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NUMERICAL CHANGES OF CHROMOSOME
• The changes in the number of chromosomes are usually
described as variations in the ploidy of the organism.
• Organisms with one or more complete sets of
chromosomes are said to be euploid.
• Organisms that carry more than two sets of chromosomes
are said to be polyploid and the level of polyploidy is described
by referring to a basic chromosome number, usually denoted n.
•
Haploids: carry a single
set of chromosomes (n)
•
Diploids: carry two chromosome sets (2n)
•
Triploids: carry three
chromosome sets (3n)
•
Tetraploids: carry four
chromosome sets (4n)
• An individual of a normally diploid species that has
only one chromosome set is called a monoploid to distinguish it from an
individual of a normally haploid species (n).
POLYPOIDS
AND ANEUPLOIDS:
• Organisms in which a particular chromosome, or
chromosome segment, is under- or overrepresented are said to be aneuploid.
These organisms usually suffer from genetic imbalance.
• Aneuploidy refers to a numerical change in part of the
genome, usually just a single chromosome, whereas polyploidy
refers to a numerical change in a whole set of chromosomes.
• Aneuploidy implies a genetic imbalance, but polyploidy
does not.
ANEUPLOIDY
• Aneuploidy describes a numerical change in part of the genome, usually a change in the dosage of a single chromosome. This includes individuals that:
•
have an extra chromosome
•
are missing a chromosome
•
have a combination of these anomalies
•
have a chromosome whose one arm has been deleted
•The under- or overrepresentation of a chromosome or a
chromosome segment can affect a phenotype.
•An organism in which a chromosome, or a piece of a
chromosome, is underrepresented is referred to as a hypoploid.
• An organism in which a chromosome or chromosome
segment is overrepresented is referred to as
a hyperploid.
Causes of
aneuploidy:
•Nondisjunction in the course of
meiosis or mitosis:
• Disjunction is another word
for the normal segregation of homologous chromosomes or chromatids to opposite
poles at meiotic or mitotic divisions.
• Nondisjunction is a failure of this process, in which
two chromosomes or chromatids incorrectly go to one pole and none to the other.
•Loss of a chromosome that has a centromeric
deletion.
•Loss of the small chromosome produced by Robertsonian
translocation.
ANEUPLOIDY TYPES
|
Nullisomic |
2n - 2 |
Missing both copies of a homolog in a diploid |
|
Monosomic |
2n - 1 |
Missing one copy of a homolog in a diploid |
|
Trisomic |
2n + 1 |
Having an extra copy of one homolog in a diploid |
|
Tetrasomic |
2n + 2 |
Having two extra copies of one homolog in a
diploid |
|
Disomic |
n + 1 |
Having an extra copy of a homolog in a haploid |
NULLISOMY
•Nullisomy occurs when both the homologs in the
otherwise diploid genome is missing.
•This is represented as 2n-2.
•The number of possible nullisomics in an organism
will be equal to the haploid chromosome number.
•Although nullisomy is a lethal condition in diploids,
an organism such as bread wheat, which behaves meiotically like a diploid
although it is a hexaploid, can tolerate nullisomy.
•The four homoeologous chromosomes apparently
compensate for a missing pair of homologs.
•In fact, all the possible 21 bread wheat nullisomics
have been produced. Their appearances differ from the normal hexaploids;
furthermore, most of the nullisomics grow less vigorously.
MONOSOMY
•Monosomy occurs
when one chromosome is missing in an otherwise diploid individual.
• It is represented as 2n – 1.
• The number of possible monosomies in an organism is
equal to the haploid chromosome number.
• When one copy of each of two non-homologous chromosomes are lost, it is called double monosomy (2n – 1 – 1).
• Suppose, gene A/a is on chromosome 2 where 'A' is dominant, and 'a' is recessive (mutant). Crosses of a/a with monosomic for chromosome 1 will give all progeny with A/a genotype. But, crosses of a/a with monosomic for chromosome 2 will give the following progenies: A/a (50%) and a/0 (50%). he appearance of recessive phenotype in the heterozygotes indicate a missing chromosome.
Turner Syndrome:
• In humans, there is only one viable monosomic, the 45,
X karyotype.
• These individuals have a single X chromosome as
well as a diploid complement of autosomes.
• Phenotypically, they are female, but they are almost
always sterile.
• 45, X individuals are usually short in stature; they
have webbed necks, hearing deficiencies, and significant cardiovascular
abnormalities.
• They originate from eggs or sperm that lack a sex
chromosome or from the loss of a sex chromosome in mitosis sometime after
fertilization.
