Revision Notes on Inheritance and Variation :-
✍️Numerical aberrations of chromosomes:
Each species has a characteristic number of chromosome. Variations or numerical changes in chromosomes (Heteroploidy) can be mainly of two types:
The somatic chromosome number in euploids is the exact multiple of basic haploid number. In euploidy an organism acquires an additional set of chromosomes over and above the diploid complement.
Monoploids possess only one set or single basic set of chromosomes. Haploids on the other hand have half the somatic chromosome number. In diploid organisms monoploids and haploids are identical while in a tetra-or hexaploid with 4n or 6n chromosomes the haploids will possess 2n or 3n chromosome whereas its monoploid will possess only one set (n) of chromosome.
(c) Polyploidy: Organism with more than two sets of chromosomes are known as polyploids. It may be triploid with three sets of chromosomes (3n) or tetraploid with four sets of chromosome (4n) and so on.
(2) Aneuploidy: Aneuploidy is the term applied for the chromosomal mutations involving only a part of a set, i.e., loss (hypoploidy) or addition (hyperploidy) of one or more chromosomes. Aneuploidy may result from non disjunction of chromosome during cell division.
(a) Monosomy: Diploid organism that are missing one chromosome of a single pair with genomic formula 2n – 1. Monosomics can form two kinds of gametes, (n) and (n –1).
(b) Nullisomy: An organism that has lost a chromsome pair is nullisomic. The result is usually lethal to diploids (2n – 2).
(c) Trisomy: Diploids which have extra chromosome represented by the chromosomal formula 2n + 1. One of the pairs of chromosomes has an extra member, so that a trivalent may be formed during meiotic prophase.
(d) Tetrasomy: In tetrasomic individual particular chromosome of the haploid set is represented four times in a diploid chromosomal complement. The general chromosomal formula for tetrasomics is 2n + 2 rather than 2n + 1+ 1. The formula 2n + 1 + 1 represents a double trisomic.
✍️Types of aneuploidy: Aneuploidy may be of following types on the basis of chromosomes involved in non disjunction.
(a) Aneuploidy involving non-disjunction in sex chromosomes: This kind of aneuploidy is brought about due to non-disjunction in sex chromosomes. It may lead to following types of syndromes:
(1) Turner’s syndrome: Such persons are monosomic for sex chromosomes i.e. possess only one X and no Y chromosome (XO). In other words they have chromosome number 2n – 1 = 45. They are phenotypic females but are sterile because they have under developed reproductive organs. They are dwarf about 4 feet 10 inches and are flat chested with wide spread nipples of mammary glands which never enlarge like those in normal woman. They develop as normal female in childhood but at adolescence their ovaries remain under developed. They lack female hormone estrogen. About one out of every 5,000 female births results in Turner’s syndrome.
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