Full chromsome analysis by microscopy (karyotyping) is a highly-skilled, labour-intensive process. It requires at least several years of experience before a trained cytogeneticist (who already has a university science degree) can master karyotyping.

Chromosomes are made up of DNA (which contains the genes that largely determine individual characteristics) combined with special DNA-stabilising proteins such as histones. Chromosomes have a central domain called a "centromere", an upper section called "p" and a lower section called "q". For example, the upper arm of chromosome 2 is identified as "2p". Special staining (called "G-banding") allows a cytogeneticist to see "bands" on a chromosome; this banding is a function of the amount and type of histone proteins binding to chromosomal DNA.


Single chromosome (#16) showing the centromere (black arrow) and three of the bands detected with special staining (red arrows)

Karyotyping is a complex process. Cells from the patient must first be grown carefully in culture. 

 

	"Cytogeneticist reviewing growth of cells in a Petri dish"
"Cytogeneticist using a laminar-flow 'biosafety' cabinet' to change culture medium in cells being grown for analysis"

After a sufficient number of cell divisions, cell division is arrested in metaphase (the time in the cell division cycle when the chromosomes are enlarged and therefore able to be differentiated from each other) with special chemicals (e.g. colchicine), and the cells with the metaphase chromosomes are fixed and stained. 

Using a microscope, TissuPath cytogeneticists then fully analyze the chromosomes of at least five metaphase cells. This is a task requiring great skill as the chromosomes in a metaphase spread are not organized or labeled, and must be differentiated on the basis of their size, relative sizes of the p and q arms, and their banding pattern.

"Scientist fixing metaphase cells for analysis"

To produce an fully-organized and interpretable picture of all chromosomes, the metaphase spreads are first subjected to computer-driven image analysis, producing an "electronic" karyotype. However, as the image analysis programs have a limited ability to correctly identify chromsomoes the electronic karyotype always requires checking and editing by our cytotogenetic specialists prior to final analysis by them.

"Cytogeneticist using computer imaging to analyze a karyotype"













After the chromosomes have been identified they are arranged in order, again using an image analysis program. The result is a karyotype. There are 22 "somatic" chromosomes and two sex chromosomes, X and Y. Normally, everyone has two copies of each somatic chromosome.  As far as the sex chromosomes are concerned, normal females have two X chromosomes; males have one X and one Y (the Y chromosome carries the genes which determine male gender). 

Karyotyping can identify a number of medically-important chromosomal alterations, including polysomy (too many of one or more chromosomes) such as Trisomy 21 or Trisomy 18, or monosomy (one instead of the normal two chromosomes) such as is seen in Turner's Syndrome.

Loss of even one "somatic" chromosome (all chromosomes other than X or Y are called "somatic" chromosomes) is usually incompatible with life, and if present in a fetus will result in fetal death and spontaneous abortion ("miscarriage"), usually early in pregnancy. Trisomy of most somatic chromosomes, except 18 and 21, is also usually incompatible with life. Most early miscarriages (2-8 weeks) are due to similar severe chromosomal abnormalities. 

Karyotyping can also detect structural alterations in chromosomes, for example "translocations" in which part of one chromosome has broken off and attached itself to another chromosome. It is possible to have a "balanced translocation" in which pieces of two separate chromosomes have exchanged. Because patients with balanced translocations technically have a normal number of genes, the effect of a translocation is difficult to predict. Some balanced translocations are entirely compatible with normal health, including normal reproductive capacity, although the translocation may be inherited by the children. If the translocation break goes through a critical part of DNA (e.g. a gene or a gene regulatory sequence), or if there has been a deletion not detectable by microscopy, significant abnormalities may result.

When structural abnormalities such as translocations are inherited (e.g. they are discovered in a fetus or a child but subsequent testing shows that one normal parent also has the abnormality) it is reasonable to assume that there will be no resulting clinical findings.