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Trisomy 21. Down syndrome (also known by the karyotype 47,XX,+21 for females and 47,XY,+21 for males) is mostly caused by a failure of the 21st chromosome to separate during egg or sperm development, known as nondisjunction. As a result, a sperm or egg cell is produced with an extra copy of chromosome 21; this cell thus has 24 chromosomes.
The trisomy 21 karyotype figure shows the chromosomal arrangement, with the prominent extra chromosome 21. Trisomy 21 is the cause of approximately 95% of observed Down syndrome, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete.
The most common aneuploidy that infants can survive with is trisomy 21, which is found in Down syndrome, affecting 1 in 800 births. Trisomy 18 (Edwards syndrome) affects 1 in 6,000 births, and trisomy 13 (Patau syndrome) affects 1 in 10,000 births. 10% of infants with trisomy 18 or 13 reach 1 year of age.
A karyotype of somebody affected with Down syndrome, depicting the presence of a full additional copy of chromosome 21. The presence of this extra copy is also referred to as Trisomy 21. The following conditions are caused by changes in the structure or number of copies of chromosome 21:
"Partial trisomy" means that there is an extra copy of part of a chromosome. "Secondary trisomy" - the extra chromosome has quadruplicated arms (the arms are identical; it is an "isochromosome"). "Tertiary trisomy" - the extra chromosome is made up of copies of arms from two other chromosomes.
Diagnosis. The simplest and easiest way to detect trisomy 8 is by a karyotype, a photograph representing all chromosomes of a cell in an orderly manner. Amniocentesis is also a technique for diagnosis. Samples from the amniotic fluid are taken from a fetus, cultured, then analyzed by a karyotype.
Karyotype of a person with trisomy 18. Three copies of the Chromosome 18 are detected. Trisomy 18 is a chromosomal abnormality characterized by the presence of an extra copy of genetic material on the 18th chromosome, either in whole ( trisomy 18) or in part (such as due to translocations ).
Down syndrome, a common chromosomal disease, is caused by trisomy of chromosome 21. Patau syndrome is caused by trisomy of chromosome 13. Trisomy 9, believed to be the 4th most common trisomy, has many long lived affected individuals but only in a form other than a full trisomy, such as trisomy 9p syndrome or mosaic trisomy 9. They often ...
A karyotype of an individual with trisomy 21, showing three copies of chromosome 21. An abnormal number of chromosomes is known as aneuploidy, and occurs when an individual is either missing a chromosome from a pair (resulting in monosomy) or has more than two chromosomes of a pair (trisomy, tetrasomy, etc.).
Triploid syndrome, also called triploidy, is a chromosomal disorder in which a fetus has three copies of every chromosome instead of the normal two. If this occurs in only some cells, it is called mosaic triploidy and is less severe. Most embryos with triploidy miscarry early in development.