Chromosome Numbers

A simple nomenclature is widely used to provide geneticists with information about chromosome numbers in different organisms. The number of unique chromosomes making up one set is referred to as "x." For example, for humans x = 23, for Arabidopsis thaliana x = 5, and for potato x = 12. The number of chromosomes in the gametes of an organism is referred to as "n." For humans n = 23, and for Arabidopsis thaliana n = 5. In potato, n = 24, half the total number of chromosomes. Note that for diploid organisms, n = x, meaning the chromosome number of the gamete will be equal to the number of unique chromosome types. By contrast, for polyploids, n will be some multiple of x, and the simple formula n/x reflects the number of different sets of chromosomes in the nucleus. For the potato, n/x = 2, indicating that the tetraploid potato carries twice the diploid number of chromosomes. Prefixes for other numbers of chromosomes are tri-(3), tetra-(4), penta-(5), hepta-(7), octo-(8), and so on.

During gamete formation, near-identical chromosomes (homologs) must pair up and undergo recombination (crossing over) before they are segregated into separate gametes. In diploid organisms, this pairing brings together the members of each homologous pair, so that (in Arabidopsis, for example), the five chromosomes from one set pair up with the five nearly identical chromosomes from the other set. In polyploid organisms, however, the number of possible pairings is larger. Scientists in fact recognize two different types of polyploidy (autopolyploidy and allopolyploidy, discussed next), based on the tendency of chromosomes from different sets to pair with one another.

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