CHROMOSOMAL ABERRATIONS
I - STRUCTURAL VARIATIONS
The occurrence of spontaneous variations in structure of number of chromosome is called chromosome aberration. The chromosome aberrations is of two types
1) Structural variations and (2) Numerical variations
Structural aberrations refers to visible changes in the structure of chromosomes. They are also called as chromosomal aberrations. They are of four types: Changes involving the number of gene loci:
1. Deletion or deficiency
2. Duplication or addition
Changes involving the arrangement of gene loci:
3. Inversion
4. Translocation.
II - NUMERICAL VARIATIONS
Diploid organism have two copies of the genome in their somatic chromosomes (2n = 2x) and their gametes have a single genome (n = x). Any change from the normal diploid condition of an organism is called as a numerical chromosome aberration, often is referred to as heteroploidy and the individuals are known as heteroploids.
Numerical aberrations are of two types 1) Aneuploidy and 2) Euploidy. Aneuploidy involves change in chromosome number by addition or deletion of few number of chromosomes. Euploidy involves changes in whole sets of chromosomes. Most common euploid is a diploid which has two sets of the same chromosome.
Monoploidy refers to the presence of only one copy of the genome and is represented by x. Haploidy refers to the gametic chromosome number of a species of a normal diploid or a polyploid and is represented by n. Haploids derived from diploid species are known as monoploids, while those obtained from polyploid species are called polyhaploids.
ANEUPLOIDY
Aneuploidy is of many types. A dipoild cell (2n) with a missing chromosome (2n-1) is called monosomic; a monosomic person has 45 chromosomes. A cell which lacks both copies of a chromosome is called as nullisomic (2n-2); a nullisomic person has 44 chromosomes. Loss of a chromosome is lost from two different pairs in a cell is called as double monosomic (2n-1-1). Aneuploidy arises due to non-disjunction of chromosomes in meiosis during formation of gametes.
Considering addition of a chromosome, a diploid cell with an extra chromosome is called as trisomic (2n+1); a trisomic person has 47 chromosomes. If there are two additional chromosomes in a cell it is called as tetrasomic (2n+2).
ANEUPLOIDY IN HUMANS
The common aneuploids in humans are sex-chromosomal aneuploids eg. Turner syndrome and Klienfelter syndrome. The common autosomal aneuploid is trisomy 21, also called Down syndrome.
Turner Syndrome (45, XO, females)
Persons with Turner syndrome are females who lacks one X chromosome (monosomy). This condition is produced when an egg with X chromosome is fertilized by a sperm without a sex chromosome resulting in a XO female. They do not undergo puberty and their female secondary sex characteristics remain immature: menstruation is usually absent, breast development is slight, pubic hair is sparse, deformed heart, horse shoe shaped kidney. This syndrome is seen in 1 of 3000 female births. Affected women are frequently short and have a low hairline, a relatively broad chest, and folds of skin on the neck. It is highly lethal in embryos and leads to abortion. 98 % of the Turner syndrome is lost during first three months of pregnancy. Most women who have Turner syndrome are sterile.
Klinefelter Syndrome (47, XXY, males)
The males has one extra X chromosome which inhibits the development of male characters. When an abnormal egg with XX chromosome fuses with normal sperm, a zygote with normal autosomes and XXY is formed, Persons with this condition has long legs, small testes, female like breast enlargement (gynecomastia), and no sperm production. They are infertile, most have normal intelligence while some are mentally retarded. Klinefelter syndrome occurs with a frequency of about 1 in 1000 male births.
This condition can be treated by surgical removal of breast. But sterility cannot be altered. Treatment with testosterone promotes development of sex organs, musculature, body hair and deeper male voice.
Down Syndrome (47, XX or XY)
It was first reported by Langdon Down in 1866. The defect is caused by trisomyof 21st chromosome. Most of the Down’s syndrome children are born to normal parents. The occurrence of Down syndrome increases with the age of the mother. The small extra chromosome is formed due to the non-disjunction of chromosomes during gamete formation where the 21st chromosomes fail to separate. Down syndrome also occurs due to translocation of chromosomes 21 and 14 or 15. This is called familial
Down syndrome.
The features are moon face, open mouth, projecting lower lip, stubby hands and feet, slanting eyes, broad forehead, congenital malformation of heart, lower blood calcium level and susceptibility to respiratory disorders.
EUPLOIDY
An organism with varying number of complete haploid chromosome set is called as an euploid. The somatic cells with two sets of chromosomes are diploids (2n) while gametes with only one set of chromosome are haploids (n). Organisms with multiple copies of haploid chromosome sets, such as three sets (3n) are triploids, four sets (4n) are tetraploids, five sets (5n) are pentaploids. They are collectively called as polyploids.
Approximately 40% of flowering plants and 70% of grasses are polyploids. Some ere species like Triticum have many states polyploidy namely diploid, tetraploid and hexaploid. Oats, cotton, potato, sugarcane and frogs, lizards, fishes are examples for polyploids.
Polyploidy is of two types namely,
1. Autopolyploidy, in which all the chromosome sets are from a single species and
2. Allopolyploidy, in which the chromosome sets are from two or more species.
AUTOPOLYPLOIDY
Presence of more copies of chromosome sets from the same species is called as autopolyploidy. An organism with three sets of the same genome is called as an autotriploid (3x); four sets as autotetraploid (4x); five sets as autopentaploid (5x) etc.
Autopolyploidy occurs in many ways. For example, the fusion of an abnormal diploid gamete with a normal haploid gamete results in a triploid. Also, fertilization of a diploid gamete by another diploid gamete results in a tetraploid. A tetraploid can also be formed by chromosome doubling of somatic tissues by using colchicine.
Since all the chromosomes are form the same species, they are homologous andtheir segregation produces unbalanced gamets with various number of chromosomesresulting in sterility. The sterility due to autopolyploidy has commercial value. Forexample, the diploid bananas have 2n=22 and produce hard , inedible seeds while,triploid bananas with 3n=33 are sterile and lack seeds. Polyploidy has been used inagriculture to produce “seedless” and “jumbo” varieties of crops. For example, seedlesstriploid watermelons are produced by crossing a tetraploid and a diploid. Jumbo macintosh apples are tetraploids.
ALLOPOLYPLOIDY
Presence of multiple copies of chromosome sets from different species is called as allopolyploidy. This can be explained by the following example with a cross involving two species with same chromosome number 2n=6. The parents are AABBCC (species 1) and GGHHII (species 2). The fusion of their gametes ABC and GHI produces a hybrid with six chromosomes, ABCGHI. Since the chromosomes are different, they will not pair during meiosis and the segregation is not normal and hence they will be sterile. As each chromosome has it homologue, this is also referred to as amphidiploid.
Korpechenko in 1920s created polyploidy by crossing cabbage Brassica oleracea (2n=18) with radish Raphanus sativus (2n=18). The new plants possessed the roots of cabbage and leaves of radish and are sterile.