Variations in chromosome
can arise from
, in which chromosomes or chromatids fail to
Aneuploidy is gain or loss of one or more chromosomes, but not a complete set.
Monosomy is loss of a single chromosome from a diploid genome.
Trisomy is gain of one chromosome in a diploid genome. Euploidy is possession of multiples of the haploid set of chromosomes. Polyploidy is possession of more than two sets: triploid is three sets and tetraploid is four sets.
Nondisjunction can occur in the first or second meiotic divisions. This can produce gametes that either contain two members of a chromosome or lack that chromosome. After fertilization by a normal gamete, monosomic, disomic (normal), or trisomic zygotes are produced.
for anything other than the
is often fatal. In
only a section of a chromosome is lost.
Individuals with Turner syndrome (X0 or 45,X) are sterile females. They are usually short and have underdeveloped breasts. Intelligence is often normal.
Partial monosomy, or segmental deletion, is the loss of a section of a chromosome. An example is Cri-du-chat syndrome (46, 5p), caused by the loss of a small part of the short ("petite", or "p") arm of chromosome 5.
Cri-du-chat (46, 5p) individuals may exhibit anatomic malformations, especially abnormal development of the glottis and larynx. They are often mentally retarded.
usually exhibits less severe effects than monosomy; many plants can be viable in this
Drawings of capsule phenotypes of the fruits of the Jimson weed Datura stramonium. These result from trisomy (2n + 1) of one of the 12 haploid chromosomes.
A trisomic cell shows irregular pairing during
: the 3 chromosomes may
to form a
Three copies of a single chromosome may synapse during prophase I, forming a trivalent. In anaphase I, one chromosome moves toward one pole and two chromosomes toward the other pole; the latter results in a n + 1 cell, preserving trisomy.
In humans, trisomy of chromosome
usually caused by nondisjunction of the
21 during meiosis.
The karyotype of trisomy-21 (47,+21), showing three members of chromosome 21.
Down syndrome individuals often exhibit an epicanthic fold of the eyelid with a flat face and round head. They are usually short and have protruding tongues and broad hands. Physical and mental development is retarded.
The frequency of trisomy-21 for maternal age 30 is about 1 in 1000. The risk increases tenfold to 1 in 100 at age 40, and continues to increase past that age.
Other examples of trisomy syndromes are
Patau syndrome (trisomy 13 (47,+13)) is associated with numerous abnormalities. The average survival time is less than four months.
Edwards syndrome (trisomy 18 (47,+18)) is associated with numerous abnormalities. The average survival time for both Edwards and Patau syndrome is less than four months.
Polyploidy (more than 2 haploid sets of chromosomes) occurs mostly in plants, and can be
Autopolyploidy involves the addition of one or more haploid chromosome sets of the same species. Triploids (AAA) have an uneven number of homologs and are usually not maintained in future generations. Tetraploids (AAAA) produce balanced gametes which can be inherited.
Allopolyploidy results from hybridization of two closely related species. If a sterile hybrid (AB) undergoes a chromosomal doubling, a fertile amphidiploid is produced, with complete chromosome sets (AABB) of both species.
Chromosomal doubling can occur if a cell fails to divide after DNA replication. This can be simulated experimentally by applying colchicine, which interferes with spindle formation, to somatic cells undergoing mitosis. This prevents replicated chromosomes from migrating to opposite poles at anaphase. When colchicine is removed, the tetraploid cell can reenter interphase.
Species 1 contains genome A consisting of three distinct chromosomes, a1, a2, and a3. Species 2 contains genome B consisting of two distinct chromosomes, b1 and b2. Following chromosome doubling of the sterile hybrid (AB), a fertile amphidiploid (AABB) can be maintained by sexual reproduction.
Cultivated American cotton, Gossypium hirsutum. This species has 26 pairs of chromosomes: 13 are large and 13 are much smaller. Old World cotton had only 13 pairs of large chromosomes, while wild American cotton has 13 pairs of small chromosomes. The origin of the cultivated cotton was reconstructed experimentally by treating a hybrid with colchicine to double the chromosome number and produce a fertile amphidiploid with characteristics similar to the cultivated variety.
Amphidiploid plants can also be produced by
Somatic[7CCel Hybridization Cells from the leaves of plants can be treated to remove their cell wall, resulting in protoplasts. These can be fused with protoplasts from a different species in culture, producing somatic hybrid cells that are amphidiploids. Sometimes entire plants can be derived from cultured protoplasts. If only stems and leaves are produced, these can be grafted onto the stem of another plant. If flowers are formed, fertilization may yield seeds which germinate into mature plants.
Rearrangements of chromosome
Errors in crossing over or chromosome breaks may cause chromosome rearrangements. A chromosome break may produce "sticky ends" that can rejoin other broken ends. The rejoined chromosome may exhibit deletions, duplications, inversions, and translocations.
