170 GREGOR MENDEL
For example, one-sixteenth of the
plants had the combination with
both recessive traits (white flower,
green seed). Mendel concluded
that the two characteristics were
inherited independently of one
another. In other words, inheritance
of flower color had no effect on
inheritance of seed color and vice
versa. The fact that heredity was
precisely proportional in this way
led Mendel to conclude that it was
not due to the blending of vague
chemical principles after all, but
happened because of discrete
“particles.” There were particles
controlling flower color, particles
for seed color, and so on. These
particles were transferred from
parents to offspring intact. This
explained why recessive traits
could hide their effects and skip
a generation: a recessive trait
would only show through if a plant
inherited two identical doses of
the particle concerned. Today we
recognize these particles as genes.Genius recognized
Mendel published the results of
his findings in a journal of natural
history in 1866, but his work failed
to make an impact in the wider
scientific world. The esoteric
nature of his title—Experiments
in Plant Hybridization—might have
restricted the readership but,
in any case, it took more than
30 years for Mendel to be properly
appreciated for what he had
done. In 1900, Dutch botanist
Hugo de Vries published the results
of plant breeding experiments
similar to those of Mendel—
including a corroboration of the
three-to-one ratio. De Vries followed
up with an acknowledgment
that Mendel had got there first.A few months later, German
botanist Carl Correns explicitly
described Mendel’s mechanism
for inheritance. Meanwhile, in
England—spurred on after reading
the papers of de Vries and
Correns—Cambridge biologist
William Bateson read Mendel’s
original paper for the first time
and immediately recognized its
significance. Bateson would
become a champion of Mendelian
ideas, and he ended up coining
the term “genetics” for this new
field of biology. Posthumously,
the Augustinian monk had at
last been appreciated.
By then, work of a different
kind—in the fields of cell biology
and biochemistry—was guiding
biologists down new avenues
of research. Microscopes
were replacing plant breeding
experiments as scientists searched
for clues by looking right inside
cells. Nineteenth-century biologists
had a hunch that the key to
heredity lay in the cell’s nucleus.
Unaware of Mendel’s work, in
1878, German Walther Flemming
identified the threadlike structures
inside cell nuclei that moved
around during cell division.
He named them chromosomes,
meaning “colored body.” Within
a few years of the rediscoveryTraits disappear entirely in
the hybrids, but reappear
unchanged in their progeny.
Gregor MendelThe first generation of peas (F^1 ) bred from
“pure” white- and purple-flowered plants all
have one particle from each parent. Purple is
dominant, so all the F^1 flowers are purple.
In the second generation (F^2 ), one plant in
four will inherit two “white” particles and
produce white flowers.
Parent generationF 1
F 2
3:1 proportionParticle for whiteParticle for purpleKEY