Achondroplasia -- A genetic disorder disturbing normal growth of cartilage, resulting in a form of dwarfism typically characterized by a normal torso and shortened limbs, and usually inherited as an autosomal dominant condition.

Autosome -- Any chromosome not involved in sex determination. The diploid human genome consists of 46 chromosomes, 22 pairs of autosomes, and 1 pair of sex chromosomes (XX or XY).

Chromosome banding -- Staining techniques developed in the late 1960s and 1970s which allow the differentiation of chromosomes on the basis of unique patterns of light and dark bands revealed by the stain.

Ehlers-Danlos syndrome -- A group of inherited disorders of the connective tissue characterized by hyperextensibility of the skin and joints, easy bruisability, fragile tissues that heal slowly, and pseudotumors. Congenital cardiac defects, especially of the atrial septum, occur in over half of affected individuals.

Electrokymography -- photography on x-ray film of the motion of the heart or other structures using a fluoroscope, a pick-up unit, and a recording instrument to graph the variations of the x-ray intensities.

Ellis-van Creveld syndrome -- An autosomal recessive skeletal dysplasia characterized by short limbs, short ribs, extra fingers, and malformed nails and teeth.

Eugenics -- A science that deals with the improvement of hereditary qualities of a breed or race, e.g., by control of human mating, or elimination of individuals deemed unfit.

Gene -- A hereditary unit consisting of a sequence of DNA occupying a specific location on a chromosome and determining a particular characteristic in an organism.

Genetic disease -- A disease that has its origin in changes to DNA; usually refers to diseases that are inherited, although non-inherited forms of cancer can result from DNA mutation.

Genetic linkage -- The proximity of two or more markers (e.g. genes) on a chromosome; the closer together the markers are, the lower the probability that they will be separated during DNA repair or replication processes, and hence the greater the probability that they will be inherited together.

Genetic markers -- The occurrence of two or more different genetically determined phenotypes in a given population, the rarest of which cannot maintain itself simply through recurrent mutations, having a simple mode of inheritance occurring with multiple alleles. Useful in family studies and studies of the distribution of genes in populations.

Genotype -- The specific allelic composition of a cell, either of the entire cell or more commonly for a certain gene or a set of genes. Also, the genes an organism possesses.

Hurler syndrome -- The prototype of the mucopolysaccharidosis type disorders, caused by a deficiency of the enzyme L-iduronidase, and characterized by early mental and physical deterioration, corneal clouding, gargoyle-like facial features, and dwarfism.

Immunology -- The branch of biomedicine concerned with the structure and function of the immune system, innate and acquired immunity, the bodily distinction of self from non-self, and laboratory techniques involving the interaction of antigens with specific antibodies.

Inborn errors of metabolism -- Inherited defects of certain components of metabolic processes, e.g., essential enzymes, which can cause developmental problems and early death.

Mapping, gene -- Determination of the relative positions of genes on a chromosome (or other DNA molecule) and of the distance, in linkage units or physical units, between them.

Marfan syndrome -- A hereditary disorder of connective tissue that is characterized by abnormal elongation of the bones and often by ocular and circulatory defects.

Melanin -- A dark brown or black pigment found in various body tissues, including hair and skin.

Mendelian inheritance -- Inheritance of characters specifically transmitted by genes, in accordance with Mendel's principles, i.e., that hereditary units occur in pairs that separate during gamete formation so that every gamete receives only one member of a pair; and that the different pairs of hereditary units are distributed to the gametes independently of each other, the gametes combine at random, and the various combinations of hereditary pairs occur in the zygotes according to the laws of chance.

Molecular biology -- The branch of biology dealing with the formation, structure, and function of macromolecules essential to life, such as DNA, RNA, and proteins, especially their role in cell replication and the transmission of genetic information.

Mutation -- The process that produces a gene or a chromosome that differs from the wild-type. This process, either through an alteration in the nucleotide sequence of the DNA coding for a gene or through a change in the physical arrangement of a chromosome, results in the creation of a new character or trait not found in the parental type.

Nosology -- A branch of medical science that deals with classification of diseases.

Osteogenesis imperfecta -- An inherited collagen disorder due to defective biosynthesis of type I collagen, and characterized by brittle, osteoporotic, easily fractured bones.

Pericarditis, constrictive -- an inflammation of the fibrous, fluid-filled membrane surrounding the heart (pericardium), complicated by scarring and thickening of the membrane which in turn restricts the heart's movement.

Peutz-Jeghers syndrome -- An autosomal dominant polyposis syndrome characterized by benign tumor-like growths in the small intestine and excessive melanin pigmentation of the skin and mucous membranes; gastrointestinal bleeding and telescoping of the intestine are common complications.

Phenotype -- The form taken by one or a group of characteristics in a specific individual; the detectable outward manifestations of a specific genotype.

Phenylketonuria, or PKU -- An inherited human metabolic disease caused by a mutation in a gene coding for a phenylalanine processing enzyme--phenylalanine hydroxylase--which leads to accumulation of phenylalanine and mental retardation if not treated.

Pleiotropism -- Multiple physical manifestations produced by mutations of one gene, e.g., the many effects of a defective connective tissue gene in Marfan syndrome.

Polyp -- A projecting mass of swollen and hypertrophied tissue or tumorous membrane.

Pseudoxanthoma elasticum -- An inherited multisystem disorder of the elastic tissue leading to skin disease and ocular and cardiovascular complications.

Recessive gene/allele -- An allele which produces little or no phenotypic effect when it occurs with a contrasting allele; two recessive alleles must occur together to express the trait.

Recombinant DNA, rDNA, hybrid strings -- Genetically engineered DNA prepared in vitro by cutting up DNA molecules and splicing together specific DNA fragments, usually from more than one type of organism, e.g., inserting pieces of virus DNA into a bacterial plasmid.

Sequencing, gene -- Determination of the order of nucleotides in a DNA or RNA molecule (sequencing can also determine the order of amino acids in a protein.)

Sex Chromosomes -- The X and Y chromosomes in human beings that determine the sex of an individual. Females have two X chromosomes in diploid cells; males have an X and a Y chromosome. The sex chromosomes comprise the 23rd chromosome pair in a karyotype.

Sickle-cell anemia -- A chronic anemia that occurs primarily in individuals of African descent who are homozygous (a gene pair that possesses identical alleles) for the gene controlling hemoglobin S. The condition is characterized by abnormal sickle-shaped red blood cells, accelerated blood cell destruction, and by episodic blocking of blood vessels by the adherence of sickle cells to the lining of the vessels and heart which can cause serious complications, such as organ failure.

Somatic Cell hybridization -- A method for isolating chromosomes for study; hybrid cells are created by fusing human cells growing in culture with mouse tumor cells. Cells like this, with too many chromosomes, get rid of some chromosomes with each new generation, and they tend to throw out the human chromosomes. Over time the hybrid cell line looks more and more like a mouse cell line: all of the mouse chromosomes replicate and are passed on when the cell divides, but only one or a few of the human chromosomes persist. If a human protein is produced by the hybrid cell line containing only, say, chromosome 7, then researchers know that the gene coding for that protein lies on chromosome 7.

Sound Spectrography -- Using a spectrograph to disperse sound waves into a spectrum then photographing or mapping the spectrum.

Syphilis -- An acute to chronic infectious disease caused by the spirochete Treponema pallidum, which is usually transmitted by sexual contact or acquired in utero.

Ventricular fibrillation -- Very rapid, irregular contractions of the muscle of the heart ventricles, resulting in a lack of synchronism between heartbeat and pulse.

X-chromosome -- See "Sex Chromosomes."