[Greg H]

Hey Darice!

Karen did you a solid with the excellent chromosome primer. Here are a couple of thoughts on your follow-on questions.

20 v. 200. The 20-cell analysis is usually called karyotype. The pathologist culls some cells from the bone marrow aspirate, grows them out in a petri dish or test tube until they are at the point of cell division (which is when you can see the chromosomes) and then picks 20 to isolate and fix up with all the chromosomes in a nice line for analysis. Deciding what's what is done by visual analysis. The pathologist has the whole set of chromosomes laying out in front of her, so she can spot irregularities wherever they occur.

The 200-cell analysis is FISH -- Fluorescence in situ hybridization analysis. Here a special "probe" is created to highlight a specific abnormality in a collection of cells. So, when you do FISH, you test for just one thing. To test for another abnormality, you have to use a different probe. The probe makes the cells that have the abnormality fluoresce in a particular way. Wikipedia has photos. FISH only looks for one irregularity at a time. So it's generally only done to look for the abnormalities associated with a particular disease.

The FISH sample is closer to being big enough to be statistically significant, so the numbers derived from FISH are probably more reliable. But there's a good bit of statistical slop in all of this.

Take care!

Greg