Cool Genes

There are 3 main types of Down syndrome.  The types are nondisjunction, or Trisomy 21, translocation and mosaicism.  In nondisjunction, there is an extra copy of the 21st chromosome because of a lack of division of gametes, a sperm or egg cell, at some point prior to conception.  Trisomy 21 is the cause of approximately 95% of observed Down syndromes, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete.  This type is not inherited.  Will was tested by a geneticist when he was about a month old and had his complete karyotype reviewed and this determined that he indeed had nondisjunction.  A karyotype is a complete look at your chromosome numbers and characteristics under a microscope to look for differences in length or number.  Current research  has shown that Trisome 21 is due to a random event during the formation of sex cells. Down syndrome occurs in all ethnic groups and among all economic classes.  There has been no evidence that it is due to parental behavior (other than age) or environmental factors.Mosaicism occurs when either there is a nondisjunction event during an early cell division in a normal embryo and leads to a fraction of the cells with trisomy 21, or a Down syndrome embryo undergoes nondisjunction and some of the cells in the embryo revert to the normal chromosomal arrangement.  This type is rare and only accounts for 1-2% of Down sydrome diagnosis' and is also not inherited.
In translocation, the long arm of chromosome 21 breaks off and reattachs itself to another chromosome and it is usually chromosome 14.  This type of Ds occurs in about 2-3% of cases and is an inherited disorder.