Glossary

Allele
One of the variant forms of a gene at a particular locus, or location, on a chromosome. Different alleles produce variation in inherited characteristics. For STR markers, each allele is the number of repeats of the short base sequence.
Base Pair
Two bases that form a "rung of the DNA ladder." A DNA nucleotide is made of a molecule of sugar, a molecule of phosphoric acid, and a molecule called a base. The bases are the "letters" that spell out the genetic code. In DNA, the code letters are A, T, G, and C, which stand for the chemicals adenine, thymine, guanine, and cytosine, respectively. In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.
Chromosome
One of the threadlike "packages" of genes and other DNA in the nucleus of a cell.
DNA
The chemical inside the nucleus of a cell that carries the genetic instructions for making living organisms.
DYS#
D=DNA, Y=Y chromosome, S=a unique DNA segment. A label for genetic markers on the Y chromosome. Each marker is designated by a number, according to international conventions. At present, virtually all the DYS designations are given to STR markers (a class often used in genetic genealogy).
Gene
The functional and physical unit of heredity passed from parent to offspring. Genes are pieces of DNA, and most genes contain the information for making a specific protein.
Genome
All the DNA contained in an organism or a cell, which includes both the chromosomes within the nucleus and the DNA in mitochondria.
Locus
A point in the genome, identified by a marker, which can be mapped by some means. It does not necessarily correspond to a gene. A single gene may have several loci within it (each defined by different markers) and these markers may be separated in genetic or physical mapping experiments. In such cases, it is useful to define these different loci, but normally the gene name should be used to designate the gene itself, as this usually will convey the most information.
Marker
Also known as a genetic marker, a segment of DNA with an identifiable physical location on a chromosome whose inheritance can be followed. A marker can be a gene, or it can be some section of DNA with no known function. Because DNA segments that lie near each other on a chromosome tend to be inherited together, markers are often used as indirect ways of tracking the inheritance pattern of genes that have not yet been identified, but whose approximate locations are known.
Microsatellite
Repetitive stretches of short sequences of DNA used as genetic markers to track inheritance in families.
Mutation
A permanent structural alteration in DNA.
Short Tandem Repeats (STR)
A genetic marker consisting of multiple copies of an identical DNA sequence arranged in direct succession in a particular region of a chromosome. Occasionally, one will mutate by the gain or loss of one repeat. (Also known as microsatellite)