84 Chapter 3. The molecular dance[[Student version, December 8, 2002]]
so across, yet it contains roughly the same massive amount of text as that compact disk, in a
package of around 10−^13 times the volume! What sort of physical object could lie behind this feat
of miniaturization? Nineteenth century science and technology offered no direct answers to this
question. But a remarkable chain of observation and logic broke this impasse, starting with the
work of Gregor Mendel, a monk trained in physics and mathematics.
Mendel’s chosen model system was the flowering pea plantPisum sativum.Hechose to study
seven heritable features (flower position, seed color, seed shape, ripe pod shape, unripe pod color,
flower color, and stem length). Each occurred in two clearly identifiable, alternative forms. The
distinctness of these features, or “traits,” endured over many generations, leading Mendel to propose
thatsufficiently simple traits are inherited in a discrete, yes/no manner.^5 Mendel imagined the
genetic code as a collection of switches, which he called “factors,” each of which could be set to
either of two (or more) settings. The various available options for a given factor are now called
allelesof that factor. Later work would show that other traits, which appear to be continuously
variable (for example hair color), are really the combined effect of so many different factors that
the discrete variations from individual factors can’t be distinguished.
Painstaking analysis of many pea plants across several generations led Mendel in 1865 to a set
of simple conclusions:
- The cells making up most of an individual (somatic cells)each carry two copies of
each factor; we say they arediploid.The two copies of a given factor may be “set”
to the same allele (the individual ishomozygousfor that factor), or different ones
(the individual isheterozygousfor that factor). - Germ cells(sperm or pollen, and eggs) are exceptional: They contain only one copy
of each factor. Germ cells form from ordinary cells by a special form of cell division,
in which one copy of each factor gets chosen from the pair in the parent cell. Today
wecall this divisionmeiosis,and the selection of factorsassortment. - Meiosis chooses each factor randomly and independently of the others, an idea now
called the “principle of independent assortment.”
Thus each of the four kinds of offspring shown in each generation of Figure 3.11 is equally likely.
After the fertilized egg forms, it creates the organism by ordinary division (mitosis), in which both
copies of each factor get duplicated. A few of the descendant cells eventually undergo meiosis to
form another generation of germ cells, and the process repeats.
If the two copies of the factor corresponding to a given trait represent different alleles, it may
bethat one allele overrides (or “dominates”) the other in determining the organism’s phenotype.
Nevertheless, both copies persist, with the hidden one ready to reappear in later generations in a
precisely predictable ratio (Figure 3.11). Verifying such quantitative predictions gave Mendel the
conviction that his guesses about the invisible processes of meiosis and mitosis were correct.
Mendel’s rules drew attention to the discrete character of inheritance; the irresistible image of
twoalternative alleles as a switch stuck in one of two possible states is physically very appealing.
Moreover, Mendel’s work showed that by far most of the apparent variation between generations
is simply reassortment of factors, which are themselves extremely stable. Other types of heritable
variations do occur spontaneously, but thesemutationsare rare. Moreover, mutations, too, are
(^5) Interestingly, Charles Darwin also did extensive breeding experiments, on snapdragons, obtained data similar to
Mendel’s, and yet failed to perceive Mendel’s laws. Mendel’s advantage was his mathematical background. Later
Darwin would express regret that he had not made enough of an effort to know “something of the great leading
principles of mathematics,” and wrote that persons “thus endowed seem to have an extra sense.”