64 THE SCIENTIST | the-scientist.com
FOUNDATIONSBRYN MAWR COLLEGE LIBRARY, SPECIAL COLLECTIONS; BIODIVERSITY HERITAGE LIBRARY, N.M. STEVENS,STUDIES IN SPERMATOGENESIS PARTIIBY JOSEPH KEIERLEBERHow Chromosomes X & Y Got Their Names, 1891
W
hy are the human sex chromosomes called “X” and
“ Y,” while the other 22 chromosomes are identified
only by numbers?
The answer begins in the late 1800s, when insect gonad
cells, whose large chromosomes are easy to view through a
microscope, were the specimen of choice for investigating the
cellular basis of heredity. In 1891, German biologist Hermann
Henking counted 11 chromosomes in firebug (Pyrrhocoris
apterus) sperm nuclei. Some nuclei, he discovered, contained
an additional large chromatin element, which he identified in
his drawings with “x.”
A few years later at the University of Kansas, Clarence
McClung observed that half of grasshopper sperm contained
a chromosome similar to Henking’s x element. Like Henking,
McClung used “x” to label this chromosome in the figures of his
1899 paper, but he coined the term “accessory chromosome” to
identify it from then on. McClung soon hypothesized that the
accessory chromosome determined male sex.
In the early 1900s, Nettie Stevens at Bryn Mawr College and
Edmund Beecher Wilson at Columbia University tackled the puzzle
of how chromosomes relate to sex differences in insects. Although
they worked independently, they followed each other’s research.
In 1905, Wilson, the more established scientist, described a pair of
unequally sized chromosomes, which segregated in a 50:50 ratio
among insect sperm. A month later, Stevens reported a similar
discovery in beetle gonads. Half of beetle sperm carried a small
chromosome, which Stevens labeled “s,” and half carried its larger
companion, “ l .” Female somatic cells contained two copies of the
large chromosome, while male cells contained one small and one
large. “This seems to be a clear case of sex determination,” Stevens
wrote, concluding that sperm carrying the small chromosome, not
McClung’s large accessory chromosome, determined male sex.
Wilson cited Stevens’s “very interesting discoveries” in
a footnote, but he was skeptical that these chromosomes
determined sex. By the time of Stevens’s death in 1912 at age
50, Wilson’s scientific stature had overshadowed Stevens’s
groundbreaking theory. In 1909, Wilson concluded that the
unequal chromosomes were indeed sex determinants. Following
Henking’s precedent, he called the large chromosome “X.” For
the small chromosome he chose “ Y.”
Improved microscopy techniques in the 1910s revealed sex-
correlated chromosomes in other animals, including humans.
By 1917, scientists were using X and Y to identify human sex
chromosomes. But for decades, they couldn’t agree on how to
name the other human chromosomes; some used numbers,
while others used letters. Nor did they agree on the number of
human chromosomes, which are small, numerous, and difficult
to count. In 1960, an international panel concluded there are 22human autosomes, and they should be numbered in an ascending
fashion, from shortest to longest. The two sex chromosomes, the
panel advised, “should continue to be referred to as X and Y,
rather than by a number, which would be an additional and
ultimately, a superfluous appellation.”
While using both letters and numbers to identify
chromosomes may be confusing, this convention can be helpful.
Robert Resta, a genetic counselor at Swedish Medical Center in
Seattle, says the XY system helps his patients understand the
inheritance patterns of sex-linked traits. “Patients remember
X and Y from their high school or college biology courses, even
if their understanding of it is imperfect,” Resta says. “They
are at least familiar terms that help make them a little more
comfortable with technical genetic discussions.”gWHAT’S IN A NAME?: Cytogeneticist Nettie Stevens (top left) drew these
images of beetle chromosomes in 1906, labeling one chromosome pair “l”
and “s” in fi gures 102 and 1 07. These chromosomes would come to be known
as the sex chromosomes, X and Y.BRYN MAWR COLLEGE LIBRARY, SPECIAL COLLECTIONS; BIODIVERSITY HERITAGE LIBRARY, N.M. STEVENS,STUDIES IN SPERMATOGENESIS PARTII