|
|
|
|
|
|||||
Cash Counts makingcontact.org
Subscribe to the Contact a Family Directory 
This site complies with the HONcode standard for trustworthy health information: verify here.
Chromosome disorders
Background
The human body is made up of billions of individual cells. Apart from the red blood cells, each cell contains a structure called a nucleus. Inside each nucleus are found long, thread-like bodies, the chromosomes. Chromosomes are made up of a short arm (designated by the letter 'p') and a long arm ('q'), joined at a point, the centromere. The ends of the two arms are called the telomeres. Chromosomes are made up of deoxyribonucleic acid (DNA) and proteins. The DNA of each cell, when stretched out, is around 2m long. In chromosomes, the DNA is coiled and much condensed. Chromosomes contain genes, which are special stretches of DNA. The genes provide the instructions that tell our bodies how to develop and function properly.
Apart from the mother's egg cells (ova) and the father's sperm cells (spermatozoa), every cell in the human body normally contains 23 pairs of chromosomes, giving 46 chromosomes in total. There are thought to be around thirty thousand genes in each cell. Of the 23 pairs of chromosomes in each of these cells, one member from each pair is normally inherited from the father and the other member of each pair is normally inherited from the mother. The first 22 pairs of chromosomes are called the autosomes and are numbered from one to 22, generally in order of length, chromosome 1 being the longest. The chromosomes in the twenty-third pair are called the sex chromosomes. They are labelled X or Y. Males normally have one copy of the X chromosome and one copy of the Y chromosome, while females normally have two copies of the X chromosome in their body cells.
A mother's egg cells each normally contain only 23 chromosomes, made up of one copy each of chromosomes 1 to 22 and one copy of the X chromosome. A father's sperm cells also normally contain only 23 chromosomes, again made up of one copy each of chromosomes 1 to 22 but also either one copy of the X chromosome or one copy of the Y chromosome. It is the father's sperm that determines whether a child will be a boy (XY) or a girl (XX).
A person's chromosomal make-up is called their karyotype which can be described by a code of letters and numbers.
