485 15.1 Changes in chromosome number In the realm of polyploids, we must distinguish between autopolyploids, which have multiple chromo-some sets originating from within one species, and allo-polyploids, which have sets from two or more different species. Allopolyploids form only between closely re-lated species; however, the different chromosome sets are only homeologous (partially homologous), not fully homologous as they are in autopolyploids. Autopolyploids Triploids are usually autopolyploids. They arise spontaneously in nature, but they can be constructed by geneticists from the cross of a 4 n (tetraploid) and a 2 n (diploid). The 2 n and the n ga-metes produced by the tetraploid and the diploid, re-spectively, unite to form a 3 n triploid. Triploids are characteristically sterile. The problem, as in monoploids, lies in pairing at meiosis. The molecular mechanisms for synapsis, or true pairing, dictate that pairing can take place between only two of the three chromosomes of
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.