3.3. Excursion: A lesson from heredity[[Student version, December 8, 2002]] 83
246810femur length (arbitrary scale)frequencyFigure 3.10:(Sketch histogram.) Results of an imaginary experiment measuring the femur lengths of a purebred
population of sheep. Selectively breeding sheep from the atypical group shown (black bar) doesn’t lead to a generation
of bigger sheep, but instead to offspring with the same distribution as the one shown.
Today we make this distinction by referring to the collection of physical characteristics of the
organism (the output of the software) as thephenotype,while the program itself is thegenotype.
It was Aristotle’s misfortune that medieval commentators fastened on his confused ideas about
physics, raising them to the level of dogma, while ignoring his correct biology. Even Aristotle,
however, could not have guessed that the genetic information carrier would turn out to be a single
molecule.
3.3.2 Identifying the physical carrier of genetic information
Nobody has ever seen a molecule with their unaided eye. We can nevertheless speak with confidence
about molecules, because the molecular hypothesis makes such a tightly interconnected web of falsi-
fiable predictions. A similarly indirect but tight web of evidence drew Schr ̈odinger’s contemporaries
to their conclusions about the molecular basis of heredity.
Tobegin, thousands of years’ experience in agronomy and animal husbandry had shown that
any organism can be inbred to the point where it will breed true for many generations. This
does not mean that every individual in a purebred lineage will be exactly identical to every other
one—certainly there are individual variations. Rather, a purebred stock is one in which there
are noheritablevariations among individuals. To make the distinction clear, suppose we take a
purebred population of sheep and make a histogram of, say, femur lengths. A familiar Gaussian-
typedistribution emerges. Suppose now that we take an unusually big sheep, from the high end
of the distribution (see Figure 3.10). Its offspring will not be unusually big, but rather will lie on
exactly the same distribution as the parent population. Whatever the genetic carrier is, it gets
duplicated and transmitted with great accuracy. Indeed, in humans some characteristic features
can be traced through ten generations.
The significance of this remark may not be immediately obvious. After all, an audio compact
disk contains nearly a gigabyte of information, duplicated and transmitted with near-perfect fidelity
from the factory. But each sheep began with a single cell. A sperm head is only a micrometer or