A karyotype. This is a photograph where all of a cell's chromosomes are separated and lined up, and this can quickly indicate whether there are an abnormal number of chromosomes, and, sometimes based solely on sight, whether certain chromosomes are abnormally short or long. For deeper analysis, the chromosomes can be stained with a radioactive or fluorescent probe to identify the presence/absence of certain genes, and the karyotype would then be analyzed based on the presence of bands or spots of color.
