Radiation Hybrid
A hybrid cell containing small fragments of irradiated human chromosomes. Maps of irradiation sites on chromosomes for the human, rat, mouse, and other genomes provide important markers, allowing the construction of very precise STS maps indispensable to studying multifactorial diseases.

Radioactivity
The emission of energy due to changes in the nucleus of an atom. Such spontaneously released radiation is a characteristic of certain elements and at some levels be harmful

Recessive
A trait or gene that is expressed in the offspring only when a copy from each parent is impaired. 2. A term applied to one member of an allele pair expressed when the other, dominant, member of the pair is present. 3. An allele (A) is recessive if the phenotype of the heteroiygote, (Aa) is the same as the homozygote (aa) for the alternative allele (a) and different from the homozygote for the recessive (AA). The allele (a) controls the heterozygote's phenotype and is called dominant. An allele may be partly, rather than fully, recessive; in that case, the heterozygous phenotype is nearer to, rather than identical with, the homozygote for the dominant allele.

Rare-cutter Enzyme
See: restriction-enzyme cutting site.

Reannealing
Reformation of double-stranded DNA from dissotiated single strands.

Receptor
A molecule that can bind to, or receive, a ligand. 2. Proteins that can bind to other specific molecles. Usually on the surface of a cell, receptors often bind to antibodies or hormones.

Random Mating
Mating between individuals without regard to genotype. 2. A mating pattern in which the probability of another individual of a particular genotype (or equals the frequency of that genotype (or phenotype) equal the frequency of that genotype (or phenotype) in the population. 3. A population mating system in which every male gamete has an equal opportunity to join in fertilizatio with every female gamete, including those gametes derived from the same individuals.