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Glossary

Locus
The position on a chromosome where a gene, or some other sequence, is located. The chromosome number, its arm, and its place designate the locus of a gene. For example, the human HLA complex is contained within approximately 4MBp region at locus 6p21.3, meaning it is at locus 21.3 of the short arm (p) of chromosome VI.

Gene
A unit of genetic material (DNA) that carries the directions a cell uses to perform a specific function, such as making a given protein.

Transcription
The first major step in gene expression, in which the information coded in DNA, is transcribed (copied) into a molecule of RNA

mRNA
messenger RNA - the end product of transcription of a gene. Both introns and exons are initially transcribed to form a pre-RNA, which undergoes splicing by nuclear enzymes to remove the introns and create mRNA, which is then used as a template for protein production on the ribosomes of the cell.

Intron
A DNA segment of largely unknown function within a gene that specifically interrupts the coding (exon) sequences of that gene. Introns are transcribed as part of the normal gene primary transcript, but intron sequences are not found in the functional mRNA. Intron sequences are removed from the primary transcript by a splicing mechanism.

Exon
The parts of a gene that are transcribed and remain in mRNA after splicing. This includes translated regions of the gene (protein coding regions) as well as 5' and 3' untranslated regions of the mRNA molecule

Splicing
The removal of introns from the sequence of mRNA. When an mRNA molecule is synthesized from a DNA template, introns are transcribed (see transcription) along with exons. In the splicing process, this material is cut out and the exons are joined together to form a continuous coding sequence

Splice-site
A small subsequence that occurs at the boundary between introns and exons that is used to remove the introns prior to translations.

Translation
The process in which the genetic code carried by mRNA directs the synthesis of proteins from amino acids. Compare transcription

HLA
The acronym for Human Leukocyte Antigens. These proteins are on the surface of most tissue cells and give an individual his or her unique tissue type. HLA factors are inherited from mother and father and the greatest chance of having the same HLA type is between siblings. On average one in four siblings is expected to share the same HLA.type. The testing for HLA antigens is referred to as "tissue typing." There are six major groups of HLA antigens A, B, C, D, Dr, and DQ.

HLA Allele
An alternative form/sequence of a HLA gene (one member of a pair) that is located at a specific HLA locus

Homozygous
Adjective apllies to diploid organism that has inherited the same allele (of any particular gene) from both parents, i.e. carries identical alleles at the corresponding sites on homologous chromosomes. Apllies also to cells or nuclei from such an organism.

Heterozygous
Adjective apllies to diploid organism that has inherited different alleles (of any particular gene) from each parent, i.e. carries different alleles at the corresponding sites on homologous chromosomes. Also apllies to cells or nuclei from such an organism. Homozygosity (noun).

SNP
Single nucleotide polymorphism. This a polymorphism where the alleles differ by the replacement of a single nucleotide in the DNA sequence. For most SNP's, only two of the four possible nucleotides (A, T, C, or G) are observed. At least two alleles with frequencies greater than 1 percent must be present in the population to call such a polymorphism a true SNP.

In/del
Insertion and/or deletion. A type of variation caused by the addition or deletion of one or more adjacent base pairs.

Polymorphism
Differences in DNA sequences that occur naturally in a population. Single nucleotide substitutions, insertions and deletions of nucleotides and repetitive sequences (microsatellites) are all examples of a polymorphism. The position at which such a sequence difference is found is a polymorphic site
At least two alleles with frequencies greater than 1 percent must be present in the population to call such a polymorphism a true SNP.

Homologous
(for genes) Refers to genes in two different species with similarity in DNA sequence that suggests a common evolutionary origin. 
(for chromosomes) Refers to a pair of chromosomes that contain the same genes at the same loci, one derived from each parent. Eg. the two copies of chromosome 1 in humans are homologous to each other