Genetics Glossary of Terms

Allele – One of a pair of genes that appear at a particular location on a particular chromosome and control the same characteristic. An individual inherits two alleles for each gene, one from each parent.

Bases pairs – Two nucleotides on complimentary DNA strands. Human DNA consists of about 3 billion base pairs, and more than 99% of those are the same in all people. The order, or sequence, of these base pairs determines the information available for building and maintaining an organism.

DNA (deoxyribonucleic acid) – Contains the genetic instructions for each individual. The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T), which reside in sequence along a backbone of deoxyribose sugar and phosphates.

Chromosomes – Long stretches of DNA, which are bundled with proteins into organized structures. Chromosomes are responsible for carrying genomic material. Every individual has 22 pairs of autosomal chromosomes and two sex chromosomes.

Genes – Specified portions within the DNA, which act as instructions to make proteins, and control when and where those proteins are made. The Human Genome Project has estimated that humans have between 20,000 and 25,000 genes.

Genome – The set of all genes that specify traits in an individual.

Genotype – The composition of an individual’s genes.

Enzyme – A biological catalyst, usually a protein that speeds up the rate of a specific chemical reaction. The body contains thousands of different enzyme molecules, each specific to a particular chemical reaction.

Exons – The expressed portion of a gene, exons contain the DNA sequences that are converted to mRNA during transcription, and thus by way of the genetic code, determine the amino acid sequence in the protein product.

Messenger RNA (mRNA) – A ribonucleic acid (RNA) version of a gene that leaves the cell nucleus and moves to the cytoplasm. During protein synthesis, the ribosome, a cytoplasmic organelle, moves along the mRNA and translates each three-base triplet into the corresponding amino acid.

Nucleotide – The DNA subunit containing a base, sugar and phosphate. Nucleotides are arranged in a double-stranded configuration, forming a ladder-like structure called a double helix.

Phenotype – The physical effect that an individual’s genotype has on their body.

Single nucleotide polymorphism (SNP) – Pronounced “snip,” a single nucleotide locus with two or more naturally occurring alleles defined by a single base pair substitution. These small differences contribute to each individual’s unique physical features, including how fast medication is metabolized.