• This latter possibility is supported by the finding
that many Turner individuals are somatic mosaics.
• People with the 45, X karyotype have no Barr bodies in
their cells, indicating that the single X chromosome that is present is not
inactivated.
TRISOMY
• Trisomies are those organisms which have an extra
chromosome.
• It is represented as 2n + 1.
• The number of possible trisomies in an organism is
equal to the haploid chromosome number.
• Suppose, gene A/a is on chromosome 2 where 'A' is dominant, and 'a' is recessive (mutant). A
trisomic (A/a/a) for chromosome 2 will give the following meiotic segregation
ratio: A (1/6), a (2/6), A/a (2/6), and a/a (1/6).
Trisomy types:
• Primary trisomies: extra chromosome
is identical to both the homologues
• Secondary trisomies: extra chromosome
is an iso-chromosome with two genetically identical arms
• Tertiary trisomies: these are the
products of translocation
• Trisomies
show irregular meiosis.
• Since the trisomies have an extra chromosome which is
homologous to one of the chromosomes of the complement, they form a trivalent.
Trisomy in plants:
• Datura stramonium
is a diploid species and has 12 pairs (=24) of chromosomes in the somatic cells.
•Albert
Blakeslee and John Belling analyzed chromosome anomalies in this plant.
•Blakeslee collected plants with altered phenotypes and
discovered that in some cases the phenotypes were inherited in an irregular
way.
•By examining the chromosomes of the mutant plants,
Belling found that in every case an extra chromosome was present.
•Altogether there were 12 different mutants, each
corresponding to a triplication of one of the Datura chromosomes.
•Such triplications are called trisomies.
Trisomy in human:
Down syndrome
•It is a
condition associated with an extra chromosome 21 (Trisomy 21).
•First described in 1866 by a British physician,
Langdon Down
•Its chromosomal basis was not clearly understood until
1959
•People
with Down syndrome are typically short in stature and loose-jointed,
particularly in the ankles; they have broad skulls, wide nostrils, large
tongues with a distinctive furrowing, stubby hands with a crease on the palm
and impaired mental abilities.
•Their life span is much shorter than that of other
people.
•They also almost invariably develop Alzheimer’s
disease.
•Trisomy 21 can be caused by chromosome nondisjunction
in one of the meiotic cell
divisions, more likely in females.
•The frequency of nondisjunction increases with
maternal age. Thus, among mothers younger than 25 years old, the risk of having
a child with Down syndrome is about 1 in 1500, whereas, among mothers 40 years
old, it is 1 in 100.
Trisomies 13 and 18
They are very rare, and
the affected individuals show serious phenotypic abnormalities and are dying
within the first few weeks after birth.
• Another viable trisomy is the triplo-X karyotype,
47, XXX.
• These individuals survive because two of the three X
chromosomes are inactivated, reducing the dosage of the X chromosome.
• Triplo-X individuals are female and are phenotypically
normal, or nearly so; sometimes they exhibit a slight mental impairment and
reduced fertility.
Klinefelter syndrome
• The 47, XXY karyotype is also a viable trisomy in
humans.
• These individuals have three sex chromosomes, two X’s
and one Y.
• Phenotypically, they are male, but they can show some
female secondary sexual characteristics and are usually sterile.
• Abnormalities associated with this condition, now
called Klinefelter syndrome; these include small testes, enlarged breasts,
long limbs, knock-knees, and underdeveloped body hair.
• The 47, XXY karyotype can originate by:
•Fertilization of an exceptional XX egg with a
Y-bearing sperm
•Fertilization of an X-bearing egg with an exceptional XY sperm.
• All individuals
with Klinefelter syndrome have one or more Barr bodies in their cells, and
those with more than two X chromosomes usually have some degree of mental impairment.
• The 47, XYY karyotype is another viable trisomy
in humans.
• These individuals are male, and except for a tendency
to be taller than 46, XY men, they do not show a consistent syndrome of
characteristics.
• All the other trisomies in humans are embryonic
lethals, demonstrating the importance of correct gene dosage.
DISOMY
• A disomic (n+1) is an aberration of a haploid
organism.
• In fungi, they can result from meiotic nondisjunction.
• In the fungus Neurospora
(a haploid), an n-1 meiotic product aborts and does not darken like a
normal ascospore
• So we may detect MI and MII nondisjunctions by
observing asci with 4:4 and 6:2 ratios of normal to aborted spores,
respectively.
• In these organisms, the disomic (n+1) meiotic product
becomes a disomic strain directly.
