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Terminology. The number of chromosomes in the cell where trisomy occurs is represented as, for example, 2 n +1 if one chromosome shows trisomy, 2 n +1+1 if two show trisomy, etc. [2] "Full trisomy", also called "primary trisomy", [2] means that an entire extra chromosome has been copied.
The extra chromosome content can arise through several different ways. The most common cause (about 92–95% of cases) is a complete extra copy of chromosome 21, resulting in trisomy 21. [93] [98] In 1–2.5% of cases, some of the cells in the body are normal and others have trisomy 21, known as mosaic Down syndrome.
Trisomy X, also known as triple X syndrome and characterized by the karyotype 47,XXX, is a chromosome disorder in which a female has an extra copy of the X chromosome. It is relatively common and occurs in 1 in 1,000 females, but is rarely diagnosed; fewer than 10% of those with the condition know they have it.
Trisomy 18 (47,XX,+18) is caused by a meiotic nondisjunction event. In nondisjunction, a pair of chromosomes fails to separate during cell division; thus, a gamete ( i.e., a sperm or egg cell) is produced with an extra copy of chromosome (for a total of 24 chromosomes).
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.).
Down syndrome, a trisomy of chromosome 21, is the most common anomaly of chromosome number in humans. The majority of cases result from nondisjunction during maternal meiosis I. [14] Trisomy occurs in at least 0.3% of newborns and in nearly 25% of spontaneous abortions .
Notice the three copies of chromosome 21. Trisomy 21 (47,XY,+21) is caused by a meiotic nondisjunction event. [3] A typical gamete (either egg or sperm) has one copy of each chromosome (23 total). When it is combined with a gamete from the other parent during conception, the child has 46 chromosomes.
The basic number of chromosomes in the somatic cells of an individual or a species is called the somatic number and is designated 2n. In the germ-line (the sex cells) the chromosome number is n (humans: n = 23). p28 Thus, in humans 2n = 46. So, in normal diploid organisms, autosomal chromosomes are present in two copies.
Full trisomy 9 is a rare and fatal chromosomal disorder caused by having three copies ( trisomy) of chromosome number 9. It can be a viable condition if trisomy affects only part of the cells of the body ( mosaicism) or in cases of partial trisomy (trisomy 9p) in which cells have a normal set of two entire chromosomes 9 plus part of a third ...
Polysomy types are categorized based on the number of extra chromosomes in each set, noted as a diploid (2n) with an extra chromosome of various numbers. For example, a polysomy with three chromosomes is called a trisomy, a polysomy with four chromosomes is called tetrasomy, etc.: