "expressed." For example, if a person has one allele for brown eyes, and one for blue, they'll have brown eyes, because that gene variation is dominant.

Genome The full DNA sequence of an organism. The human genome is about three billion chemical base pairs long.

Genetics The branch of biology that studies heredity and variation in organisms.

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• What's Genetic Engineering?
• Human Genome Turns 10: 5 Lessons Learned

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