224
CHROMOSOMES
PLAY A ROLE IN
HEREDITY
THOMAS HUNT MORGAN (1866–1945)
D
uring the 19th century,
biologists observing cells
divide under a microscope
noticed the appearance of pairs of
tiny threads in every cell’s nucleus.
These threads could be stained by
dyes for observation, and came to
be called chromosomes, meaning
“colored bodies.” The biologists
soon began to wonder whether
chromosomes had something to
do with heredity.
In 1910, experiments conducted
by American geneticist Thomas
Hunt Morgan would confirm theroles of genes and chromosomes
in inheritance, explaining evolution
at a molecular level.Particles of inheritance
By the early 20th century, scientists
had traced the chromosome’s
precise movements at cell division,
and noticed that the number of
chromosomes varied between
species, but that the number in the
body cells of the same species were
usually the same. In 1902, German
biologist Theodor Boveri, having
studied the fertilization of a seaIN CONTEXT
BRANCH
BiologyBEFORE
1866 Gregor Mendel describes
laws of inheritance, concluding
that inherited characteristics
are controlled by discrete
particles, later called genes.1900 Dutch botanist Hugo de
Vries reaffirms Mendel’s laws.1902 Theodor Boveri and
Walter Sutton independently
conclude that chromosomes
are involved in inheritance.AFTER
1913 Morgan’s student Alfred
Sturtevant constructs the first
genetic “map,” of the fruit fly.1930 Barbara McClintock
discovers that genes can shift
positions on chromosomes.1953 James Watson and
Francis Crick’s double-helix
model of DNA explains how
genetic information is passed
on during reproduction.Chromosomes
play a role
in heredity.When cells divide, their chromosomes split and
replicate in ways that parallel the emergence
of inherited characteristics.This suggests that genes controlling these
characteristics occur on the chromosomes.Some characteristics depend
on the sex of the organism,
so must be controlled by sex-
determining chromosomes.