Short notes on polypolids and explain the various types

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Classification of polyploids:

There are mainly three categories of polyploids-

1. Autopolyploids

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2. Allopolyloids and

3. Autoallopolyploids

Autopolyploids:

Here the same chromosomes set are multiplied. For instance if the diploid set in AA, auto polypoid would be AAAA. Autopolyploids are produced when there is somatic doubling of chromosomes. If for example during mitotic cell division, after the duplication of chromo­somes if the two groups fail to migrate to opposite poles, a polypoid cell would result.

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Autopolyploids are widely distributed in the flowering plants. The common Indian grass Cynodon dactylon is an autotriploid and survives mainly by vegetative propagation.

Allopolyploids:

These are produced by crossing two different species with no chromosome homology.

The F1 hybrid AB would be sterile as the chromosomes of A and B are completely non homologus and cannot pair. If the chromosomes of the F1 hybrid are doubled they become AABB and the individual is called an allopolyploid. This individual is fertile as A can pair with A and so also B. As distinct from autopolyploids, allopolyploids have multiplication of dif­ferent genomes (chromosomes).

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A classical example of allopolyploid is provided by Raphanobrassica re­ported in 1927 by the Russian scientist Karpachenko. He crossed Raphanus sativus and Brassica oleracea and obtained an F1 hybrid which was com­pletely sterile. The sterility was due to complete non homology between the genomes of Raphanus and Brassica. But some plants of Raphanobrassica were fertile obviously due to the doubling of the chro­mosomes.

Autoallopolyploids:

These, represent a higher series of polypoidy (octoploidy) in which there are 8 sets of homologues. It can be genetically represented as AAAA BBBB.

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