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Download Summary Variations in chromosome number, also called as
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Transcript
Summary Variations in chromosome number, also called as heteroploidy, are of two types, viz. euploidy and aneuploidy. Euploidy is a condition where one or more complete sets of chromosomes are involved. Euploids are classified with respect to the basic chromosome number of a species. Thus an organism can be ‘X’ (monoploid or haploid), 2X (diploid), 3X (triploid), 4X (tetraploid) etc. Two main types of euploidy include: monoploidy or haploidy and polyploidy. Monoploids have a single chromosome set designated by ‘X’, whereas haploids have half of the somatic chromosome number designated by ‘n’. However, the term haploidy is more commonly used than monoploidy. Haploids originate spontaneously by parthenogenetic development of egg, synergids or antipodal cells or can be artificially produced by distant hybridization and anther / pollen culture. Meiotic bevahiour of haploids is irregular. Haploids are important for production of homozygous diploid lines, production of aneuploids, hybrid sorting, gene transfer etc. Polyploidy is of two types, viz. autopolyploidy and allopolyploidy. Autopolyploids have the same basic set of chromosomes multiplied. e.g. autotriploids (AAA), autotetraploids (AAAA) etc., where ‘A’ represents a single chromosome set. Autopolyploids originate spontaneously due to mitotic and meiotic irregularities leading to chromosome doubling. The most common method of production of autopolyploids is by using colchicine – a drug which inhibits spindle organization resulting in chromosome doubling without cell division. Meiotic behaviour of autotriploids is irregular, whereas autotetraploids and autohexaploids have comperatively normal meiotic behaviour. Autopolyploids have an important role in crop improvement. Triploids are produced in those crop plants where seed is not the commercial requirement, whereas tetraploids and hexaploids are highly valuable in various fruit crops, vegetables, ornamentals, forage crops etc. Allopolyploids contain genomes from two or more than two different species, e.g. allotetraploid (AABB), allohexaploid (AABBCC or AAABBB) etc. Allopolyploids are frequently produced in nature due to hybridization between two different species followed by chromosome doubling of the hybrid. They can also be produced by hybridization between two or more species followed by colchicine treatment, leading to chromosome doubling of the hybrid. Allopolyploids are also called as amphidiploids due to their normal diploid like meiotic behaviour. Allopolyploids have tremendous role in the evolution of crop plants because many of our important crops, such as, wheat, brassica, tobacco, cotton, triticale, etc. are allopolyploids.