What is a Karyotype? - Rainbows Down Under

A karyotype is simply a photograph of the chromosomes from a single cell. From that photograph a description of the characteristics of the chromosomes as they are within the nucleus of the cell is able to be prepared. These characteristics are described in terms of the number, form, size and arrangement within the nucleus of the cell. The word karyotype is derived from the Greek word karyo - meaning of the nucleus and typos - meaning mark.

To prepare a karyotype a sample of cells is taken. Usually these are white blood cells (lymphocytes) from a blood draw. They may however be from other sources such as amniocytes taken during an amniocentesis, or fibroblasts from a skin biopsy. After preparation a microphotograph is then taken of the chromosomes during cell division.

An example of a microphotograph of chromosomes
The chromosomes often look bent or twisted. This is completely normal and does not reflect anything about the chromosomes themselves, rather it is simply the way that the chromosomes are sitting on the slide. Chromosomes are very flexible.

Often the cytogeneticist will sort them into size and label them as in this diagram.

In order to communicate what they have been able to see in the photograph, a written report is prepared describing the characteristics. The terms used in these reports are standardised so that others can understand what has been written. However to the lay person these reports often looks very complicated because many different terms and abbreviations are often used. To help I have listed some of these abbreviations.

add - Addition material of unknown origin del - Deletion de novo - A chromosome abnormality which has not been inherited der - Derivative Chromosome dic - Dicentric dup - Duplication fra - Fragile Site idic - Isodicentric chromosome ins - Insertion inv - Inversion i or iso - Isochromosome mar - Marker chromosome mat - Maternal origin Minus sign (-) - Loss mos - Mosaic p - Short arm of chromosome pat - Paternal origin Plus sign (+) - Gain q - Long arm of chromosome r - Ring chromosome rcp - Reciprocal rea - Rearrangement rec - Recombinant chromosome rob - Robertsonian translocation t - translocation tel - Telomere (end of chromosome arm) ter - Terminal end of chromosome upd - Uniparental disomy ? - Uncertain

Here is an example of an actual karyotype and report describing a balanced translocation between chromosome's 5 and 13. Part of the 13th chromosome is attached to the 5th chromosome.

Here is an example of the written report describing a male with Trisomy 18 mosaicism. The blacked out details are simply the patients name and other identifying items. In this case 100 cells were examined and 65 had an additional 18th chromosome making 47 chromosomes in each cell. 35 contained the usual compliment of 46. The actual karyotype, or photograph, would be of two different cells, one with the additional 18th chromosome and one without.



What is Mosaicism?	What is a Phenotype?


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?

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What is a Trisomy Index