225
See also: Gregor Mendel 166–71 ■ Barbara McClintock 271 ■
James Watson and Francis Crick 276–83 ■ Michael Syvanen 318–19
A PARADIGM SHIFT
urchin, concluded that an organism’s
chromosomes had to be present in
a full set for an embryo to develop
properly. Later that same year, an
American student named Walter
Sutton concluded from his work on
grasshoppers that chromosomes
might even mirror the theoretical
“particles of inheritance” proposed
in 1866 by Gregor Mendel.
Mendel had done exhaustive
experiments in the breeding of pea
plants and, in 1866, suggested that
their inherited characteristics were
determined by discrete particles.
Four decades later, to test the
link between chromosomes and
Mendel’s theory, Morgan embarked
on research that would combine
breeding experiments with modern
microscopy, in what came to be
known as the “Fly Room” at
Columbia University, New York.
From peas to fruit flies
Fruit flies (Drosophila) are gnat-
sized insects that can be bred in
small glass bottles and can produce
the next generation—with a great
many offspring—in just 10 days.
This made the fruit fly ideal for
studying inheritance. Morgan’s
team isolated and crossbred flies
with particular characteristics, and
then analyzed the proportions of
variations in the offspring—just as
Mendel had done with his peas.
Morgan finally corroborated
Mendel’s results after he spotted
a male with eyes that were white
rather than the normal red. Mating
a white-eyed male with a red-eyed
female produced only red-eyed
offspring, which suggested that
red was a dominant trait and
white was recessive. When those
offspring were crossbred, one in
four of the next generation was
white-eyed, and always male. The
“white gene” must be linked to sex.
When other traits linked to sex
appeared, Morgan concluded that
all these traits must be inherited
jointly and the genes responsible
for them must all be carried on
the chromosome that determines
sex. The females had a pair of X
chromosomes, while males had
an X and a Y. During reproduction,
the offspring inherits an X from the
mother, and an X or a Y from the
father. The “white gene” is carried
by the X. The Y chromosome has
no corresponding gene.
Further work led Morgan to
the notion that specific genes
were not only located on specific
chromosomes, but occupied
particular positions on them. This
opened up the idea that scientists
could “map” an organism’s genes. ■Thomas Hunt Morgan
Born in Kentucky, US, Thomas
Hunt Morgan trained as a
zoologist before going
on to study the development
of embryos. After moving to
Columbia University in New
York in 1904, he began to
focus on the mechanism of
inheritance. Initially sceptical
of Mendel’s conclusions, and
even of Darwin’s, he focused
his efforts on the breeding of
fruit flies to test his ideas
about genetics. His success
with fruit flies would lead
many researchers to use them
in genetics experiments.
Morgan’s observation of
stable, inherited mutations in
fruit flies eventually led him to
realize that Darwin was right,
and in 1915, he published a
work explaining how heredity
functioned according to
Mendel’s laws. Morgan
continued his research at
the California Institute of
Technology (Caltech) and,
in 1933, he was awarded the
Nobel Prize in Genetics.Key works1910 Sex-limited Inheritance
in Drosophila
1915 The Mechanism of
Mendelian Heredity
1926 The Theory of the GeneCrossbreeding fruit flies over two
generations shows how the white-eyed
trait is passed only to some males,
through the sex chromosomes.Second Generation (F2)Male FemaleFirst Generation (F1)