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Trisomy
Trisomy is the presence of one extra copy of a chromosome, such that there are 47 chromosomes in total compared to the normal number of 46 chromosomes. Trisomy is common. It has been estimated that approximately 20% of all human conceptions are trisomic (Hassold and Jacobs, 1984). Down syndrome is the most common type of trisomy that babies may be born with. Down syndrome is caused by the presence of an extra chromosome 21. Down syndrome is also called trisomy 21, "tri" for three copies of chromosome 21. The karyotype of Down syndrome is written 47, XX + 21 or 47, XY + 21. 80% of trisomy 21 conceptions result in early miscarriage. Individuals who are born with Down syndrome tend to have facial features that look similar to each other. These individuals will experience mild to severe mental retardation, heart problems and other health concerns throughout their lifetime. Tell me more about Down syndrome.
There are other extra chromosomes that children can be born with. There can be an extra copy of chromosome 13, which is called trisomy 13 or Patau syndrome. Or there can be an extra copy of chromosome 18, which is called trisomy 18 or Edward syndrome. Trisomy 13 and trisomy 18 are very severe chromosome changes and both usually lead to early infant death, often within the first year of life. Most trisomy 13 and 18 conceptions result in early miscarriage.
An extra copy of any of the other autosomes results in early miscarriage.
Changes in the number of sex chromosomes has a less severe effect on development and sometimes individuals with extra sex chromosomes can even be asymptomatic. Examples of the inheritance of an extra sex chromosome include: XXY, XXX or XYY. Klinefelter syndrome is the inheritance of two X chromosomes and one Y chromosome (47, XXY). Klinefelter's is characterized by male development, infertility and potential mild learning problems. Tell me more about Klinefelter syndrome.
How does the extra chromosome get there? (more scientific details)
Monosomy
In general, loss of chromosome material has a more severe effect on the growth of a baby than does the addition of chromosome material. Monosomy is the presence of only one copy of a chromosome when there is usually two copies, such that there are 45 chromosomes in the cell compared to the normal number of 46 chromosomes. Monosomy is more severe than trisomy. Most monosomies probably do not survive to implantation. This explains why monosomy is not often observed even in early pregnancy losses. Other than a few rare reports of monosomy for chromosome 21, only monosomy for the X chromosome is observed in clinically recognized pregnancies. 99% of monosomy X pregnancies end in an early miscarriage.
However, monosomy X can be found in healthy liveborn babies. Turner syndrome is caused by the inheritance of only one copy of the X chromosome. The karyotype of Turner syndrome is written 45, X0. Individuals with Turner syndrome develop as females. Females with Turner syndrome are usually short in stature, have a characteristic webbing of the neck, are infertile and have normal intelligence. (The majority of those that do survive to term are probably ‘mosaics’ i.e. they have a normal cell line in some tissues.)
Show me a picture of changes in the number of chromosomes.
Polyploidy
Triploidy is when there is one complete extra set of chromosomes. An extra copy of each chromosome for a total of 69 chromosomes. Tetraploidy is when there is two extra sets of chromosomes. Two extra copies of chromosomes makes a total of 92 chromosomes. Both of these chromosome changes generally lead to miscarriage, however some triploids can survive to the third trimester of pregnancy. Marker chromosome
A marker chromosome is an extra piece of a chromosome of unidentified origin. This extra piece of chromosome can also be called an extra structurally abnormal chromosome, a supernumery chromosome or an accessory chromosome, particularly once its origin is known. The effect of this extra chromosome material depends on the chromosome involved and the size of the piece.
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