310 Margaret Morrison
techniques developed by the early population geneticists, especially R. A. Fisher
(1890-1962). In that sense then one needs to understand the history of popula-
tion genetics as coincident with another developing science of the early twentieth
century — statistics.
Genes are the physical units of heredity and in modern terms are typically
defined as a segment of genetic material that determines the sequence of amino
acids in specific polypeptides. Gregor Mendel’s work on crossbreeding pea plants
[1865] is usually referred to as the beginning of genetics proper since it was in
that context that his basic principles of inheritance were specified. Leaving aside
questions of what exactly Mendel discovered and what constitutes “Mendelism”, it
is nevertheless important to mention some of the basic features of the story. What
Mendel set forth in his 1865 paper was essentially a law of serial development based
on straightforward combinatory analysis which stated that a hybrid of a particular
character would produce an offspring distributed according to specific proportions.
Based on his experiments with garden peas he analysed the numerical data for pure
parental forms A and a and the hybrid Aa and found that the first generation from
hybrid were distributed according to the ratio 1A:2Aa:1a. This analysis was then
extended to several independent characters or traits. He went on in a later part of
the paper entitled ‘the reproductive cells of hybrids’ to explain the combination of
these traits as the result of the union of reproductive cells, each of which carries a
particular trait. In that sense there was a correspondence between the combination
series of the traits and the union of the gametes during fertilization. What this
means is that if the individual is a hybrid then the two characters oranlagen
will act separately. Although Mendel himself didn’t specify the two independent
laws with which he is frequently associated (segregation, which states that a cross
should be defined in terms of a pair of antagonistic characters and independent
assortment which claims that characters assort independently) the conclusions on
which they are based can clearly be found in his work. And, while he used the
terms dominant and recessive he did not formulate a specific law of dominance as
such.
For thirty-five years Mendel’s work went largely unnoticed, then in 1900 it was
“rediscovered” independently by Hugo De Vries (1848–1935), a Dutch botanist,
Carl Correns (1864–1933), a German biologist, and E. von Tschermak (1871–
1962), an Austrian naturalist. With this rediscovery came new interpretations
and extensions of Mendel’s original results, some of which will be discussed below.
However, even before 1900 other work was being done on the notion of heredity
and its physiological and statistical bases. In order to appreciate the implications
of these early accounts of heredity and the notion of discontinuous variation for
the theory of human evolutionary development we need to go back to 1865, but
to a different context — the work of Francis Galton (1822-1911). Galton was the
first cousin of Charles Darwin and studied medicine at Cambridge. He abandoned
a medical career to explore Africa and in 1853 received a gold medal from Royal
Geographical Society for his achievements. In the early 1860s he did extensive
work with weather maps, using graphical methods for the analysis of multivariate