Gene mutations. They are a result of changes in the primary structure
of DNA or RNA (when it is the basic genetic material) within the framework
of one gene. They are divided into point and block. Point mutations are due
to the changes of single nucleotides in the molecules of DNA or RNA. The
changes can be replacing of one nucleotide by another (transition),
dropping out of one nucleotides (deletion) and inserting of nucleotides
(insertion). Block mutations include a greater number of nucleotides or
whole segments of the polynucleotide chain of gene. They can arise
because of dropping out of certain segment (deletion), shifting of a segment
to other place in the gene (translocation), its turning to 180° (inversion) and
repetition of a nucleotide group in one gene (duplication).
Chromosome mutations (aberrations). This type of mutations arise
as a result of changes in the structure of chromosomes and can be
observed under light microscope. They are intrachromosomal and
interchromosomal. Intrachromosomal mutations are due to dropping out of
terminal or inner parts of the chromosome (deficiency and deletions),
doubling of chromosomal parts (duplications), turning to 180° of
chromosomal parts (inversions), and the interchromosomal — to exchange
of parts between two homologous chromosomes, which can be ordinary or
reciprocal (Fig. 2–76).
Genome mutations. They are a result of changes in the number of
chromosomes or in the genome as a whole. As it was already mentioned,
the haploid set of chromosomes (1n) is a fixed number. Except haploid, the
chromosomes set can be diploid (2n), triploid (3n), tetraploid (4n), etc.
Genome mutations are subdivided into three types: polyploidy or
euploidy — with increased genome number; haploidy — with reduced
genome number; aneuploidy (heteroploidy) — connected with changes in
the number of chromosomes. If increasing the genome number is within
one species, the obtained mutations are called autopolyploids, and when