Genetics: The Science of Variation
1) Excerpts from Genetics: An Introduction to the Study of Heredity (1913)
by Herbert Eugene Walter, Assistant Professor of Biology, Brown University
Within a generation the center of biological interest has gradually been swinging from the
origin of species to the origin of the individual. The nineteenth century was Darwin’s century.
His monumental work “On the Origin of Species by Means of Natural Selection,” which
appeared in 1859, not only dominated the biological sciences but also influenced profoundly
many other realms of thought, particularly those of philosophy and theology.
Now, at the beginning of the twentieth century, a particular emphasis is being laid upon the
study of heredity. The interpretation of investigations along this line of research had been made
possible through the cumulative discoveries of many things that were not known in Darwin’s
day. Trained students have been patiently and persistently bending over improved microscopes,
untangling the mysteries of the cell, while an increasing host of investigators, inspired by the
Austrian monk Mendel, have been industriously devoting their energies to breeding animals
and plants with an insight denied to breeders of preceding centuries.
The study of the origin of the individual . . . forms the subject-matter of heredity, or, to use
the more definitive word of [William] Bateson, of genetics.
It is not with the individual as a whole that genetics is chiefly concerned, but rather with
characteristics of the individual.
Three factors determine the characteristics of an individual, namely, environment, training,
and heritage as expressed diagrammatically in Figure 1. It may indeed be said that an
individual is the
result of the
interaction of
these three
factors since he
may be
modified by
changing any
one of them.
Although no
one factor can
possibly be
omitted, the
student of
genetics places
the emphasis upon heritage as the factor of greatest importance. Heritage, or “blood,”
expresses the innate equipment of individual. It is what he actually is even before birth. It is
his nature. It is what determines whether he shall be a beast or a man. Consequently in the
diagram (Fig. 1), the triangle of life is represented as resting solidly upon the side marked
“heritage” for its foundation.
Environment and training, although indispensable, are both factors which are subsequent
and secondary. Environment is what the individual has, for example, housing, food, friends and
enemies, surrounding aids which may help him and obstacles which he must overcome. It is the
particular world into which he comes . . .
Training, or education, on the other hand, represents what the individual does with his
heritage and environment. Lacking a suitable environment a good heritage may come to
naught like good seed sown upon stony ground, but it is nevertheless true that the best
environment cannot make up for defective heritage . . . Consequently the biologist holds that,
although what an individual has and does is unquestionably of great importance, particularly to
the individual himself, what he is, is far more important in the long run. Improved environment
and education may better the generation already born. Improved blood will better every
generation to come. [pp. 1-4]
Diagram showing dominant and recessive characters in a variety of organisms
2) Excerpts from “The Factors of Organic Evolution,” from The Nature of the World and of
Man (1927) by Horatio Hackett Newman, University of Chicago
While the main highways of evolution are now well marked out, some of the bypaths are
still obscure. All the facts so far presented may be organized and interpreted on an evolutionary
assumption. Thousands of other facts have been satisfactorily explained on similar grounds, and
there are at present no known facts contrary to the principle of evolution. For these reasons
scientists the world over agree that the validity of the principle has been amply
demonstrated. Many go so far as to rank evolution as a law of nature and assign to it a rank
equal to that of the law of gravitation, the Copernican theory, the atomic theory, and the other
great scientific generalizations. The basis of its validity is the same as theirs, for they all
severally owe their high place in human regard to the fact that they explain and rationalize and
unify the facts of nature . . . Let us then rest assured that the truth of evolution is
demonstrated . . .
While it may be generally agreed that certain causal factors co-operate to bring about
evolution, few evolutionists would attach the same relative importance to the various
factors. This is what is really meant when it is said that biologists are not at all in agreement
about evolution. All are agreed as to the general fact and as to its main trends, but there is much
disagreement of a technical sort as to the relative values of the co-operating causal factors . . .
This is an age of rapid discovery in the field of evolution; most of our knowledge regarding
its mechanism has come within the present century . . .
A great biological problem.—The units of life as we know them are individuals, or
organisms. Even within the limits of a species one encounters the utmost individual diversity.
No two individuals are exactly alike; not even are two leaves on a tree or two scales on a
fish exactly alike. Hence every individual is unique. Our first problem is that of
determining just why each individual is just as it is. Broadly speaking, the peculiarities of an
individual result from the co-operation of two factors, an intrinsic factor, heredity, and an
extrinsic factor, environment. Some writers would include training as a third factor. Logically,
however, there is no middle ground between intrinsic and extrinsic. Therefore, it seems best to
consider training as one aspect of the environment.
Heredity and environment.—The heredity of an organism includes everything that is
passed along from one generation to another through the protoplasmic organization of the germ
cells, both paternal and maternal. Typically, a new individual starts out in life as a single cell
(zygote) produced by the union of two parent germ cells (gametes) known respectively as the
egg and the sperm. The zygote is capable of developing only under certain definite
environmental conditions. The environment not only conditions development but also
determines to some extent the course and the character of development. Every peculiarity
of an organism is the product of a long series of interactions not only between the organism
and the environment but among different parts of the organism. A change in the
environment may make a fish zygote develop into a somewhat different fish, but it cannot
produce a new species of fish. Nor can similarity of environment cause the zygotes of two
different species of fish to become alike. Environment than can modify development only
within certain limits . . .
