Any numerical or structural change in the usual chromosome complement of a cell or organism is called chromosome aberration. There are two types of chromosomal aberrations:
(a) Changes in the number of chromosomes: Aneuploidy and euploidy
(b) Changes in structure of chromosome: deficiency (deletion), duplication, inversion and translocation
Change in Chromosome Number
In some case, number of chromosome in an affected person can are increased or decreased. Following types of chromosomal abnormalities are produced due to change in chromosomal number.
The random abnormal number of chromosomes in the animals is called as aneuploidy. Aneuploidy is mostly caused by non disjunction. The members of a pair of homologous chromosomes do not move apart properly during non disjunction. Therefore, the sister chromatids fail to separate at meiosis II. In these cases, one gamete receives the same type of chromosome and another receives no copy. The other chromosomes are usually distributed normally. If these abnormal gametes unite with a normal one, the offspring will have an abnormal chromosome number. It causes aneuploidy.
Types of aneuploidy
There are following types of aneuploidy:
I. Monosomic: (2n — I): If a chromosome is missing the aneuploidy is monosomic. Therefore, the cell has 2n — I chromosomes. Human genetic disorders arising from monosomy are:
(a) XO (Turner syndrome)
(b) cri du chat syndrome: It is a partial monosomy caused by a deletion of the end of the short (p) arm of chromosome 5.
(c) I p36 Deletion Syndrome: It is a partial monosomy caused by a deletion at the end of the short (p) arm of chromosome I.
- Disomic: 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, the disomy means two copies of the chromosome from one of the parents (with no contribution from the other parent).
- Trisomic (2n + I): If the chromosome is present in triplicate in the fertilized egg the aneuploid cell is called trisomic. The cell has a total of 2n + I chromokome. There are two types of trisomy:
(a) Partial trisomy: In this case, part of an extra chromosome is attached to one of the other chromosomes. Sometimes, one of the chromosomes has two copies of part of its chromosome.
(b) Mosaic trisomy: In this case, an extra chromosomal material exists in only some of the organism’s cells.
The most common types of trisomy that survive without spontaneous abortion in humans are:
- Trisomy 21 (Down syndrome)
- Trisomy 18 (Edwards syndrome)
- Trisomy 13 (Patau syndrome)
- Trisomy 9
- XXX (Triple X syndrome)
- XXY (Klinefelter’s syndrome)
- XYY (XYY syndrome)
- Tertrasomics: If the chromosome is present in tetrad form in the fertilized egg the aneuploid cell is called tertrasomics for that chromosome. The cell has a total of 211 + 2 chromosomes.
- Nullisomics: If a set of chromosome is missing the aneuploidy is nullisomics. Therefore, the cell has 2n —0 chromosomes.
Affects of aneuploidy
Aneuploidy is produced in sperm or egg. Later it is transmitted to the zygote. Mitosis transmits the aneuploidy to all embryonic cells. Such organism may not survive. If the organism survives, it becomes abnormal. Non disjunction can also occur during mitosis. The early aneuploid condition is passed to a large number of cells by mitosis. It has greater effect on the organism.
A condition in which the organisms have more than two complete sets of chromosomes is called polyploidy. The natural occurrence of polyploids in the animal kingdom is extremely rare. But polyploidy can be induced experimentally in certain animals, such as frogs and rabbits. It occurs by two ways:
Autopolyploids are polyploids in with chromosomes derived from a single species. It has two forms:
(a) Natural Autopolyploidy: Autopolyploids can arise from a
naturally occurring spontaneous genome doubling (for example potato). Bananas and apples can be found as triploid autopolyploids. The giant tree Sequoia semperviretts or Coast Redwood has a hexaploid (6n) genome. It is a autoallopolyploid (AAAABB). Cabbage is a very interesting example of a fertile allotetraploid crop.
(b) Artificial Autopolyploidy: Polyploidy can be induced in cell culture by some chemicals like colchicine. Colchicine causes chromosome doubling.
Allopolyploids are polyploids with chromosomes derived from different species. Triticale is an example of an allopolyploid. It has six chromosome sets. Four are taken from wheat (Triticum turgidum) and two are taken from rye (Secale cereale).
Types of polyploidy
There are following types of polyploidy:
- Triploidy (3n): They have an extra haploid set of chromosome
in each pair. It may be produced by the fertilization of an
abnormal diploid egg with sperm produced by nondisjunction.
- Tetraploidy (4n): They have increase of one diploid set of
chromosome in each pair. Sometimes, there is the failure of a 2n zygote to divide after replicating its chromosomes. Subsequent mitosis will then produce a 4n embryo.
- Mosaic polyploidy: It is more common than complete polyploid animals. In this case, the animals have patches of polyploidy cells.Polyploidy in humansPolyploidy also occurs in humans. But these occur much less frequently. 1 e polyploid conditions observed in humans are triploidy (69.,(XX ) and tetraploidy (92,XXXX). Additional
chromosomes contain a large amount of surplus gene product. It
causes multiple disease like defects of the heart and central
Poly ploid crops
Polyploid plants are stronger than diploids. In the breeding of crops, those plants that are stronger and tougher are selected. Thus polyploids of many crops are produced:
(a) Triploid crops: banana. apple
(b) Tetraploid crops: wheat, maize, cotton. potato, cabbage. leek. tobacco, peanut
(c) Hexaploid crops: chrysanthemum, bread wheat, oat
(d) Octaploid crops: strawberry, dahlia, pansies. sugar cane.
Some crops are found in a variety of ploidy. Apples. tulips and lilies are commonly found as both diploid and triploid. Daylilies (hemerocallist are available as either diploid or tetraploid.
Comparison of aneuploidy and polyploidy
Polyploids are more normal in appearance than aneuploids. Most of polyploids are fertile. One extra (or missing) chromosome disrupts genetic balance in aneuploids.