Chromosomes are thread-like bodies in the cell nucleus of all plants and animals that hold the genes--the blueprints of heredity. Each chromosome carries a single strand of deoxyribonucleic acid (DNA) that threads together about 1,000 genes.

Not much was known about chromosomes prior to the 1880s, due to the lack of adequate cell staining techniques and poor microscopes. In 1879, however, Walther Flemming, using new synthetic dyes, was able to discern bodies in cells that had previously gone undetected. He noticed that some material scattered throughout the nucleus heavily absorbed the dye and coined the word chromatin to describe this dark, stainable substance. Upon further observation, he noted that when a cell divided into two daughter cells, the chromatin first doubled, then split lengthwise, leaving each daughter cell with the same amount of chromatin as the parent cell. By 1882, Flemming had identified all the stages of this process, a fundamental...