Normal human cells have a single pair of each chromosome, with one inherited from the mother and one from the father. Trisomy is a genetic condition in which there are three of a given chromosome instead of two. Human chromosomes are numbered 1 through 23; trisomy 21, for example, is the condition in which there are three copies of chromosome 21 instead of two, resulting in Down syndrome. Other trisomy conditions arise in the same manner due to the defective duplication of a chromosome.
How Trisomy Occurs
Two cells are created when either egg or sperm cells divide, each having exactly one of each chromosome of the original pair. The resulting cells have 23 single chromosomes rather than 46, or 23 pair. Occasionally, some chromosomes fail to separate completely, leaving 22 unpaired chromosomes and 1 paired chromosome rather than the normal 23 unpaired chromosomes. When the egg or sperm cell with the double complement of this chromosome meets a normal egg or sperm cell, the resulting embryo will contain three of that chromosome, resulting in trisomy.
When Trisomy Occurs
Trisomy is caused by chromosomal non-disjunction, which occurs when the chromosomes simply fail to divide properly. A significant factor in some trisomy conditions is maternal age: A woman is born with a lifetime supply of egg cells, but by the time she reaches her late thirties the egg cells lose the ability to divide properly. Maternal age is considered to be the major contributing factor to the development of Down Syndrome.
Chromosomal Rearrangement
Trisomy can occur due to chromosomal rearrangement in rare instances. This happens when one piece of a single chromosome pairs with another single, non-matching chromosome, resulting in severe birth defects such as cleft lip and palate, club foot and lung and kidney abnormalities. Chromosomal rearrangement typically leads to fetal death.
Full and Partial Trisomy
Full trisomy occurs when the entire chromosome pair fails to separate, leaving the individual with an three whole chromosomes rather than two. Partial trisomy happens when only a part of one of the chromosome pairs fails to separate fully, resulting in a single pair plus a piece of a third one. The severity of the symptoms in either full or partial trisomy corresponds to the amount of the third chromosome present as well as the number of genes on that chromosome.
Conditions Caused by Trisomy
Embryos with trisomy often do not survive; however, if chromosomes 21, 18 or 13 are involved, survival to birth is probable. Down syndrome is caused by trisomy of chromosome 21, Edward's syndrome involves chromosome 18 and Patau's syndrome indicates trisomy of chromosome 13. The symptoms of these types of trisomies differ, but each results in some level of retardation as well as distinctive differences in physical characteristics. Trisomy can also occur in sex cells, leading to individuals who are XXX, XXY and XYY rather than the normal XY, with mild retardation and infertility. The cause of these types of trisomy is the same as in autosomal (non-sex) cells.
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