Chromosomal trisomies

Aneuploidy in live births and abortions arises from aneuploid gametes during germ cell meiosis. Trisomy or monosomy of large chromosomes leads to early embryonic death. Trisomy of the smaller chromosomes allows survival but is detrimental to the health of an affected person, for example, Down s syndrome (trisomy 21), Patau syndrome (trisomy 13) and Edward s syndrome (trisomy 18). Sex chromosome trisomies (Klinefelter s and XXX syndromes) and the sex chromosome monosomy (XO), known as Turner s syndrome, are also compatible with survival. 

Down syndrome Down syndrome is the leading cause of mental retardation, occurring in about 1 of every 800 live births the mother s age strongly influences its occurrence. Mental and physical problems, including heart and eye defects, are the result of the formation of three chromosomes (trisomy), usually number 21, instead of a pair. 

Chromosomal Disorders Autosomal Trisomy 21 Trisomy 18 Trisomy 13 Translocations Deletions ... 

It Is done under circumstances In which there appears to be a significantly Increased risk of having a child with a chromosome abnormality For the most part, this Increased risk Is associated with trisomy 21, although It can. In certain rare Instances,... 

Down s syndrome A congenital learning disorder arising from a chromosomal abnormality (trisomy 21 - an extra chromosome 21). 

Aneuploidy in somatic cells is involved in the formation of human tumors. Up to ten percent of tumors are monosomic and trisomic for a specific chromosome as the single observable cytogenetic change. Most common among such tumors are trisomy 8, 9, 12, and 21 and monosomy for chromosomes 7, 22, and Y. 

Detection of chromosome fragments. Individuals with Turner s syndrome are, in some cases, mosaic for a portion of or for the entire Y chromosome (46,XY/45,X). Since such individuals may be at increased risk for gonadal tumors, Southern blot or PCR analysis has been used to detect the presence of Y chromosome segments in studies of DNA from peripheral blood samples. Similarly, the fetal sex as well as the presence of some aneuploid states (e.g., trisomy 18) can be determined by analysis of DNA from chorionic villi or amniotic fluid cells. 

The most common form of aneuploidy is trisomy, or the presence of an extra chromosome in each cell. "Tri-" is Greek for "three" people with trisomy have three copies of a particular chromosome in each cell instead of the normal two copies. Down syndrome is an example of a condition caused by trisomy—people with Down syndrome typically have three copies of chromosome 21 in each cell, for a total of 47 chromosomes per cell. 

Aneuploidy The chromosomal constitution of cells which deviate from the normal by the addition or subtraction of chromosomes or chromosome pairs. In a normally diploid cell the loss of a chromosome pair is termed nullisomy (symbol 2N-2), the loss of a single chromosome is monosomy (symbol 2N-1), the addition of a chromosome pair is tetrasomy (symbol 2N+2), the addition of a single chromosome is trisomy (symbol 2N+1). [nih]... 

Trisomy The possession of a third chromosome of any one type in an otherwise diploid cell. 

Down Syndrome People with Down syndrome have 3 chromosomes, a trisomy, of chromosome 21. The result is lower cognitive ability and physical stature. The disorder also causes affected people to have a higher incidence of heart, intestinal, and thyroid problems. 

Aneuploidy, which indicates a deviation from the euploid number of chromosomes, is the term used to describe the loss or gain of specific chromosomes. Two major types of aneuploidy are observed trisomy (three copies of a specific chromosome) and monosomy (one copy of a specific chromosome). Monosomies and trisomies are usually caused by nondisjunction (the failure of the two members of the chromosome pair to disjoin or separate) during meiosis (Fig II-3-2). All autosomal monosomies are lethal, but trisomies of three different autosomes (13,18, and 21) are compatible with survival to term in at least some cases. This difference illustrates the fact that the body tolerates extra genetic material more successfiiUy than a loss of genetic material. 

As its name implies, a translocation is reciprocal when genetic material is exchanged between two chromosomes. For example, parts of the short arms of chromosomes 2 and 8 could be exchanged (Fig II-3-4). The individual who carries the reciprocal translocation (46,XX,t[2p 8p]) will not usually be affected clinically because he or she has the normal complement of genetic material. However, his or her offspring can inherit unbalanced chromosome material (e.g., a copy of the normal 8 and a copy of chromosome 2 that contains the translocated piece of 8, resulting in a partial trisomy of the 8th chromosome and partial monosomy of the second chromosome [Fig II-3-4]). 

Pinkel, D., Landegent, J., Collins, C., Fuscoe, J., Segraves, R., Lucas, J., and Gray, J. W. (1988) Fluorescence in situ hybridization with human chromosome specific libraries detection of trisomy 21 and translocations of chromosome 4. Proc. Natl. Acad. Sci. USA 85, 9138-9142. 

Chromosomal abnormalities included the presence of aneuploidy, a second Ph chromosome, and trisomy 8. The loss of one p53 allele by an alteration of the short arm of chromosome 17 was seen in seven patients, and new reciprocal translocations were seen in two patients. In eight cases, multiple cytogenetic abnormalities were also present (40). 

The gene for human (Cu,Zn)-SOD is located on chromosome 21 (two alleles have actually been described) In trisomy 21 (Down syndrome) an increase of about 50 % in SOD activity was observed in erythrocytes and in blood platelets... 

This is a controversial area with regard to humans where there is currently little hard data. Theoretically it is possible for a foreign compound to cause mutations in male germ cells, which result in malformations or the development of abnormal offspring. This is similar to the situation in which inherited mutations or chromosomal aberrations lead to the birth of abnormal offspring, such as occur in Down syndrome, for example, where an extra chromosome occurs (Trisomy 21). 

The unequal partition of chromosomes or nondisjunction is a serious effect if the affected daughter cells survive. Down syndrome or Trisomy 21 is the result of chromosome nondisjunction in humans, those affected having 47 chromosomes instead of 46. This unequal partitioning of chromosomes can occur at mitosis in germ cells or during meiosis in the production of sperm or ova. 

CITE, Chromosomal aberrations, human trisomy 21-lymphoeytes in vitro + NT 80 Sehulz et al. (1982)... 

---