A chromosomal abnormality, chromosomal anomaly, chromosomal aberration, chromosomal mutation, or chromosomal disorder, is a missing, extra, or irregular portion of chromosomal DNA. These can occur in the form of numerical abnormalities (euploidy, aneuploidy), where there is an abnormal number of chromosomes, or as structural abnormalities (insertion, deletion, duplication, inversion and translocation), where one or more individual chromosomes are altered.
Change in Chromosome Number
The phenotypes in
many organisms are affected by changes in the number of chromosomes in their
cells. Sometimes, even changes in part of a chromosome can be significant.
These numerical changes are described as variations in the ploidy of the
organism (from Greek word meaning fold, as in "twofold").
(a) Euploid: Organisms with complete, or normal sets of
chromosomes are called euploid (from Greek words meaning good and fold).
(b) Polyploid: Organisms that carry extra sets of chromosomes are
called polyploid (from Greek words meaning many and fold). The level of
polyploidy is described by referring to a basic chromosome number. It is denoted
n. Thus:
·
Diploids have two basic chromosome sets. They have 2n
chromosomes.
·
Triploids have three sets. They have 3n.
·
Tetraploids have four sets. They have 4n and so forth.
(c) Aneuploid: Organisms in which a particular chromosome, or chromosome segment, is under- or over-represented are called aneuploid (from Greek words meaning not good, and fold). Therefore, these organisms suffer from specific genetic imbalance.
Difference between aneuploidy and polyploidy:
Ø Aneuploidy is a numerical change in part of the genome. It usually occurs in just a single chromosome. But polyploidy is a numerical change in a whole set of chromosomes.
Ø Aneuploidy causes a genetic imbalance. But polyploidy does not do so.
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| Fig. 1: Chromosomal Aberrations |
EUPLOIDY
The condition in which one or more full sets of chromosomes are present in an organism is called euploidy. Euploids may be Monoploids, Diploids or Polyploids.
MONOPLOIDY
The condition in which single basic set of chromosomes is present is called monopoloidy. Such organisms are called monoploids. One set of chromosome is equal to haploid number of chromosome of an organism. Haploids have half of the somatic number of chromosomes. Here each chromosome is represented once. Haploids are produced due to parthenogenesis.
§ This occurs in honey bees. In honey bees, queen and workers are diploids. Drones are haploids. Drones are produced by parthenogenesis. In parthenogenesis, egg is changed into zygote without fertilization.
§ Some flowering plants also produce haploids. They are produced by apomixes (parthenogenesis). Its example is cotton.
§ There are some androgenic haploids. The haploids which are produced from other reproductive cells and not from egg are called androgenic haploids. Here haploids may be produced from synergids or antipodal cells of embryo sac. They may be produced from pollen tube.
POLYPLOIDY
The presence of extra chromosome sets is called polyploidy. It is common in plants but very rare in animals.
(a) One-half of all known plant genera contain polyploid species. About two-thirds of all grasses are polyploids. Many of these species reproduce asexually.
(b) In animals, reproduction is primarily by sexual means. Therefore, polyploidy is rare. It is because it interferes with the sex-determination mechanism.
Importance of Polyploidy:
i. One general effect of polyploidy is that cell size is increased due to more chromosomes in the nucleus. This increase in size is correlated with an overall increase in the size of the organism.
ii. Polyploid species are larger and more active than their diploid counterparts. These characteristics have a practical significance for humans. Human depend on many polyploid plant species for food. These species produce larger seeds and fruits. Therefore, they provide greater yields in agriculture.
iii. Wheat, coffee, potatoes, bananas, strawberries, and cotton are all polyploid crop plants.
iv. Many ornamental garden plants, including roses and tulips, are also polyploid.
A. Sterile Polyploids:
Many polyploid species are sterile. Extra sets of chromosomes segregate irregularly in meiosis. It causes grossly unbalanced (aneuploid) gametes. If such gametes unite in fertilization, the resulting zygotes almost always die. This inviability among the zygotes explains why many polyploid species are sterile. For example, a triploid species has three identical sets of n chromosomes. Therefore, total number of chromosomes is 3n. When meiosis occurs, each chromosome will try to pair with its homologues.
(a) One possibility is that two homologues will pair completely along their length. It leaves the third without a partner; this solitary chromosome is called a univalent.
(b) Another possibility is that all three homologues will synapse. They form a trivalent. In trivalent, each member is partially paired with each of the others.
