Final answer:
The additive used for chromosome analysis during karyotyping is colchicine, which is used to arrest cells in metaphase. The chromosomes are then stained, often with Giemsa stain, for visualization.
Step-by-step explanation:
The additive used by phlebotomists when performing a chromosome analysis, such as for a karyotype test, is a chemical called colchicine. During this process, white blood cells from a blood sample or other tissue are collected and encouraged to divide.
Colchicine is applied to the culture to halt the process of cell division during metaphase, ensuring that the chromosomes are condensed and more visible under a microscope. The cells are then treated with a hypotonic solution to make them swell and spread out the chromosomes. Finally, the sample is fixed and applied to a slide for further analysis using stains that reveal distinct banding patterns of the chromosomes for visual examination under bright-field microscopy, commonly employing the Giemsa stain for this purpose.