The Science Book

A CENTURY OF PROGRESS 171

Hugo de Vries discovered the 3:1

ratio of characteristics in experiments

with a variety of plants in the 1890s.

He would later concede that Mendel

had a claim to priority in the discovery.

of Mendel’s work, biologists had
demonstrated that Mendel’s
“particles of inheritance” were real
and that they were carried
on chromosomes.

Laws of inheritance refined
Mendel had established two laws
of inheritance. First, the fixed
proportions of characteristics in
offspring led him to conclude that
the particles of inheritance came
in pairs. There was a particle pair
for flower color, a pair for seed color,
and so on. Pairs were formed at
fertilization because one particle
came from each parent—and
separated again when the new
generation reproduced to form
its own sex cells. If the particles
coming together were different
varieties (such as those for purple
and white flower), only the dominant
particle would be expressed.
In modern terms, the different
varieties of genes are called alleles.
Mendel’s first law became known

as the Law of Segregation because

the alleles segregated to form

sex cells. Mendel’s second

law arose when he considered

two characteristics. The Law of

Independent Assortment suggests

that the relevant genes for each

trait are inherited independently.

Mendel’s choice of plant species

was, it turns out, fortuitous. We

now know that the characteristics

of Pisum sativum follow the

simplest pattern of inheritance.

Each characteristic—such as

flower color—is under the control of

a single type of gene that comes in

different varieties (alleles). However,

many biological characteristics—

such as human height—are the

outcome of the interactions of

many different genes.

Furthermore, the genes

Mendel studied were inherited

independently. Later work would

show that genes can sit side-by-

side on the same chromosome.

Each chromosome carries

I suggest...the term

Genetics, which sufficiently

indicates that our labours are

devoted to the elucidation of

the phenomena of heredity

and variation.

William Bateson

hundreds or thousands of genes

on a string of DNA. Chromosome

pairs separate to create sex

cells, and the chromosome is

then passed on whole. This

means that the inheritance of

traits controlled by different

genes on the same chromosome

is not independent. Each pea

characteristic studied by

Mendel is due to a gene on a

separate chromosome. If they

had been on the same chromosome,

his results would have been more

complex and harder to interpret.

In the 20th century, research

would reveal the exceptions to

Mendel’s laws. As scientists probed

more deeply into the behavior of

genes and chromosomes, they

confirmed that inheritance can

happen in more complicated ways

than Mendel had found. However,

these discoveries build on, rather

than contradict, Mendel’s findings,

which laid the foundation for

modern genetics. ■