What is trisomy 18? : What is trisomy 18? Trisomy 18, also called Edwards syndrome, is a chromosomal condition that occurs when there are three copies of chromosome 18 in each cell instead of the normal two copies. Infants born with trisomy 18 have an extremely high mortality rate—only 5 to 10 percent of these infants survive the first year of life.
Characteristics : Characteristics Trisomy 18 severely affects all organ systems of the body. Includes:
Nervous system and brain - mental retardation and delayed development, high muscle tone, seizures, and physical malformations such as brain defects
Head and face - small head (microcephaly), small eyes, wide-set eyes, small lower jaw
Heart - congenital heart defects such as ventricular septal defect
Bones - severe growth retardation, clenched hands with 2nd and 5th fingers on top of the others, and other defects of the hands and feet
Malformations of the digestive tract, the urinary tract, and genitals
Slide 4: Trisomy 18 occurs in about 1 in 5,000 newborns. Approximately 80 percent of newborns affected by this disorder are female; probably because males with trisomy18 are more likely to be miscarried. Although women of all ages can have a child with trisomy 18, the chance of having a child with this condition increases as a woman gets older.
Can trisomy 18 be inherited? : Can trisomy 18 be inherited? Most cases of trisomy 18 are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction can result in reproductive cells with an abnormal number of chromosomes.
Nondisjunction- Failure of chromosome pairs to separate properly during cell division. This could arise from a failure of homologous chromosomes to separate in meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis. The result of this error is a cell with an imbalance of chromosomes. Such a cell is said to be aneuploid. Loss of a single chromosome (2n-1), in which the daughter cell(s) with the defect will have one chromosome missing from one of its pairs, is referred to as a monosomy. Gaining a single chromosome, in which the daughter cell(s) with the defect will have one chromosome in addition to its pairs is referred to as a trisomy.
mistake in meiosis
Numerical Chromosome Abnormality : Numerical Chromosome Abnormality A plus (+) or minus (-) sign is placed before a chromosome to indicate a gain or loss of that particular chromosome.
47,XX,+18 or 47,XY,+18