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Μ. J. D. WHITE
some number are easily explained by polyploidy. In these at least simple
fragmentation and simple fusion seem to be excluded. As far as animals
are concerned the bulk of the evidence seems decidedly against the healing of broken chromosome ends.
If the telomere concept applies generally, although perhaps with some
special exceptions, it follows that certain types of structural rearrangements which might theoretically be expected, such as terminal inversions, probably do not occur (supposed instances of terminal inversions
being actually subterminal). And other rearrangements, which would
leave a chromosome without a centromere, or with only one telomere,
may occur, but cannot persist through a series of cell divisions.
Thus single chromosome breaks, although they probably occur spontaneously with a rather high frequency, cannot apparently give rise to
viable rearrangements, at least in groups with monocentrie chromosomes. The spontaneous occurrence of three or more breaks in the same
nucleus will be negligibly rare. Consequently we must expect that almost all the rearrangements which participate in the evolutionary process
will be two-break ones. Where it appears that more complicated rearrangements have taken place, it will usually be found on close analysis
that several two-break rearrangements have occurred in succession.
Two-break rearrangements where both breaks are in the same chromosome may be inversions or deletions of chromosome segments. In the
case of monocentrie chromosomes, we may distinguish between rearrangements in which both breaks are on the same side of the centromere (paracentric)
or on opposite sides
(pericentric).
Rearrangements where the breaks are in different chromosomes are
translocations.
Translocations between homologous chromosomes may
give rise to deletions and duplications. A practical difference of some
importance exists between translocations in which the breaks are closely
adjacent to the centromeres or telomeres and those in which the breaks
are situated in the chromosome arms at some distance from either the
centromere or the telomere. The former give rise to what are, effectively, whole-arm transpositions, whereas in the latter the integrity of
the chromosome arms is lost.
One type of whole-arm transposition which is of particular importance
in evolution may be called a centric fusion. It results in the joining together of two acrocentric chromosomes to produce a metacentric.
There may be several ways in which this can come about (Fig. 1). The
first method is for one chromosome to break in the long arm very close
to the centromere while the other breaks in the minute "short arm."
Rejoining of the freshly broken ends "the other way round" will give a
large V-shaped metacentric with a single intact centromere and a minute
Μ. J. D. WHITE
some number are easily explained by polyploidy. In these at least simple
fragmentation and simple fusion seem to be excluded. As far as animals
are concerned the bulk of the evidence seems decidedly against the healing of broken chromosome ends.
If the telomere concept applies generally, although perhaps with some
special exceptions, it follows that certain types of structural rearrangements which might theoretically be expected, such as terminal inversions, probably do not occur (supposed instances of terminal inversions
being actually subterminal). And other rearrangements, which would
leave a chromosome without a centromere, or with only one telomere,
may occur, but cannot persist through a series of cell divisions.
Thus single chromosome breaks, although they probably occur spontaneously with a rather high frequency, cannot apparently give rise to
viable rearrangements, at least in groups with monocentrie chromosomes. The spontaneous occurrence of three or more breaks in the same
nucleus will be negligibly rare. Consequently we must expect that almost all the rearrangements which participate in the evolutionary process
will be two-break ones. Where it appears that more complicated rearrangements have taken place, it will usually be found on close analysis
that several two-break rearrangements have occurred in succession.
Two-break rearrangements where both breaks are in the same chromosome may be inversions or deletions of chromosome segments. In the
case of monocentrie chromosomes, we may distinguish between rearrangements in which both breaks are on the same side of the centromere (paracentric)
or on opposite sides
(pericentric).
Rearrangements where the breaks are in different chromosomes are
translocations.
Translocations between homologous chromosomes may
give rise to deletions and duplications. A practical difference of some
importance exists between translocations in which the breaks are closely
adjacent to the centromeres or telomeres and those in which the breaks
are situated in the chromosome arms at some distance from either the
centromere or the telomere. The former give rise to what are, effectively, whole-arm transpositions, whereas in the latter the integrity of
the chromosome arms is lost.
One type of whole-arm transposition which is of particular importance
in evolution may be called a centric fusion. It results in the joining together of two acrocentric chromosomes to produce a metacentric.
There may be several ways in which this can come about (Fig. 1). The
first method is for one chromosome to break in the long arm very close
to the centromere while the other breaks in the minute "short arm."
Rejoining of the freshly broken ends "the other way round" will give a
large V-shaped metacentric with a single intact centromere and a minute