Re: Re: HYB: pink probabilities

Linda Mann <lmann@volfirst.net> wrote:
I never have been able to understand why probability of pink from one
tttt parent and one TTtt parent is 1/6 instead of 1/4 (which is what a
simple 4 x 4 Punnett square tells me).
 
 
Linda.
Neil's explaination is quite accurate, but I found it helped me to lable each gene and follow them through making gametes.
 
TTtt can be labled as T' T" t' t"
So the resulting gametes will be, when a cell divides;
T' T"    and  t' t"   or
T' t'     and T" t"  or
T't"     and  T" t'
 
Count them up and you'll get one TT, four  Tt, and one tt.  Six total.  1/6, 1/6.
This works for single recessive genes in autotertaploid with chromosome segregation.  An alternative, which Neil mentioned, though not by name, is chromatid segragation.
Chromatid segregation happens when the chromosome has duplicated but not divided so there are 8 sets still connected as four pair.  The good news is that chromatid segragation gives you a little better odds that the 1/6 of chromosme segregation.
As for going to the second or later generations, I used to work with autotetraploid sorghum.  In the F2 from a cross, a single recessive showed up 1/36 of the time.  Two recessives, a double recessive, showed up, or should have 1/1296, or about 1 in 1300.  But in the F3, the double recessives could be found in reasonable numbers.
Of course, sorghum is easily selfed, so it was easier to breed than iris.
Walter
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