Genetic Disorders is a collective classification for syndromes, diseases, and congenital anomalies that result from alterations of the genes and chromosomes. There are four general categories of genetic disorders, each relating to the way in which the alterations manifest.
The normal human GENOME contains 23 paired chromosomes. CHROMOSOMAL DISORDERS occur when there are disruptions in these pairings in which there is an extra CHROMOSOME (trisomy) or a missing chromosome (monosomy). Chromosomal disorders also occur when large segments of a chromosome are damaged or missing (deletion syndromes). Less often, a broken segment of a chromosome attaches itself to another chromosome (TRANSLOCATION). Common chromosomal disorders include DOWN SYNDROME, EDWARDS SYNDROME, PATAU’S SYNDROME, TURNER SYNDROME, and KLINEFELTER’S SYNDROME.
Each GENE encodes, or directs, a specific action within the body. Single-gene disorders, in which a MUTATION of a gene or set of genes causes malfunctions of the proteins that carry out the gene’s instructions, cause conditions of faulty encoding, either because the gene’s protein messenger is missing or incomplete. The disorders that result often become more severe over time as the malfunction continues to repeat itself. CYSTIC FIBROSIS, SICKLE CELL DISEASE, Duchenne’s MUSCULAR DYSTROPHY, HUNTINGTON’S DISEASE, and MARFAN SYNDROME are single-gene disorders.
Researchers suspect many health conditions arise as a result of an interplay between genetic and environmental factors. Geneticists call such conditions multifactorial disorders because it appears a certain combination of events must take place for disease to result; doctors may refer to them as conditions of GENETIC PREDISPOSITION. These are the conditions that tend to run in families; some family members develop them and others do not. Health conditions in which genetics, lifestyle, and other variables participate in the development of disease are numerous. Some of those that are common include CORONARY ARTERY DISEASE (CAD), HYPERTENSION (high BLOOD PRESSURE), CANCER, DIABETES, GALLBLADDER DISEASE, and NEPHROLITHIASIS (kidney stones), and numerous other conditions.
Mitochondria are self-replicating structures within a cell (called organelles) that carry out the metabolic functions of the cell. Mitochondria have their own DNA that directs their specific functions, and have multiple copies; MITOCHONDRIAL DNA (MTDNA) is different from the cell’s nuclear DNA. MITOCHONDRIAL DISORDERS, which are the rarest of the genetic disorders, occur when the genes that encode mitochondrial activity contain mutations or when there are defects in the mtDNA. Mitochondrial disorders tend to vary widely among individuals, causing different symptoms because they affect different, and usually multiple, organs or structures. Doctors define the disorder as a complex of symptoms. Some forms of ENCEPHALOPATHY, MYOPATHY, and CARDIOMYOPATHY are mitochondrial disorders.
See also CONGENITAL ANOMALY; INHERITANCE PATTERNS; MOSAICISM.
Resource: Facts On File Encyclopedia Of Health And Medicine
RNA - the abbreviation for ribonucleic acid. RNA is a single-strand molecule consisting of ribose; a sugar; and nucleotides made up of the nitrogen bases adenine, uracil, guanine, and cytosine. RNA exists in a number of forms, all of which serve as biochemical messengers that carry the instructions of DNA to the ribosomes, structures in the cell’s
DNA - the abbreviation for deoxyribonucleic acid. DNA is the molecule of heredity; its sequences form the body’s GENETIC CODE. Each cell in the body contains DNA within the chromosomes in its nucleus (except erythrocytes, which do not have nuclei). DNA has a characteristic double-helix structure that resembles a gently twisting ladder. The supporting
Gene is a segment of coding DNA (DNA that instructs the structure and function of cells throughout the body) composed of a specific sequence of nucleotides. The gene is the basic unit of inheritance that directs every facet of the body’s appearance and functions. Genes align along chromosomes in pairs. Each CHROMOSOME (AUTOSOME) contains thousands of
Stem Cell - an undifferentiated, primal cell that has the capability to endlessly divide and develop into numerous types of cells. Totipotent stem cells exist primarily in the early EMBRYO (blastocyst) and can differentiate into (become) virtually any type of cell in the body. As the body becomes more complex and develops beyond the blastocyst stage, stem
Allele - any of the variations of a GENE that may occupy the same position (locus) on a CHROMOSOME. The gene controlling a particular trait or function always occupies the same locus on the same chromosome. Genes occur as pairs, with one gene coming from each parent. The pairing determines how the gene’s traits are expressed in the individual. For
Apoptosis - the natural mechanism through which a cell engages in actions that lead to its death, often called programmed cell death or cell suicide. Apoptosis appears linked to SENESCENCE, an inherent limitation on the number of times a cell can divide. Both apoptosis and senescence play significant roles in the aging process. Once the cell initiates
Autosomal Trisomy is a chromosomal disorder in which there are three instead of the normal two copies of an AUTOSOME (nonsex chromosome). An autosomal trisomy may be complete (affect all cells) or mosaic (affect only some cells). The most commonly occurring complete autosomal trisomies that are survivable are those involving chromosomes 21, 18, and 13,
Autosome is a CHROMOSOME that appears as a pair in which both chromosomes are the same in either sex, also called a nonsex chromosome. In contrast, the sex chromosomes appear as a pair that is different in males and females. The human GENOME contains 22 autosomes and one pair of sex chromosomes for a total complement of 46 chromosomes as 23 pairs.
Chromosome is a coiled DNA molecule within the cell’s nucleus that carries an individual’s GENETIC CODE. Most of the time the chromosome’s structure is loose and indistinguishable. Only in the stage of cell division immediately before the cell divides (the metaphase) does the chromosome draw itself into a compact, rodlike structure the
Cloning - the creation of exact copies of a GENE, cell, or entire organism. Such exact copies occur naturally when a ZYGOTE divides to become identical multiples such as twins or, less commonly, triplets. Manipulated cloning is primarily a research method at present, though scientists use cloning for therapeutic applications in creating RECOMBINANT DNA