The Science Book

271

See also: Gregor Mendel 166–71 ■ Thomas Hunt Morgan 224–25 ■

James Watson and Francis Crick 276–83 ■ Michael Syvanen 318–19

I

n the early 20th century, the

laws of inheritance that had

been described by Gregor

Mendel in 1866 were refined as new

discoveries were made about the

particles of inheritance, identified

as genes, and the microscopic

threads that carry them, called

chromosomes. In the 1930s,

American geneticist Barbara

McClintock first realized that

chromosomes were not the stable

structures previously imagined,

and that the position of genes in

chromosomes could alter.

Exchanging genes

McClintock was studying

inheritance in corn plants. A

corncob has hundreds of kernels,

each colored yellow, brown, or

streaked, according to the cob’s

genes. A kernel is a seed—a single

offspring—so studying many cobs

gives a range of data on the

inheritance of kernel color.

McClintock combined breeding

experiments with microscope work

on chromosomes. In 1930, she found

that, during sexual reproduction,

chromosomes paired up when sex

cells were formed, creating an X

shape. She realized that these

X-shaped structures marked

locations where chromosome pairs

were exchanging segments. Genes

that were once linked together on

the same chromosome were shuffled

around, which resulted in new

traits, including variable colors.

This shuffling of genes—called

genetic recombination—produces

a far greater genetic variety in the

offspring. As a result, the chances

of survival in different environments

are enhanced. ■

FUNDAMENTAL BUILDING BLOCKS

JUMPING GENES

BARBARA MCCLINTOCK (1902–1992)

IN CONTEXT

BRANCH

Biology

BEFORE
1866 Gregor Mendel describes
inheritance as a phenomenon
determined by “particles”—
later called genes.

1902 Theodor Boveri and
Walter Sutton independently
conclude that chromosomes
are involved in inheritance.

1915 Thomas Hunt Morgan’s
fruit fly experiments confirm
earlier theories and show that
genes can be linked together
on the same chromosome.

AFTER
1953 James Watson and
Francis Crick’s double-helix
model of the DNA that makes
up chromosomes shows how
genetic material is replicated.

2000 The first human genome
is published, cataloguing the
location of 20,000–25,000
genes on humans’ 23 pairs
of chromosomes.

Variable colors in corn prompted

McClintock to trace the genetic

recombinations responsible for this

variety, which she reported in 1951.