FACTS AND THEORIES OF HEREDITY
For a long time mankind has been aware of the general facts about heredity. This is attested
by the numerous aphorisms, such as "Blood will tell"; "He is a chip off the old block"; "He
comes from good (or bad) stock." . . .
The first attempt to systematize the available data about heredity was that of Charles Darwin.
In his famous book, Variation of Animals and Plants under Domestication, he presented a large
amount of data about heredity, but this attempt to interpret the facts was largely a failure, for
two reasons. First, there was no definite understanding about germ cells and the processes of
sexual reproduction; second, in all hybridization experiments the individual as a whole was
considered as the unit of heredity . . .
How are the characters of the individual represented in the germ plasm? This is perhaps
the leading problem of genetics. [August] Weismann thought that there was a specific
determiner in the germ plasm for every character of the organism and that in the course of
development the various determiners were segregated into different groups of body cells in such
a way that each kind of cell got its own specific determiner. The germ cells, however, remain
unchanged from generation to generation, each containing all the determiners characteristic of
the species. This theory . . . has been profoundly modified by subsequent discoveries.
The work of Gregor Mendel actually preceded that of Weismann by a good many years, but it
was not appreciated until after the germ-plasm theory became current. Mendel . . . performed an
extensive series of hybridization experiments with a number of varieties of cultivated peas.
His analysis of results, a model of scientific method, was presented before an obscure local
scientific society in 1865. Unfortunately it was virtually lost to the world for thirty-five years
but was rediscovered in 1900. The rediscovery of Mendel's paper, Researches on PlantHybrids, was a great event, for it marked the beginning of scientific genetics. The early
hybridizers before Mendel thought of the individual as the unit of heredity and considered
that hybrid offspring were merely intermediate in type, that there was a sort of an
averaging up of hereditary differences. Mendel came to the conclusion that the individual is
not the unit of heredity, but that each individual is a complex of many independently
heritable characters that may be separated from one another and recombined in the
various offspring in a great variety of different ways. There is a regularity and definiteness
about the separation and recombination of character differences that has come to be expressed in
the form of certain rules or laws that are now called Mendel's laws.
MENDEL'S LAWS
The law of dominance.—Almost all of the unit characters show two or more different forms
of expressions. Thus there are brown eyes, blue eyes, and hazel eyes; straight, kinky, and
wavy hair; tall, short, and medium stature; broad and narrow nose. As a rule, two different
expressions or forms of the same character are mutually exclusive, any single individual showing
one or the other. When two individuals, differing with reference to a single unit character,
mate, their offspring will show one or the other character expression. Thus a pure black
guinea-pig when mated with a pure white or albino individual will have none but black offspring.
That character difference which appears in the offspring to the exclusion of the other is
known as dominant; the one that fails to appear, as recessive . . .
The law of segregation.—This law is . . . undoubtedly the most significant and far-reaching
of all the laws of heredity. We may illustrate the workings of this law by a continuation of the
experiment involving the mating of black and white guinea-pigs. The first generation black
hybrids (now universally known by the shorthand expression, F1) are interbred and, strangely
enough, some of the offspring of the next generation (F2) are black and some white, in a
very definite ratio of three blacks to one white. Thus the hybrids have split up in their
offspring into the two forms that were originally bred together . . . It is a universal law that in
the F2 generation there will appear on the average three dominants to one recessive . . .
To black and white fur in guinea-pigs we may add smooth and rough coat. "Rough" is
dominant over "smooth." Mate a black, smooth-haired guinea-pig with a white, rough-haired
one [Figure 119].
The F1 generation will be all "black rough," for F1 individuals show only the dominant
characters. Interbreed the F1 hybrids and we get the following F2 ratio: nine black rough
to three black smooth to three white rough to one white smooth. In order to explain this ratio
we need merely assume that black and white lie in one pair of chromosomes, rough and smooth
lie in a different pair. Also we must assume that each pair behaves as if it were alone and
uninfluenced by the other pair. The "black smooth" parent will give only black smooth gametes
(Bs), the "white rough" parent will give only white rough gametes (wR) . . . If these unite with
each other on a pure chance basis we may represent the result by the checkerboard method
[See diagram below].
Wherever both B and R are present the individual will be "black rough" whether or not
the recessive characters are also present. It is a simple matter to count nine out of sixteen are
black rough, three that are black smooth, three that are white rough, and only one that contains
no dominant genes and is therefore white smooth . . .
THE ORIGIN OF NEW HEREDITARY CHARACTERS
Differences in developed characters have been shown to be due to differences in the
chemical packets or genes and to differences in the environment. Only by breeding
successive generations under a constant environment is it possible to note changes in the genes.
In the fruit fly, Drosophila melanogaster, [Thomas Hunt] Morgan and his collaborators
have observed about four hundred gene changes in the last two decades. These so-called
gene mutations affect changes in practically all parts of the body, the most obvious being those
concerned with the color, shape, size, and texture of the eyes; size, shape, and texture of the
wings; number, shape, and distribution of the bristles; changes in legs, body regions, internal
organs; and numerous physiological characteristics such as general health or viability. Many
changes are so serious in character as to shorten life or prevent the completion of development.
Similar mutations of the gene type have been observed in many other animals and in not a
few plants. In general, it may be said that any hereditary change that follows the laws of
Mendelian heredity is due to a gene mutation . . . [pp. 381-400]