|
|
||
|
|
|
|
|
|
Chromosomes are tiny threadlike structures, which are contained in the nucleus of every human cell. There are a few exceptions for example mature red blood cells which have no nucleus. |
|
|
|
|
|
|
|
A photograph of a single chromosome |
|
|
|
Each chromosome is made up of DNA (deoxyribonucleic acid), which is simply a large acid molecule. It is shaped like a double helix, and carries our genetic information encoded in genes. |
|
|
|
|
|
|
|
A pictorial example of DNA |
|
|
|
In general every human cell contains 22 pairs of chromosomes (also called autosomes) and two sex chromosomes. Together these 46 chromosomes contain some 50 - 100,000 genes. So it is these chromosomes which are the genetic blueprint of our life and which cause every person to be a unique individual. |
|
|
|
|
|
|
|
|
|
|
|
A photograph of the 46 chromosomes in a human cell |
|
|
|
|
|
|
|
Through research and the development of staining techniques in the 1950's, scientists were for the first time able to view the human chromosome. Although they appear disorganised within the cell, scientists have been able to identify them and so have numbered them from 1-22 in order of size. (see What is a karyotype?) |
|
|
|
|
|
|
|
A schematic diagram of the 22 pairs of chromosomes and the sex chromosomes X & Y. |
|
|
|
Each cell also contains a pair of sex chromosomes. In a male this combination is an X and a Y (XY), and in a female there are two X chromosomes (XX). Together with the 22 pairs of autosomes they make a total of 46 chromosomes. The sex chromosome X is inherited from the mother and the sex chromosome inherited from the father is either an X (female) or a Y (male). It is this chromosome which determines the sex of the baby. You can have three of one kind of sex chromosome, for example XXX or XYYY however these are not true trisomic conditions. This is because a trisomy refers to three autosomes (tri = three, some = autosome) and the sex chromosomes are NOT autosomes. If there are three copies of the X chromosome it is referred to as Triple X rather than trisomy X. If however there are three copies of an autosome, that is any of the chromosomes numbered 1 to 22, this is referred to as a trisomy. Which trisomy is identified by the chromosome number, For example Trisomy 18 refers to three copies of the 18th chromosome, Trisomy 3, refers to three copies of the 3rd chromosome. |
|
|
|
|
|
|
|
|
|
|
|
||
 |
||
|
What is a Trisomy Index |
||
 What is a chromosome? |
 The human egg & sperm cell |
|
|
How does it happen? |
What is "full" trisomy? |
|
|
What is a partial trisomy? |
What is a translocation? |
|
|
What is mosaicism? |
What is a karyotype? |
|
|
What is a Phenotype? |
What is prenatal testing?
|
|
|
How is a diagnosis made?
|
Will my baby die?
|
|
|
Why did this happen to me? |
What are the risks of it happening again? |
|
|
|
|
|
|
|
|
|
|
If you or your family have been effected by a trisomy diagnosis and would like to share experiences with others who are travelling similar journeys visit Trisomy Online - a free email based support group for families effected by the rarer trisomy disorders.
Families helping families |
||
|
|
||
|
|
| Home | What is a Trisomy? | What is Trisomy 18? | What is Trisomy 13? || Alex's Story | Stories From Special Friends | In Memory of Special Friends | | Special Occasions | Awards | Shapiro's Syndrome | SOFT Australia | Trisomy Listerv | |
|
|
|
Visitors Since 18th November 2000 FastCounter by bCentral |
|
|
|
|
|
|
|
|
|
