3.3. Excursion: A lesson from heredity[[Student version, December 8, 2002]] 85
Figure 3.11:(Diagram.) (a)Purebred red and white flowers are cross-pollinated to yield offspring, each with one
chromosome containing the “red” allele and one with the “white” allele. If neither allele is dominant, the offspring
will all be pink. For example four-o’clocks (a flower) exhibit this “semidominance” behavior. (b)Interbreeding the
offspring of the previous generation, we recover pure white flowers in one out of four cases. Even in other species,
for which the red allele is dominant, one in four of the second-generation offspring will still be white. [Cartoon by
George Gamow, from (Gamow, 1961).] [Copyrighted figure; permission pending.]
discrete events, and once formed, a mutation spreads in the population by the same Mendelian rules
listed above. Thus factors are switches that can snap crisply into new positions, but not easily;
once changed by mutation, they don’t switch back readily.
The history of biology in this period is a beautiful counterpoint between classical genetics and
cell biology. Cell biology has a remarkable history of its own; for example many advances had
to await the discovery of staining techniques, without which the various components of cells were
invisible. By about the time of Mendel’s work, E. Haeckel had identified the nucleus of the cell
as the seat of its heritable characters. A recently fertilized egg visibly contained two equal-sized
“pronuclei,” which soon fused. In 1882, W. Flemming noted that the nucleus organized itself into
threadlikechromosomesjust before division. Each chromosome was present in duplicate prior to
mitosis, as required by Mendel’s rules (see Figure 3.11), and just before division each appeared to
double, after which one copy of each was pulled into each daughter cell. Moreover, E. van Beneden
observed that the pronuclei of a fertilized worm egg each had two chromosomes, while the ordinary
cells had four. van Beneden’s result gave visible testimony to Mendel’s logical deduction about the
mixing of factors from both parents.
By this point, it would have been almost irresistible to conclude that the physical carriers of
Mendel’s genetic factors were precisely the chromosomes, had anyone been aware of Mendel. Un-
fortunately Mendel’s results, published in 1865, languished in obscurity, not to be rediscovered
until 1900 by H. de Vries, C. Correns, and E. von Tschermak. Immediately upon this rediscov-
ery W. Sutton and T. Boveri independently proposed that Mendel’s genetic factors were physical
objects—“genes”—physically located on the chromosomes. (Sutton was a graduate student at the