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RE: ploidy


Rick:

>However, there are four sets of chromosomes in a tetraploid, rather 
>than two, so that you have five possible combinations of alleles at 
>each locus, rather than three.

Allow me to throw in some other considerations.  I think Ben got 
confused because he was thinking about segregation ratios from the 
point of a autotetraploid rather then an ampidiploid.  Segeration 
ratios from autotetraploids can be variable.  However, autotetraploids 
are rarely fertile to any extent.  With an amphidiploid every gene is 
basically duplicated, like in an autotetraploid, but you get some 
addition factors to consider with the five possible combinations you 
mentioned.

In an amphidiploid the combination BBbb comes in two forms - BB//bb 
and Bb//Bb, where the "//" seperates the two genomes.  If you self the 
Bb//Bb plant you can recover the recessive phenotype (15:1 
dominant:recessive; 1/4 x 1/4), but if you self the BB//bb plant all 
the progenies are dominant and of genotype BB//bb.

If you take a dominant complementary gene and throw in an inhibitor 
gene and then duplicate that on the second genome, you can quickly see 
how figuring out segregation ratios in an amphidiploid can become very 
difficult.  And a dominant complimentary gene with an inhibitor is a 
SIMPLE system!  Then, throw in some transposable elements to mess 
things up and....

Joe Halinar

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