It is easier to observe the removal of genes through death or through
failure to reproduce than to observe their production through mutation.
It is particularly difficult to study their production in human beings,
since men have comparatively long lifetimes and few children, and since
their mating habits cannot well be controlled.
For this reason, geneticists have experimented with species much simpler
than man—smaller organisms that are short-lived, produce many offspring,
and that can be penned up and allowed to mate only under fixed
conditions. Such creatures may have fewer chromosomes than man does and
the sites of mutation are more easily pinned down.
An important assumption made in such experiments is that the machinery
of inheritance and mutation is essentially the same in all creatures and
that therefore knowledge gained from very simple species (even from
bacteria) is applicable to man. There is overwhelming evidence to
indicate that this is true in general, although there are specific
instances where it is not completely true and scientists must tread
softly while drawing conclusions.
The animals most commonly used in studies of genetics and mutations are
certain species of fruit flies, called _Drosophila_. The American
geneticist, Hermann J. Muller, devised techniques whereby he could study
the occurrence of lethal mutations anywhere along one of the four pairs
of chromosomes possessed by _Drosophilia_.
A lethal gene, he found, might well be produced somewhere along the
length of a particular chromosome once out of every two hundred times
that chromosome underwent replication. This means that out of every 200
sex cells produced by _Drosophilia_, one would contain a lethal gene
somewhere along the length of that chromosome.
[Illustration: _Geneticist Hermann J. Muller studying_ Drosophila _in
his laboratory. Dr. Muller won a Nobel Prize in 1946 for showing that
radiation can cause mutations. (See page 34.)_]
That particular chromosome, however, contained at least 500 genes
capable of undergoing a lethal mutation. If each of those genes is
equally likely to undergo such a mutation, then the chance that any one
particular gene is lethal is one out of 200 × 500, or 1 out of 100,000.
This is a typical mutation rate for a gene in higher organisms
generally, as far as geneticists can tell (though the rates are lower
among bacteria and viruses). Naturally, a chance for mutation takes
place every time a new individual is born. Fruit flies have many more
offspring per year than human beings, since their generations are
shorter and they produce more young at a time. For that reason, though
the mutation rate may be the same in fruit flies as in man, many more
actual mutations are produced per unit time in fruit flies than in men.
This does not mean that the situation may be ignored in the case of man.
Suppose the rate for production of a particular deleterious gene in man
is 1 out of 100,000. It is estimated that a human being has at least
10,000 different genes, and therefore the chance that at least one of
the genes in a sex cell is deleterious is 10,000 out of 100,000 or 1 out
of 10.
Furthermore, it is estimated that the number of gene mutations that are
weakly deleterious are four times as numerous as those that are strongly
deleterious or lethal. The chances that at least one gene in a sex cell
is at least weakly deleterious then would be 4 + 1 out of 10, or 1 out
of 2.
Naturally, these deleterious genes are not necessarily spread out evenly
among human beings with one to a sex cell. Some sex cells will be
carrying more than one, thus increasing the number that may be expected
to carry none at all. Even so, it is supposed that very nearly half the
sex cells produced by humanity carry at least one deleterious gene.
Even though only half the sex cells are free of deleterious genes, it is
still possible to produce a satisfactory new generation of men. Yet one
can see that the genetic load is quite heavy and that anything that
would tend to increase it would certainly be undesirable, and perhaps
even dangerous.
We tend to increase the genetic load by reducing the rate at which
deleterious genes are removed, that is, by taking care of the sick and
retarded, and by trying to prevent discomfort and death at all levels.
There is, however, no humane alternative to this. What’s more, it is, by
and large, only those with slightly deleterious genes who are preserved
genetically. It is those persons with nearsightedness, with diabetes,
and so on, who, with the aid of glasses, insulin, or other props, can go
on to live normal lives and have children in the usual numbers. Those
with strongly deleterious genes either die despite all that can be done
for them even today or, at the least, do not have a chance to have many
children.
The danger of an increase in the genetic load rests more heavily, then,
at the other end—at measures that (usually inadvertently or
unintentionally) increase the rate of production of mutant genes. It is
to this matter we will now turn.