B. Fertile Polyploids:
The meiotic uncertainties also occur in tetraploids with four identical chromosome sets. Therefore, such tetraploids are also sterile. However, some tetraploids are able to produce viable progeny. These species contain two distinct sets of chromosomes. Each set has been duplicated. Thus, fertile tetraploids arise by chromosome duplication in a hybrid that was produced by a cross of two different diploid species. Most often these species have the same or very similar chromosome numbers. Its best examples are modern bread wheat, Triticum aestivum. This important crop species is a hexaploid containing three different chromosome sets. There are seven chromosomes in each set. There are total of 21 in the gametes and 42 in the somatic cells.
Types of polyploids:
(a) Allopolyploids: Polyploids created by hybridization between different
species are called allopolyploids. The chromosomes from different
species are less likely to interfere with each other's segregation during
meiosis. Therefore, polyploids arising from hybridizations between different
species are fertile.
(b) Autopolyploids: Polyploids created by chromosome duplication within a species are called autopolyploids. One possible mechanism for this event is for a cell to go through mitosis without going through cytokinesis (the cytoplasmic division of a cell into two daughter cells at the end of mitosis or meiosis). Such a cell will have twice the number of chromosomes. Through subsequent divisions, it could then give rise to a polyploid clone of cells.
ANEUPLOIDY
The random abnormal number of chromosomes in the organisms is called as aneuploidy. Or the phenomenon in which particular chromosome is under- or over- represented in organisms is called aneuploidy. The individuals that have an extra chromosome or missing a chromosome, or have a combination of these anomalies are called aneuploids.
Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. When an individual is either missing a chromosome from a pair (resulting in monosomy) or has more than two chromosomes of a pair (trisomy, tetrasomy, etc.).
Aneuploidy can occur with sex chromosomes or autosomes. Most cases of aneuploidy in the autosomes result in miscarriage, and the most common extra autosomal chromosomes among live births are 21, 18 and 13. Chromosome abnormalities are detected in 1 of 160 live human births. An example of trisomy in humans is Down syndrome, which is caused by an extra copy of chromosome 21; the disorder is therefore also called Trisomy 21. An example of monosomy in humans is Turner syndrome, where the individual is born with only one sex chromosome, an X.
Most cells in the human body have 23 pairs of chromosomes, or a total of 46 chromosomes. One copy of each pair is inherited from the mother and the other copy is inherited from the father. The first 22 pairs of chromosomes (called autosomes) are numbered from 1 to 22, from largest to smallest. The 23rd pair of chromosomes are the sex chromosomes. Normal females have two X chromosomes (XX), while normal males have one X chromosome and one Y chromosome (XY). The characteristics of the chromosomes in a cell as they are seen under a light microscope are called the karyotype (Fig. 2).
Aneuploidy is mostly caused by nondisjunction. The members of a pair of homologous chromosomes do not move apart properly during nondisjunction.
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| Fig. 2: Human male karyotype |
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| Fig. 3: Non-disjunction |
Types of aneuploidy:
There are following
types of aneuploidy:
(i)
Monosomics or Monosomy (2n – 1)
(ii)
Disomics or Disomy (2n)
(iii)
Trisomics or Trisomy (2n + 1)
(iv)
Tetrasomics or Tetrasomy (2n + 2)
(v) Nullisomics (2n – 2)
1. Monosomic or Monosomy (2n – 1)
If a chromosome is missing the aneuploidy is called monosomic. Therefore, the cell has 2n – 1 chromosomes.
1.1. Monosomy in Humans:
Many human genetic
disorders arise from monosomy. One monosomic genetic disorder is XO (Turner
syndrome). In this case a sex chromosome is missing in female. Thus the
females fail to develop normally. These individuals have a single X chromosome
with diploid autosomes. Phenotypically, they are female. But their ovaries are
rudimentary. Hence they are almost always sterile. Such individuals are short
stature. They have webbed necks, hearing deficiencies, and significant
cardiovascular abnormalities.
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| Fig. 4: Turner's syndrome karyotype |
Henry H. Turner first described the condition in 1938. Thus it is now called Turner syndrome. 45, X individuals can originate from eggs or sperm that lack a sex chromosome or from the loss of a sex chromosome in mitosis sometime after fertilization. Thus many Turner individuals are somatic mosaics (an individual composed of two genetically different types of cells). These people have two types of cells in their bodies. Some have 45, X and others have 46, XX. It occurs when an X chromosome is lost during the development of a 46, XX zygote. All the descendants of the cell in which the loss occurred are 45, X (Fig. 5).
1.2. Monosomy in Drosophila:
Monosomy also
occurs in Drosophila at different intensities:
(a) XO monosomy also occurs in Drosophila. The affected
fly has normal appearance. But it is sterile male.
