Variations in chromosome
can arise from
, in which chromosomes or chromatids fail to
for anything other than the
is often fatal. In
only a section of a chromosome is lost.
usually exhibits less severe effects than monosomy; many plants can be viable in this
A trisomic cell shows irregular pairing during
: the 3 chromosomes may
to form a
In humans, trisomy of chromosome
usually caused by nondisjunction of the
21 during meiosis.
Other examples of trisomy syndromes are
Polyploidy (more than 2 haploid sets of chromosomes) occurs mostly in plants, and can be
Amphidiploid plants can also be produced by
Rearrangements of chromosome
(or a deficiency) can occur near one end (
) or from the interior of the chromosome (
A deletion in recessive mutants may
in a heterozygote.
is a repeated segment of DNA caused by
during meiosis or through a replication error prior to meiosis.
gene may exhibit phenotypic variations in genes that exhibit
involves a rearrangement of the linear gene
and may arise from chromosomal
; the inversion can be
is a movement of a chromosomal segment to a new location in the genome.
involves the exchange between
chromosomes; its unusual
can also produce inviable gametes.
A common human rearrangement is
Some chromosome sites are
and susceptible to breakage, causing disorders such as Fragile
which is the most common form of inherited mental retardation.