Allele.
One of a series of possible alternative forms of a given gene that differs in DNA sequence.

Carrier.
An individual who is heterozygous for a single recessive gene.

Cloning.
Genetically engineered replicas of DNA sequences.

Codon.
Nucleotide triplet in messenger RNA that specifies the amino acid to be inserted at a specific position during translation.

Crossing over.
Exchange of genetic material between homologous chromosomes.

Deletion.
Loss of a DNA segment.

Deuteranopic dichromat.
A person lacking green-cone pigment.

Dominant gene.
A gene in a diploid organism that is phenotypically manifest in the heterozygous state.

Exon.
Portion of a split gene that is included in the transcript of a gene and survives processing of the RNA in the cell nucleus.

Functional cloning.
Strategy of gene cloning based on the relationship between the pathophysiology of a disease and the biochemical properties of the genes studied.

Genetic mapping.
Determining the linear arrangement and location of mutable sites on a chromosome as deduced from recombination experiments.

Genotype.
The genetic constitution of an organism as opposed to its physical appearance (phenotype).

Heterozygous.
Having one or more pairs of dissimilar alleles.

Homologous.
Referring to structures that show a fundamental similarity.

Homozygous.
Having identical alleles in corresponding loci of homologous chromosomes.

Insertion.
A chromosomal or DNA abnormality in which an extra portion of chromosome or DNA is added.

Intron.
A segment in split genes that is transcribed into nuclear RNA but is subsequently removed.

Linkage.
Greater association in inheritance of nonallelic genes than is to be expected from independent assortment.

Locus.
Position of a gene in a chromosome.

Marker.
Gene with a known location on a chromosome and clear-cut phenotype used as a reference point when mapping a new mutant gene.

Missense mutation.
DNA mutation that alters an amino acid.

Mutation.
Process by which a gene undergoes structural change.

Nonsense mutation.
DNA mutation that results in a premature termination codon.

Penetrance.
Proportion of individuals with a given genotype that show the expected phenotype.

Phenotype.
Observable properties of an organism produced by the genotype in conjunction with the environment.

Polymerase chain reaction.
Technique for copying the complementary strands of a target DNA molecule simultaneously for a number of cycles until the desired amount is obtained. Primers are synthesized that are complementary to the DNA that flanks the target region. The DNA is heated to separate complementary strands and then cooled to let the primers bind to the flanking sequences. Heat-stable DNA polymerase is added and the reaction is allowed to proceed for many replication cycles.

Polymorphism.
Existence of two or more genetically different classes in the same interbreeding population.

Positional cloning.
The main strategy of gene cloning in reverse genetics. Genes are cloned based on linkage analysis and overlapping DNA clones. No prior knowledge of the gene defect or its chromosomal location is necessary.

Probe.
Biochemical labeled with radioactive isotope used to isolate a gene, gene product, or protein.

Protanopic dichromat.
A person lacking red-cone pigment.

Recessive gene.
A gene in a diploid organisms that is phenotypically manifest in the homozygous state but is masked in the presence of its dominant allele.

Recombination.
Occurrence of progeny with combinations of genes other than those of the parents because of independent assortment or crossing over.

Restriction fragment length polymorphisms.
Variations occurring within a species in the length of DNA fragments generated by specific endonucleases. Variations can be generated by mutations that create or abolish enzyme recognition sites.

Southern blotting.
A technique for transferring electrophoretically resolved DNA segments from an agarose gel to a nitrocellulose filter paper sheet using capillary action. The DNA fragment of interest can then be probed with a radioactive, complementary nucleic acid and its position determined by autoradiography.

Splice junctions.
Segments containing a few nucleotides that reside at the ends of introns functioning in excision during the processing of transcripts.

Transcription.
Formation of an RNA molecule from a DNA template. This is the first step in gene expression.

Translation.
Formation of a protein directed by messenger RNA.

Trichromat.
A person with normal color vision.

Tritanopic dichromat.
A person missing blue-cone pigment.

X-linkage.
The presence of a gene located on the X chromosome. Also known as sex linkage.