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Glossary

Allele: One of the variant forms of the DNA sequence at a particular locus, or location, on a chromosome. Different alleles can produce variation in inherited characteristics such as hair color or blood type. In an individual, one form of the allele (the dominant one) may be expressed more than another form (the recessive one). Other alleles may have no affect (are silent) but may tag genes or other nearby alleles that are causative or contribute to a genetic characteristic, such as a disease or disease susceptibility.

Autosome: Any chromosome other than the sex chromosomes, X and Y. Humans have 22 pairs of chromosomes.

cDNA: DNA synthesized by the enzyme reverse transcriptase using mRNA as a template, either experimentally or in vivo.

Chromosome: a threadlike body in the cell nucleus that carries the genes in a linear order

Clone: DNA clones are populations of identical DNA molecules which have been purified by cell-based cloning methods or by PCR.

DNA (Deoxyribonucleic Acid): The chemical inside the nucleus of a cell that carries the genetic instructions for making living organisms. DNA is a double stranded molecule composed of sugar, phosphate and 4 different bases adenine, thymine, cytosine and guanine.

DNA library: A collection of DNA clones which is meant to collectively represent a starting population of DNA. For a genomic DNA library, the starting DNA is the total DNA from a given cell population (which shows little variation between different cell types). In the case of cDNA library, the starting DNA is cDNA prepared using reverse transcriptase from single-stranded RNA from a specific tissue.

EST: An expressed STS (sequence tagged site) obtained by randomly selecting a cDNA clone for sequencing and designing specific primers for specifically PCR amplifying the corresponding fragment from genomic DNA.

Enzyme: A protein produced by a living organism, capable of catalyzing a chemical reaction. Almost all processes in living organisms require some form of enzyme to cause the reactions to occur at a rate sufficient to support life. There are a very wide variety of enzymes, each specifically catalyzing a different chemical reaction, the sum of which cause the bulk of the physiological changes observed as life processes.

Gene: A gene is a segment of the DNA molecule that contains the instructions for how, when and where our bodies make each of the many thousands of proteins required for life. Each gene is comprised of thousands of combinations of four letters that make up our genetic code: A,T,C, and G. Each gene's code combines the "letters" A, T, C and G in various ways, spelling out the "words" that specify which amino acid is needed at every step in the process of making the proteins required for our bodies to develop and function.

Genetic map: A map which relies on tracing the inheritance of phenotypes and/or polymorphic markers, through generations. Polymorphic loci are positioned relative to one another on the basis of the frequency with which they recombine during meiosis. The unit of distance is 1 centiMorgan (1cM) which denotes a 1% chance of recombination.

Genome: the collective name for the different DNA molecules found in the cells of a particular species. In humans, the genome comprises of a complex nuclear genome with about 30000 genes, and a very simple mitochondrial genome with 37 genes.

Genome Scan: Analysis of the genome by typing it for multiple sequence variations.
Homologs (chromosomes): The two copies of a chromosome in a diploid cell. Unlike sister chromatids, homologous chromosomes are not copies of each other. One was inherited from the father and the other from the mother.

Homologs (genes): Two or more genes whose sequences are significantly related because of a close evolutionary relationship, either between species (orthologs) or within a species (paralogs)

Heterozygote: An individual that has two different forms of a particular gene, one inherited from each parent.

Homozygote: An individual having two identical alleles at a particular locus.

Impaired Glucose Tolerance (IGT): According to the criteria of the World Health Organization and the American Diabetes Association, impaired glucose tolerance is defined as two-hour glucose levels of 140 to 199 mg per dL (7.8 to 11.0 mmol) on a 75-g oral glucose tolerance test. A patient is said to be under the condition of IGT when he/she has an intermediately raised glucose level after 2 hours, but less than would qualify for type 2 diabetes mellitus. The fasting glucose may be either normal or mildly elevated.

Locus: A unique chromosomal location defining the position of an individual gene or DNA sequence.

Microsatellite marker: Small run (usually less than 0.1kb) of tandem repeats of a very simple DNA sequence, usually 1-4bp. Markers of this type can be very polymorphic.

Mutations: Changes to the base pair sequence of the genetic material. Gene mutations have varying effects on health depending on where they occur and whether they alter the function of the genes and their products, the essential proteins.

Phenotype: The observable traits or characteristics of an individual, for example hair color, weight, or the presence or absence of a disease.

Polymorphic markers: Polymorphic (genetic) markers are DNA sequences which show variation between individuals and which are used in constructing genetic maps by following how alleles segregate in large families. Markers may be located within coding sequences or other gene components but are mostly located in noncoding DNA. Commonly used markers are microsatellites and SNPs.

Promoter: A combination of short sequence elements, normally just upstream of a gene, to which RNA polymerase binds in order to initiate transcription of the gene.

SNP (single nucleotide polymorphism): SNPs provide a type of DNA marker. They occur very frequently in DNA and can be typed very easily by automated methods, allowing very large numbers of samples to be analyzed at a time.

STS (sequence tagged site): Any short (usually <500bp) sequence which is uniquely represented in a genome and for which primers have been designed enabling specific PCR amplification of that sequence.


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