Shifts in chromosome combinations, with or without crossovers, can
produce unique organisms with characteristics not quite like any
organism that appeared in the past nor likely to appear in the
reasonable future. They may even produce novelties in individual
characteristics since genes can affect one another, and a gene
surrounded by unusual neighbors can produce unexpected effects.
Matters can go further still, however, in the direction of novelty. It
is possible for chromosomes to undergo more serious changes, either
structural or chemical, so that entirely new characteristics are
produced that might not otherwise exist. Such changes are called
_mutations_.
We must be careful how we use this term. A child may possess some
characteristics not present in either parent through the mere shuffling
of chromosomes and not through mutation.
Suppose, for instance, that a man is heterozygous to eye color, carrying
one gene for brown eyes and one for blue eyes. His eyes would, of
course, be brown since the gene for brown eyes is dominant over that for
blue. Half the sperm cells he produces would carry a single gene for
brown eyes in its half set of chromosomes. The other half would carry a
single gene for blue eyes. If his wife were similarly heterozygous (and
therefore also had brown eyes), half her egg cells would carry the gene
for brown eyes and half the gene for blue.
It might follow in this marriage, then, that a sperm carrying the gene
for blue eyes might fertilize an egg carrying the gene for blue eyes.
The child would then be homozygous, with two genes for blue eyes, and he
would definitely be blue-eyed. In this way, two brown-eyed parents might
have a blue-eyed child and this would _not_ be a mutation. If the
parents’ ancestry were traced further back, blue-eyed individuals would
undoubtedly be found on both sides of the family tree.
If, however, there were no record of, say, anything but normal color
vision in a child’s ancestry, and he were born color-blind, that could
be assumed to be the result of a mutation. Such a mutation could then be
passed on by the normal modes of inheritance and a certain proportion of
the child’s eventual descendants would be color-blind.
A mutation may be associated with changes in chromosome structure
sufficiently drastic to be visible under the microscope. Such
_chromosome mutations_ can arise in several ways. Chromosomes may
undergo replication without the cell itself dividing. In that way, cells
can develop with two, three, or four times the normal complement of
chromosomes, and organisms made up of cells displaying such _polyploidy_
can be markedly different from the norm. This situation is found chiefly
among plants and among some groups of invertebrates. It does not usually
occur in mammals, and when it does it leads to quick death.
Less extreme changes take place, too, as when a particular chromosome
breaks and fails to reunite, or when several break and then reunite
incorrectly. Under such conditions, the mechanism by which chromosomes
are distributed among the daughter cells is not likely to work
correctly. Sex cells may then be produced with a piece of chromosome (or
a whole one) missing, or with an extra piece (or whole chromosome)
present.
In 1959, such a situation was found to exist in the case of persons
suffering from a long-known disease called Down’s syndrome.[2] Each
person so afflicted has 47 chromosomes in place of the normal 46. It
turned out that the 21st pair of chromosomes (using a convention whereby
the chromosome pairs are numbered in order of decreasing size) consists
of three individuals rather than two. The existence of this chromosome
abnormality clearly demonstrated what had previously been strongly
suspected—that Down’s syndrome originates as a mutation and is inborn
(see the figure on the next page).
[Illustration: _Karyotype of a female patient with Down’s syndrome
(Mongolism). During meiosis both chromosomes No. 21 of the mother,
instead of just one, went to the ovum. Fertilization added the father’s
chromosome, which made three Nos. 21 instead of the normal pair.
(Compare with the normal karyotype on page 4.)_]
Most mutations, however, are not associated with any noticeable change
in chromosome structure. There are, instead, more subtle changes in the
chemical structure of the genes that make up the chromosome. Then we
have _gene mutations_.
The process by which a gene produces its own replica is complicated and,
while it rarely goes wrong, it does misfire on occasion. Then, too, even
when a gene molecule is replicated perfectly, it may undergo change
afterward through the action upon it of some chemical or other
environmental influence. In either case, a new variety of a particular
gene is produced and, if present in a sex cell, it may be passed on to
descendants through an indefinite number of generations.
Of course, chromosome or gene mutations may take place in ordinary cells
rather than in sex cells. Such changes in ordinary cells are _somatic
mutations_. When mutated body cells divide, new cells with changed
characteristics are produced. These changes may be trivial, or they may
be serious. It is often suggested, for instance, that cancer may result
from a somatic mutation in which certain cells lose the capacity to
regulate their growth properly. Since somatic mutations do not involve
the sex cells, they are confined to the individual and are not passed on
to the offspring.