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Re: HYB: Pigments 101

In a message dated 11/28/00 6:20:11 AM Mountain Standard Time, 
wmoores@watervalley.net writes:

 Am I reading this correctly about the 'ancestors are seedlings for 
 which no description is available?'  Are these parentages those with 
 just seedling numbers and no named cultivars?  Interesting! >>

No, these numbered seedlings must trace back to named cultivars, but it IS 
possible to use this technique to assess an ancestor that was a numbered 
seedling of unknown appearance.  Just HOW MUCH you can dig out this way 
depends on how many unknowns you're dealing with.  

Here's a fictional example so simple you probably wouldn't need to chart it.  
Let's call the cultivar we are assessing "POTENTIAL PINK BREEDER" 
[conventionally abbreviated "PPB"]:

1.  We know that, although yellow itself, POTENTIAL PINK BREEDER has pink in 
its ancestry.  It has other traits we want to incorporate, so we want to know 
how likely it is to produce pinks if it is crossed to pink. 

2.  PPB's pollen parent is a named cultivar, and is pink.  So we know that it 
has 4 doses of the t-factor and PPB must have inherited two doses from it.  
We can follow this line back further, out of curiousity, but we've already 
determined what we were after in terms of the t-factor.

3.  PPB's pod parent is NUMBERED SEEDLING, description not available -- but 
we know it wasn't pink because PPB isn't.  At this point, we can only say 
that this mysterious seedling has 0,1,2, or 3 doses of the t-factor and PPB 
itself could have inherited either 0 or 1 dose from it.  That's not good 
enough, because we want to assess the chances that PB actually carries a 
total of three doses.

4.  So we look at the parents of NUMBERED SEEDLING, and find that one is pink 
and the other is yellow.  PINK GRANDPARENT [PGP] would have contributed 2 
doses to NS.  YELLOW GRANDPARENT [YGP] could have contributed 0, 1, or 2 --  
but in this case we know it was no more than 1 because we've already figured 
out that NS itself wasn't pink.  

So it's time to assess the odds:    

NS could be TTtt or Tttt.  

If it's TTtt, it would produce gametes in a 1:4:1 ratio -- but we already 
know that NS is not tttt so we can say the odds are 4 to 1 that it is Tttt.  

If it's Tttt, it would produce gametes in a 1:1 ratio -- but we know that NS 
is not tttt so it must be Tttt.  

This does not prove that NS is Tttt, but does show that in the worst case the 
odds are 4 in 5 that it is Tttt rather than TTtt and in the best case it can 
only be Tttt.  In a program for pink, we would thus favor it over others with 
lesser potential.  If we want to assess the chances further, for example, if 
PPB is competing with others of similar potential, we can carry the analysis 
back another generation or two from YGP.  

This chart alone does not tell us for sure whether PPB is Tttt or TTtt, but 
if we go ahead and use it with pinks the ratio of pink to yellow among its 
offspring can answer that question.

This technique for charting ancestors is clearly most powerful when combined 
with sibling and descendant analysis. 

Sharon McAllister

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