(or a deficiency) can occur near one end (
) or from the interior of the chromosome (
A terminal chromosome deletion occurs when a segment of DNA is lost from one end of a chromosome.
An intercalary chromosome deletion occurs when a segment of DNA is lost from the interior of a chromosome. Such a deletion involving may exhibit a deletion loop during meiosis I.
For synapsis to occur between a chromosome with a large intercalary deficiency and a normal complete homolog, the unpaired region of the normal homolog must form a deletion or deficiency loop during meiosis I.
A deletion in recessive mutants may
in a heterozygote.
If a deletion covers dominant genes, a heterozygous individual may exhibit the recessive phenotype in those loci due to pseudodominance, as shown by genes linked to the Notch locus in D. melanogaster. D. melanogaster possesses polytene chromosomes in the salivary glands that allows visualizing the banding pattern of the deficiency loop.
Giant polytene chromosomes, found in some fly larvae, represent paired homologs. Each chromosome undergoes many rounds of DNA replication, but without strand separation or cell division. Over 1000 strands may remain attached to one large centromere, exhibiting banding patterns under a light microscope.
is a repeated segment of DNA caused by
during meiosis or through a replication error prior to meiosis.
The tetrad at left is mispaired during synapsis. A single unequal crossover between chromatids 2 and 3 results in deficient and duplicated chromosome regions.
gene may exhibit phenotypic variations in genes that exhibit
Bar-eye (B) is a dominant X-linked mutation with reduced number of facets in the eye (wild type B^+ has about 800 facets). Homozygous females show semidominance: a more pronounced phenotype (68 facets) than heterozygotes (358 facets). B^D (double-Bar) females show even fewer facets (45), due to triplication of region 16A of the X chromosome.
Wild-type (B^+) flies possess one copy of region 16A of the X chromosome. This region is duplicated in B flies. Unequal crossing over in a B individual can result in a B^D chromatid with a triplicated region 16A and a B^+ wild-type, producing a 2:1:1 ratio in the gametes.
involves a rearrangement of the linear gene
and may arise from chromosomal
; the inversion can be
An inversion requires two breaks of a chromosome loop, where the "sticky" ends are rejoined so that the rejoined segment is turned around 180° This diagram shows a pericentric inversion: the inverted segment includes the centromere.
A paracentric inversion does not include the centromere, and the ratio of the lengths of the two chromosome arms is unchanged. A pericentric inversion often changes the arm ratio, which may sometimes be detected in metaphase chromosome.
A single crossover (SCO) within a paracentric inversion produces two parental chromatids and two recombinant chromatids with duplications and deletions. One recombinant chromatid is acentric (lacking a centromere), the other is dicentric (two centromeres); both pose problems in anaphase. The acentric chromatid may move randomly to one pole, or it may be lost, The dicentric chromatid is pulled in two directions and breaks apart, resulting in deletions.
A single crossover (SCO) within a pericentric inversion also produces two recombinant chromatids, both with duplications and deletions. No acentric or dicentric chromatids are produced. In both paracentric and pericentric inversion, 1/2 the gametes will inherit duplications and deletions, which reduces viability of the offspring.
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 reciprocal translocation can occur when two breaks near the ends of two nonhomologous chromosome arms cause an exchange of segments.
Synapsis in a translocation heterozygote results in a "cross" pairing of the homologs, producing unbalanced gametes, as with inversions, but even without crossing over.
Following meiosis and independent assortment, two segregation patterns are possible for gamete formation. Alternate segregation leads to a normal (1,4 - no translocations) and a balanced (2,3 - reciprocal translocation) gamete. Adjacent segregation leads to gametes (1,3 and 2,4) containing duplications and deletions.
A common human rearrangement is
[6CA[6CRobertsonian translocation (or centric fusion) involves breaks at the extreme ends of the short arms of two nonhomologous acrocentric haploid chromosomes (13, 14, 15, 21, and 22). Centric fusion of the long arms creates a larger submetacentric or metacentric chromosome plus 2 acentric fragments. Familial Down syndrome is an example of this rearrangement.
Familial Down syndrome. One parent contains a 14/21 translocation and has only 45 chromosomes, and is a phenotypically normal carrier. 1/4 of the individual's gametes will have almost 2 copies of chromosome 21. The resulting zygote has 46 chromosomes, but almost 3 copies of chromosome 21, and exhibits Down syndrome.
Some chromosome sites are
and susceptible to breakage, causing disorders such as Fragile
which is the most common form of inherited mental retardation.
A normal human X chromosome (left) contrasted with a fragile X chromosome (right), which is prone to breakage in culture. The syndrome is associated with excessive repeats of the trinucleotide sequence CGG in the FMR1 gene, which inactivates this gene. The FMR1 gene codes for an RNA-binding protein; absence of this protein presumably affects brain development.