The Science Book

A CENTURY OF PROGRESS 169

Mendel’s discovery
The breakthrough in understanding
inheritance came nearly a century
before the chemical structure of
DNA was established—and
less than a decade after Darwin
published On the Origin of Species.
Gregor Mendel, an Augustinian
monk in Brno, was a teacher,
scientist, and mathematician who
succeeded where many better-
known naturalists had failed. It
was, perhaps, Mendel’s skills in
mathematics and probability
theory that proved the difference.
Mendel conducted experiments
with the common pea, Pisum
sativum. This plant varies in
several identifiable ways, such
as height, flower color, seed color,
and seed shape. Mendel started
looking at the inheritance of one
characteristic at a time and applied
his mathematical mind to the
results. By breeding pea plants,
which were easily cultivated in
the monastery grounds, he could
conduct a series of experiments
to obtain meaningful data.
Mendel took critical precautions
in his work. Recognizing that
characteristics can skip and hide
through generations, he was careful
to start with pea plants of “pure”
stock—such as white-flowered
plants that only produced white-
flowered offspring. He crossed
pure white-flowered plants
with pure purple-flowered ones,
pure tall with pure short, and so
on. In each case, he also precisely
controlled the fertilization: using
tweezers, he transferred pollen from
unopened flower buds to stop them
from scattering indiscriminately.
He performed these breeding
experiments many times
and documented the numbers

and characteristics of plants in the

next generation, and the generation

after that. He found that alternate

varieties (such as purple flower

and white flower) were inherited

in fixed proportions. In the first

generation, only one variety, such

as purple flower, came through; in

the second generation, this variety

accounted for three-quarters

of the offspring. Mendel called this

the dominant variety. He called the

other variety the recessive variety.

In this case, white flower was

recessive, and made up a quarter of

the second generation plants. For

each characteristic—tall/short;

seed color; flower color; and seed

See also: Jean-Baptiste Lamarck 118 ■ Charles Darwin 142–49 ■ Thomas Hunt Morgan 224–25 ■
James Watson and Francis Crick 276–83 ■ Michael Syvanen 318–19 ■ William French Anderson 322–23

This is explained if inheritance

is controlled by pairs of particles

inherited from the parents.

A pea’s flower may

be white or purple.

Purebred purple

peas crossed with

purebred white peas

produce a first generation

of peas that are

all purple.

Breeding the first

generation of purple plants

with each other produces a

second generation with

both purple and white

in a proportion

of 3 to 1.

Purple is the

dominant characteristic.

White is the recessive

characteristic.

shape—it was possible to identify

dominant and recessive varieties

according to these proportions.

The key conclusion

Mendel went further and tested the

inheritance of two characteristics

simultaneously—such as flower

color and seed color. He found that

offspring ended up with different

combinations of traits and—once

again—these combinations

occurred in fixed proportions. In

the first generation, all plants

had both dominant traits (purple

flower, yellow seed), but in the

second generation there was a

mixture of combinations. ❯❯