A blood sample was drawn from Matthew to run a chromosome analysis test on the day he was born when the doctors suspected Down syndrome upon physical examination. A chromosome test would reveal any duplication, deletion or translocation of genetic material in each cell. A person without Down syndrome has 46 chromosomes while a person with Down syndrome has 47 chromosomes.
The picture of systematically arranged chromosomes, which is taken using a digital camera connected to a computer with special software, is called a karyotype. Matthew's karyotype shows an extra 21st chromosome, which indicates that he has Trisomy 21, the most common type of Down syndrome. This extra genetic material was present in all sample cells that were tested.
There are two other types of Down syndrome: mosaicism and translocation. Mosaicism is diagnosed when an individual has some cells with 46 chromosomes and some cells with 47 chromosomes. Translocation is diagnosed when the extra 21st chromosome has attached itself to another numbered chromosome. These are the very basic definitions. The downsyn website offers a more detailed explanation of the types of Down syndrome.
Here is Matthew's karyotype. I requested a copy of it from our geneticist. The little arrow right above the last row of chromosomes is pointing to the extra 21st chromosome. A person without Down syndrome would only have 2 strands of the 21st chromosome, not 3 as shown below. I find it very interesting and fascinating. Each of these chromosomes are found in every cell of Matthew's body. They contain the genetic information from both Bill and I, making a very unique Matthew.
Down Syndrome is named after John Langdon Down, a British physician who described the condition in a report in 1866. Up until the 1950's, the diagnosis of Down syndrome was based on finding certain physical characteristics that were commonly associated with the syndrome. It wasn't until the 1950's that karyotyping techniques were discovered. In 1959, Jérôme Lejeune discovered that Down syndrome resulted from an extra 21st chromosome.
(fact source: "Down Syndrome" on Wikipedia).