Trisomy 21

We went back to the Geneticist for the result of the Karyotyping Test.The result was P has Trisomy 21.

What is Trisomy 21? Trisomy 21: The Story of Down Syndrome The formal story began in 1866, when a physician named John Langdon Down published an essay in England in which he described a set of children with common features who were distinct from other children with mental retardation. Down was superintendent of an asylum for children with mental retardation in Surrey, England when he made the first distinction between children who were cretins (later to be found to have hypothyroidism) and what he referred to as "Mongoloids." Send Me Email Down based this unfortunate name on his notion that these children looked like people from Mongolia, who were thought then to have an arrested development. This ethnic insult came under fire in the early 1960s from Asian genetic researchers, and the term was dropped from scientific use. Instead, the condition became called "Down's syndrome." In the 1970s, an American revision of scientific terms changed it simply to "Down syndrome," while it still is called "Down's" in the UK and some places in Europe. In the first part of the twentieth century, there was much speculation of the cause of Down syndrome. The first people to speculate that it might be due to chromosomal abnormalities were Waardenburg and Bleyer in the 1930s. But it wasn't until 1959 that Jerome Lejeune and Patricia Jacobs, working independently, first determined the cause to be trisomy (triplication) of the 21st chromosome. Cases of Down syndrome due to translocation and mosaicism (see definitions of these below) were described over the next three years. The Chromosomes Chromosomes are thread-like structures composed of DNA and other proteins. They are present in every cell of the body and carry the genetic information needed for that cell to develop. Genes, which are units of information, are "encoded" in the DNA. Human cells normally have 46 chromosomes which can be arranged in 23 pairs. Of these 23, 22 are alike in males and females; these are called the "autosomes." The 23rd pair are the sex chromosomes ('X' and 'Y'). Each member of a pair of chromosomes carries the same information, in that the same genes are in the same spots on the chromosome. However, variations of that gene ("alleles") may be present. (Example: the genetic information for eye color is a "gene;" the variations for blue, green, etc. are the "alleles.") Human cells divide in two ways. The first is ordinary cell division ("mitosis"), by which the body grows. In this method, one cell becomes two cells which have the exact same number and type of chromosomes as the parent cell. The second method of cell division occurs in the ovaries and testicles ("meiosis") and consists of one cell splitting into two, with the resulting cells having half the number of chromosomes of the parent cell. So, normal eggs and sperm cells only have 23 chromosomes instead of 46. Many errors can occur during cell division. In meiosis, the pairs of chromosomes are supposed to split up and go to different spots in the dividing cell; this event is called "disjunction." However, occasionally one pair doesn't divide, and the whole pair goes to one spot. This means that in the resulting cells, one will have 24 chromosomes and the other will have 22 chromosomes. This accident is called "nondisjunction." If a sperm or egg with an abnormal number of chromosomes merges with a normal mate, the resulting fertilized egg will have an abnormal number of chromosomes. In Down syndrome, 95% of all cases are caused by this event: one cell has two 21st chromosomes instead of one, so the resulting fertilized egg has three 21st chromosomes. Hence the scientific name, trisomy 21. Recent research has shown that in these cases, approximately 90% of the abnormal cells are the eggs. The cause of the nondisjunction error isn't known, but there is definitely connection with maternal age. Research is currently aimed at trying to determine the cause and timing of the nondisjunction event. Three to four percent of all cases of trisomy 21 are due to Robertsonian Translocation. In this case, two breaks occur in separate chromosomes, usually the 14th and 21st chromosomes. There is rearrangement of the genetic material so that some of the 14th chromosome is replaced by extra 21st chromosome. So while the number of chromosomes remain normal, there is a triplication of the 21st chromosome material. Some of these children may only have triplication of part of the 21st chromosome instead of the whole chromosome, which is called a partial trisomy 21. Translocations resulting in trisomy 21 may be inherited, so it's important to check the chromosomes of the parents in these cases to see if either may be a "carrier." The remainder of cases of trisomy 21 are due to mosaicism. These people have a mixture of cell lines, some of which have a normal set of chromosomes and others which have trisomy 21. In cellular mosaicism, the mixture is seen in different cells of the same type. In tissue mosaicism, one set of cells, such as all blood cells, may have normal chromosomes, and another type, such as all skin cells, may have trisomy 21.