Heredity 33law of segregation, which states that pairs of genes separate
and keep their individuality and are passed on to the next
generation, unaltered. Another of his laws—independent
assortment—states that different traits (under the control of
distinct genes) are inherited independently of one another.
Mendel based his laws on statistical frequencies of ob-
served characteristics, such as color and texture in gen-
erations of plants. His inferences about the mechanisms of
inheritance were confirmed through the discovery of the
cellular and molecular basis of inheritance in the first half of
the 20th century. When chromosomes, the cellular struc-
tures containing the genetic information, were discovered
at the start of the 20th century, they provided a visible ve-
hicle for transmission of traits proposed in Mendel’s laws.
It was not until 1953 that James Watson and Francis
Crick found that genes are actually portions of moleculesunderstanding of biological inheritance. He went a step far-
ther, though, in that he recognized the need for theoretical
explanations, so at age 34, he began careful breeding experi-
ments in the monastery garden, starting with pea plants.
Over eight years, Mendel planted over 30,000 plants—
controlling their pollination, observing the results, and
figuring out the mathematics behind it all. This allowed
him to predict the outcome of hybridization, or breeding
that combined distinct varieties of the same species, over
successive generations, in terms of basic laws of heredity.
Though his findings were published in 1866 in a respected
scientific journal, no one seemed to recognize the impor-
tance of Mendel’s work during his lifetime.
Interestingly, a copy of this journal was found in Darwin’s
own library with the pages still uncut (journals were printed
on long continuous sheets of paper and then folded into
pages to be cut by the reader), an indication that the journal
had never been read. In 1900, cell biology had advanced to
the point where appreciation of Mendel’s laws was inevitable,
and in that year three European botanists, working indepen-
dently of one another, rediscovered not only the laws but also
Mendel’s original paper. With this recognition, the science of
genetics began. Still, it would be another fifty-three years be-
fore the molecular mechanisms of heredity and the discrete
units of inheritance would be discovered. Today, a compre-
hensive understanding of heredity, molecular genetics, and
population genetics supports evolutionary theory.
Heredity
In order to understand how evolution works, one has to
have some understanding of the mechanics of heredity,
because heritable variation constitutes the raw material for
evolution. Our knowledge of the mechanisms of heredity
is fairly recent; most of the fruitful research into the mo-
lecular level of inheritance has taken place in the past five
decades. Although some aspects remain puzzling, the out-
lines by now are reasonably clear.
The Transmission of Genes
Today we define a gene as a portion of the DNA molecule
containing a sequence of base pairs that encodes a particu-
lar protein; however, the molecular basis of the gene was
not known at the turn of the 20th century when biologists
coined the term from the Greek word for “birth.” Mendel had
deduced the presence and activity of genes by experiment-
ing with garden peas to determine how various traits are
passed from one generation to the next. Specifically, he dis-
covered that inheritance was particulate, rather than blend-
ing, as Darwin and many others thought. That is, the units
controlling the expression of visible traits come in pairs, one
from each parent, and retain their separate identities over the
generations rather than blending into a combination of pa-
rental traits in offspring. This was the basis of Mendel’s first
gene A portion of the DNA molecule containing a sequence of
base pairs that is the fundamental physical and functional unit
of heredity.
law of segregation The Mendelian principle that variants
of genes for a particular trait retain their separate identities
through the generations.
law of independent assortment The Mendelian principle
that genes controlling different traits are inherited indepen-
dently of one another.
chromosomes In the cell nucleus, the structures visible dur-
ing cellular division containing long strands of DNA combined
with a protein.British scientist Rosalind Franklin’s pioneering work in x-ray crystal pho-
tography played a vital role in unlocking the secret of the genetic code
in 1953. Without her permission, Franklin’s colleague Maurice Wilkins
showed one of her images to James Watson. In his book The Double He-
lix, Watson wrote, “The instant I saw the picture my mouth fell open and
my pulse began to race.” While her research was published simultane-
ously in the prestigious journal Nature in 1953 alongside that of James
Watson, Francis Crick, and Maurice Wilkins, only the gentlemen received
the Nobel Prize for the double-helix model of DNA in 1962.© Vittorio Luzzati/National Portrait Gallery, London