Each cell is a tiny chemical factory in which several thousand different
kinds of chemical changes are constantly taking place among the numerous
sorts of molecules that move about in its fluid or that are pinned to
its solid structures. These chemical changes are guided and controlled
by the existence of as many thousands of different _enzymes_ within the
cell.
Enzymes possess large molecules built up of some 20 different, but
chemically related, units called _amino acids_. A particular enzyme
molecule may contain a single amino acid of one type, five of another,
several dozen of still another and so on. All the units are strung
together in some specific pattern in one long chain, or in a small
number of closely connected chains.
Every different pattern of amino acids forms a molecule with its own set
of properties, and there are an enormous number of patterns possible. In
an enzyme molecule made up of 500 amino acids, the number of possible
patterns can be expressed by a 1 followed by 1100 zeroes (10¹¹⁰⁰).
Every cell has the capacity of choosing among this unimaginable number
of possible patterns and selecting those characteristic of itself. It
therefore ends with a complement of specific enzymes that guide its own
chemical changes and, consequently, its properties and its behavior. The
“instructions” that enable a fertilized ovum to develop in the proper
manner are essentially “instructions” for choosing a particular set of
enzyme patterns out of all those possible.
The differences in the enzyme-guided behavior of the cells making up
different species show themselves in differences in body structure. We
cannot completely follow the long and intricate chain of
cause-and-effect that leads from one set of enzymes to the long neck of
a giraffe and from another set of enzymes to the large brain of a man,
but we are sure that the chain is there. Even within a species,
different individuals will have slight distinctions among their sets of
enzymes and this accounts for the fact that no two human beings are
exactly alike (leaving identical twins out of consideration).
Each chromosome can be considered as being composed of small sections
called _genes_, usually pictured as being strung along the length of the
chromosome. Each gene is considered to be responsible for the formation
of a chain of amino acids in a fixed pattern. The formation is guided by
the details of the gene’s own structure (which are the “instructions”
earlier referred to). This gene structure, which can be translated into
an enzyme’s structure, is now called the _genetic code_.
[Illustration: _Stained section of one cell from salivary gland of_
Drosophila, _or fruit flies, reveals dark bands that may be genes
controlling specific traits_.]
If a particular enzyme (or group of enzymes) is, for any reason, formed
imperfectly or not at all, this may show up as some visible abnormality
of the body—an inability to see color, for instance, or the possession
of two joints in each finger rather than three. It is much easier to
observe physical differences than some delicate change in the enzyme
pattern of the cells. Genes are therefore usually referred to by the
body change they bring about, and one can, for instance, speak of a
“gene for color blindness”.
A gene may exist in two or more varieties, each producing a slightly
different enzyme, a situation that is reflected, in turn, in slight
changes in body characteristics. Thus, there are genes governing eye
color, one of which is sufficiently important to be considered a “gene
for blue eyes” and another a “gene for brown eyes”. One or the other,
but not both, will be found in a specific place on a specific
chromosome.
The two chromosomes of a particular pair govern identical sets of
characteristics. Both, for instance, will have a place for genes
governing eye color. If we consider only the most important of the
varieties involved, those on each chromosome of the pair may be
identical; both may be for blue eyes or both may be for brown eyes. In
that case, the individual is _homozygous_ for that characteristic and
may be referred to as a _homozygote_. The chromosomes of the pair may
carry different varieties: A gene for blue eyes on one chromosome and
one for brown eyes on the other. The individual is then _heterozygous_
for that characteristic and may be referred to as a _heterozygote_.
Naturally, particular individuals may be homozygous for some types of
characteristics and heterozygous for others.
When an individual is heterozygous for a particular characteristic, it
frequently happens that he shows the effect associated with only one of
the gene varieties. If he possesses both a gene for brown eyes and one
for blue eyes, his eyes are just as brown as though he had carried two
genes for brown eyes. The gene for brown eyes is _dominant_ in this case
while the gene for blue eyes is _recessive_.