MAKING EXCEPTIONS: In the drawings illustrating one of his 1916 Genetics articles, Calvin Bridges compared the karyotypes of wild-type fruit flies (1–4) with those expressing traits contrary to the normal pattern of sex-linked inheritance (5–24). Normal flies have four pairs of chromosomes, including a pair of X chromosomes for females (1–3) and one X and one Y (hook-shape chromosome) for males (4; the extra spot here was thought to be of no significance). Bridges found that females expressing their mothers’ recessive X-linked traits always had an XXY karyotype as a result of nondisjunction during meiosis (5–19). Figures 20–24 depict the XXYY karyotype of a female produced by mating an XXY female and an XYY male, the parents themselves offspring of an XXY female.
View at full size: PDFC.B. BRIDGES, “NON-DISJUNCTION AS PROOF OF THE CHROMOSOME THEORY OF HEREDITY(concluded),” GENETICS, 1:107-63, 1916.In an era of whole-genome sequencing, epigenetic profiling, and precision gene editing, it’s easy to take for granted the basics of genetic inheritance. But 100 years ago, it was not at all clear how heritable information was passed from one generation to the next. Although cytologists who studied cell division hypothesized that chromosomes were involved in heredity, they lacked experimental data to back up the idea. It took an observant student named Calvin Bridges and some rule-breaking fruit flies (Drosophila melanogaster) to confirm the hunch.
In 1910, Bridges began working in the famous Fly Room at Columbia University as a member of Thomas Hunt Morgan’s lab. Morgan’s group had already described sex-linked inheritance: male fruit flies, which receive just one X chromosome from their mothers, express any recessive genes on that chromosome, such as an unusual eye color. Females, on the other hand, receive an X chromosome from each parent, so recessive X-linked traits emerge only among homozygotes—at least, that’s what happens most of the time. Bridges encountered rare errors in this pattern. About 1 in every 1,600 heterozygotic females expressed an unusual eye color or some other recessive X-linked trait borne by her mother, suggesting she had not received an X chromosome from her father. And an equal number ...
