Genes form the physical hereditary link between generations. A typical human body cell contains about 40,000 genes. The genes do not exist as a separate unit, but are arranged in a linear order on thread like bodies known as chromosomes, within the nucleus of a cell. Chromosomes become shortened and thickened during cell division and can be seen clearly under the microscope.
Chromosomes are microscopic filamentous structures that contain an individual’s genetic material. This genetic material serves as the “instruction manual” for the body, containing the “directions” the body needs in order to form and function properly. Human cells have a total of 46 chromosomes, which are arranged into 23 pairs. We inherit one member of each pair from our biological mother, and the other member of each pair from our biological father. The first 22 pairs of chromosomes are called “autosomes” and the last pair is called the “sex chromosomes”. Females typically have two “X” sex chromosomes, while males typically have one “X” and one “Y” sex chromosome (Tseng. 1995).
Let us know what is a chromosome analysis?
Chromosome analysis is a study of the number and general structure of all 46 chromosomes, it is also known as a karyotype. In a standard karyotype, chromosomes from cells in the body are counted to ensure that the cells have the correct number of chromosomes, and their structure is analysed to ensure that there are no large pieces of material that are missing (deleted), extra (duplicated), or rearranged in any way. It is important to realise that standard chromosome analysis may not be able to detect tiny deletions or duplications of genetic material and will not be able to detect single gene conditions, such as sickle cell disease. Hundreds of different types of chromosome abnormalities causing well described syndromes have been reported in humans. They fall into two categories:
Numerical Chromosome Abnormality means that a person has a total number of chromosomes different from 46; usually 47 or sometimes 45 chromosomes, in each cell of their body, respectively. An example of a numerical chromosome abnormality is Down syndrome, which is caused by having an entire extra chromosome 21, for a total of three copies of chromosome 21 instead of two (www.genetics.emory.edu).
ü Structural Chromosome Abnormality means that a portion of the genetic material has been rearranged in some way; for example, a piece of one chromosome may be attached to another chromosome (translocation), or a piece of a chromosome may be turned upside down (inversion). A rearrangement may or may not result in obvious health problems. This depends on whether the structural problem ultimately results in a net gain or loss of chromosome material. If the chromosome material is simply in a rearranged fashion, but all of the genetic information is present, the person may have no symptoms and this is known as a balanced rearrangement (www.genetics.emory.edu).
Requirement for chromosome analysis:
Chromosome analysis is recommended as a routine diagnostic procedure for a number of indications, including the following:
- Problems noted during early growth/development,
- Stillbirths and neonatal deaths,
- Fertility problems,
- Pregnancy in women 35 years or older at the time of delivery, and
- Family History (Rowley. 2001).