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A trisomy is a genetic abnormality in which there are three copies, instead of the normal two, of a particular chromosome
Chromosome

A chromosome is an organized structure of DNA and protein that is found in Cell . A chromosome is a single piece of DNA that contains many genes, regulatory sequence and other genetic sequence....
.

Description and Causes
Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one chromosome inherited from each parent. In such organisms, a process called meiosis
Meiosis

In biology or life science, meiosis is a process of reductional division in which the number of chromosomes per cell is halved. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores....
 creates cells called gametes (eggs or sperm) that have only one set of chromosomes.






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A trisomy is a genetic abnormality in which there are three copies, instead of the normal two, of a particular chromosome
Chromosome

A chromosome is an organized structure of DNA and protein that is found in Cell . A chromosome is a single piece of DNA that contains many genes, regulatory sequence and other genetic sequence....
.

Description and Causes


Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one chromosome inherited from each parent. In such organisms, a process called meiosis
Meiosis

In biology or life science, meiosis is a process of reductional division in which the number of chromosomes per cell is halved. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores....
 creates cells called gametes (eggs or sperm) that have only one set of chromosomes. The number of chromosomes is different for different species. Human beings have 46 chromosomes (i.e. 23 pairs of chromosomes). Human gametes have only 23 chromosomes.

If the chromosome pairs fail to separate properly during cell division the egg or sperm may have a second copy of one of the chromosomes. (See non-disjunction.) If such a gamete results in fertilization and an embryo, the resulting embryo may also have an entire copy of the extra chromosome.

Terminology

"Full trisomy" means that an entire extra chromosme has been copied. "Partial trisomy" means that there is an extra copy of part of a chromosome.

Trisomies are sometimes characterised as "Autosomal trisomies" (trisomies of the non-sex chromosomes) and "Sex-chromosome trisomies." Autosomal trisomies are described by referencing the specific chromosome that has an extra copy. Thus, for example, the presence of an extra chromosome 21, which is found in Down syndrome
Down syndrome

Down syndrome, Down's syndrome, or trisomy 21 is a chromosomal disorder caused by the presence of all or part of an extra chromosome 21 ....
, is called trisomy 21.

Human trisomy

Trisomies can occur with any chromosome
Chromosome

A chromosome is an organized structure of DNA and protein that is found in Cell . A chromosome is a single piece of DNA that contains many genes, regulatory sequence and other genetic sequence....
, but often result in miscarriage. For example, Trisomy 16
Trisomy 16

Trisomy 16 is a chromosomal abnormality in which there are three copies of chromosome 16 rather than two. It is the most common chromosomal cause of miscarriage during the first trimester of pregnancy....
 is the most common trisomy in humans, occurring in more than 1% of pregnancies. This condition, however, usually results in spontaneous miscarriage
Miscarriage

Miscarriage or spontaneous abortion is the spontaneous end of a pregnancy at a stage where the embryo or fetus is incapable of surviving, generally defined in humans at prior to 20 weeks of gestation....
 in the first trimester.

The most common types of autosomal
Autosome

An autosome is a non-sex chromosome. It is an ordinarily paired type of chromosome that is the same in both sexes of a species . For example, in humans, there are 22 pairs of autosomes....
 trisomy that survive to birth in humans are:

  • Trisomy 21 (Down syndrome
    Down syndrome

    Down syndrome, Down's syndrome, or trisomy 21 is a chromosomal disorder caused by the presence of all or part of an extra chromosome 21 ....
    )
  • Trisomy 18 (Edwards syndrome
    Edwards syndrome

    Trisomy 18 or Edwards Syndrome is a genetic disorder caused by the presence of all or part of an extra chromosome 18 . It is named after John H....
    )
  • Trisomy 13 (Patau syndrome
    Patau syndrome

    Patau syndrome, also known as trisomy 13, is a chromosome abnormality, a syndrome in which a patient has an additional chromosome 13 due to a non-disjunction of chromosomes during meiosis....
    )
  • Trisomy 12 (Chronic Lymphocytic Leukemia
    Chronic lymphocytic leukemia

    B-cell chronic lymphocytic leukemia , is a type of leukemia, or cancer of the white blood cells . CLL affects a particular lymphocyte, the B cell, which originates in the bone marrow, develops in the lymph nodes, and normally fights infection....
    )
  • Trisomy 9
    Trisomy 9

    Trisomy 9 is a Genetic disorder caused by having three copies of chromosome number 9. It can appear with or without mosaicism....
  • Trisomy 8 (Warkany syndrome 2
    Warkany syndrome 2

    Warkany syndrome 2, also known as Aneuploidy 8, is a Chromosome aberration that has severe effects on the fetus.Trisomy is also found in some cases of chronic myeloid leukaemia, potentially as a result of karyotypic instability caused by the bcr:abl fusion gene....
    )


The most common forms of autosomal
Autosome

An autosome is a non-sex chromosome. It is an ordinarily paired type of chromosome that is the same in both sexes of a species . For example, in humans, there are 22 pairs of autosomes....
 trisomy are trisomy of chromosome 21 which results in Down syndrome
Down syndrome

Down syndrome, Down's syndrome, or trisomy 21 is a chromosomal disorder caused by the presence of all or part of an extra chromosome 21 ....
 and trisomy of chromosome 18 which results in Edwards syndrome
Edwards syndrome

Trisomy 18 or Edwards Syndrome is a genetic disorder caused by the presence of all or part of an extra chromosome 18 . It is named after John H....
. In rare cases, a fetus with trisomy of chromosome 13 can survive, giving rise to Patau syndrome
Patau syndrome

Patau syndrome, also known as trisomy 13, is a chromosome abnormality, a syndrome in which a patient has an additional chromosome 13 due to a non-disjunction of chromosomes during meiosis....
. Autosomal trisomy can be associated with birth defects, mental retardation and shortened life expectancy.

Trisomy of sex chromosomes can also occur:

  • XXX (Super female / XXX syndrome)
  • XXY (Klinefelter syndrome
    Klinefelter's syndrome

    Klinefelter's syndrome, 47,XXY or XXY syndrome is a condition in which males have an extra X sex chromosome.While females have an XX chromosomal makeup, and males an XY, Affected individuals have at least two X chromosomes and at least one Y chromosome....
    )
  • XYY (Jacobs syndrome / XYY syndrome)


See also


  • Chromosome Abnormalities
    Chromosome abnormalities

    A chromosome anomaly reflects an atypical number of chromosomes or a structural abnormality in one or more chromosomes. Chromosome anomalies usually occur when there is an error in cell division following meiosis or mitosis....
  • Aneuploidy
    Aneuploidy

    Aneuploidy is defined as an abnormal number of chromosomes. Syndromes caused by an extra or missing chromosome are among the most widely recognized genetic disorders in humans....
  • Karyotype
    Karyotype

    A karyotype is the characteristic chromosome complement of a eukaryote species. The preparation and study of karyotypes is part of cytogenetics....
  • Sexual Reproduction
    Sexual reproduction

    Sexual reproduction is characterized by processes that pass a Genetic recombination of Genetics material to offspring, resulting in Genetic diversity....