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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 2 m 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 twenty-three pairs of chromosomes, giving forty-six chromosomes in total. There are thought to be around thirty thousand genes in each cell. Of the twenty-three 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 twenty-two pairs of chromosomes are called the autosomes and are numbered from one to twenty-two, 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 twenty-three 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 twenty-three 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.
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