Polyploid:
An organism which possesses more than two sets of chromosomes.
Polypoidy:
Phenomenon of having one or more genomes in addition to the normal number for the species.
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A polyploid on the basis of number of chromosome set present may be triploid (3N), tetraploid (4N), pentaploid (5N), hexaploid (6N), septaploid (7N), octaploid (8N) etc. The polyploid species are of two types :
(i) Autopolyploid species/individual:
Contains more than two copies of a single genome.
(ii) Allopolyploid species/individual:
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Contains two or more genomes and each genome is ordinarily present in two copies.
Autopolyploidy:
The polyploidy which takes place within a species and results due to multiplication of the same set of chromosomes. It can be induced artificially by the use of a chemical called colchicine which arrests and breaks the spindle so that nuclear division is not followed by wall formation.
All autopolyploids exhibit gigas effect i.e., larger size, more juice, more sugar, more grains etc. Triploids are sexually sterile which are propagated vegetatively e.g., many banana varieties, seedless water melon, triploid sugar beet with more sugar and larger roots, some apple and pear varieties with large sized and more juicy fruits. An autotriploid (three copies of a single genome) variety of tea has also been developed in India.
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The polyploids which arise as a result of chromosome doubling following hybridisation of two distinctly different species are called allopolyploids. It is also called interspecific polyploidy because increase in number of genomes or chromosome sets is due to pooling from individuals of two or more species caused by hybridisaion followed by chromosome doubling e.g., AABB, AABBCC.
Among artificially produced allopolyploidy Triticale is the first man made crop derived by crossing wheat (Triticum) and rye (Secale). The chromosome number of the resulting F 1 was doubled to produce triticale.
Haploidy:
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The haploid bears only one set of chromosomes i.e., only one copy of each chromosome. Haploids of higher plants are sterile and generally do not survive. However, they are highly useful in producing pure homozygous lines for breeding programmes. Such plants are developed through a culture technique developed by Guha and Maheshwari (1964). It is called androgenic haploid culture. Gynogenic haploids are also developed by using unfertilised ovules.