(b) Monosomy for one autosome is not tolerated in Drosophila.
There can be loss of chromosome 4. It results in slow development of the
individual. It has reduced body size. Mostly such flies do not survive long.
(c) Similarly, loss of autosome 2 or 3 is lethal in Drosophila.
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| Fig. 5: Origin of monosomy at fertilization |
1.3. Monosomy in Plants:
Monosomy occurs in plants like wheat, maize, tobacco etc. Such monosomic plants are less viable than normal diploid plants. In plants, monosomy can occur for any of its set of chromosome. For example, wheat has 21 pairs of chromosome. Thus 21 monosomies are possible in wheat. Monosomy can also occur in ployploids. A polyploid has several chromosomes of same type. Therefore, loss of one chromosome does not affect such plants.
1.a. Double Monosomics (2n – 1 – 1):
It is loss of two non-homologous chromosomes. It is different from nullisomics, where set of homologous chromosomes is lost.
1.b. Triple Monosomics (2n – 1 – 1 – 1):
It is loss of three non-homologous chromosomes. It can occur in polyploids like wheat. Here it is well tolerated due to presence of extra sets of chromosomes.
2. Disomic or Disomy (2n):
A disomy is the presence of a pair of chromosome. For diploid organisms, such as humans, it is the normal condition. For organism that are normally triploid or above, disomy is an aneuploidy. It can occur in cells that are normally haploid, such as gametes. In uniparental disomy, disomy means two copies of the chromosome from one of the parents (with no contribution from the other parent).
3. Trisomic or Trisomy (2n + 1):
If the chromosome is present in triplicate in the fertilized egg the aneuploid cell is called trisomic. The cell has a total of 2n + 1 chromosome. Addition of an extra chromosome produces more viable individuals in both animals and plants.
3.1. Trisomy in Humans:
There are most common types of trisomy in humans. Such individuals can survive without spontaneous abortion. But it causes morphological abnormalities in the affected individuals.
3.1.1. Trisomy 21 (Down syndrome):
It is also called mongolism.
Down syndrome was first described in 1866 by a British physician, Langdon
Down. People with Down syndrome are typically short in stature and
loose-jointed, particularly in the ankles. They have broad skulls, wide
nostrils and large tongues with a distinctive furrowing. They have stubby hands
with a crease on the palm. They have impaired mental abilities. The life span
of people with Down syndrome is much shorter than that of other people. It
affects 1 in 800 live births.
The extra
chromosome 21 in Down syndrome is an example of a trisomy. Fig. 6 shows the
karyotype of a male Down patient. They have 47 chromosomes including X and Y
chromosomes and extra chromosome 21. The karyotype of this individual is 47, XY,
21.
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| Fig. 6: Down's syndrome karyotype |
3.1.2. Trisomy 18 (Edward’s syndrome)
There is an extra
copy of chromosome 18 (Fig. 7). This affects the growth of the baby and the
development of their organs. It affects 1 in 6,000 live births.
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| Fig. 7: Edward's syndrome karyotype |
3.1.3. Trisomy 13 (Patau syndrome) – It affects 1 in 10,000 live births.
3.1.4. Trisomy 9
3.1.5. XXX (Triple X syndrome)
3.1.6. XYY (XYY syndrome)
3.1.7. XXY (Klinefelter's syndrome):
Phenotypically, affected persons are male. But they can show some female secondary sexual characteristics. They are usually sterile. In 1942 H. F. Klinefelter described the abnormalities associated Klinefelter syndrome. These include small testes, enlarged breasts, long limbs, knock-knees, and underdeveloped body hair. The XXY karyotype can originate by fertilization of an exceptional XX egg with a Y-bearing sperm or by fertilization of an X-bearing egg with an exceptional XY sperm.
3.2. Trisomy in Drosophila:
Trisomy can also occur in Drosophila. Female Drosophila can have three X chromosomes with its autosomes (XXX : 2A). It may be fertile. But it is less viable than females (XX: 2A).
4. Tertrasomic or Tetrasomy (2n + 2):
If the chromosome is present in tetrad form in the fertilized egg the aneuploid cell is called tertrasomics for that chromosome. Tetrasomics have a particular chromosome present in four doses. The cell has a total of 2n + 2 chromosomes, 21 possible tetrasomics are identified in wheat.
5. Nullisomic (2n – 2):
If a set of chromosome is missing the aneuploidy is nullisomics. Therefore, the cell has 2n – 2 chromosomes. Humans with this condition will not survive.
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