Where in the cell do genes actually live?

Genes live on chromosomes. A chromosome is a package of DNA that holds part of an organism's genetic material, and human cells carry 23 pairs of them, 46 in all. The chromosome theory of inheritance says that the paired "factors" in Mendel's laws are chromosomes: they come in matched pairs, one from each parent, and genes sit at specific sites, called loci, along them.

The proof arrived in pieces. Around 1902, Theodor Boveri found in sea urchins that all the chromosomes had to be present for an embryo to develop properly, and Walter Sutton found in grasshoppers that chromosomes come in matched maternal and paternal pairs that separate during meiosis. In 1905 Nettie Stevens showed that a special chromosome in mealworm sperm decided the sex of the offspring.

The clincher came from a doubter. The biologist Ernst Mayr counted T. H. Morgan among the famous geneticists who hotly contested the theory. Then, in 1910, Morgan noticed a white-eyed male among his red-eyed fruit flies. The trait followed sex in its inheritance, so he concluded the gene was probably carried on a sex chromosome, and that other genes probably sat on specific chromosomes as well. By 1915 his fly work gave incontrovertible evidence, and in 1933 he won the Nobel Prize for the role of chromosomes in heredity.

Even counting the chromosomes was hard. In 1923 Theophilus Painter published a count of 48 in humans, other scientists copied his error, and the true number, 46, was only established in 1956.