Re: status update 28Jan04

My two cents.

Things are possibly a little more complicated than this:

Facts: Single weak dominance is what is most communly observed in nonSa x Sa crosses. Austin says often 30%.

Comments: Single dominant Aaaa x aaaa should produce 50% Aaaa seedlings. One can infere from this example that seven recessive genes are needed to equal a single dominant.

AAaa x aaaa gives more than 96% Aaaa+AAaa.

Then there is eventual other genes interferences: this is not obvious as then we should observe quite variable percent SA seedlings. Much more than is found.

Another possibility is some distortion by unequal gamete success. Gametes (eggs or pollen) are competing and not equally successfull. They may be superseded by stronger gametes or produce non developing and/or non germinating seeds or non viable seedlings.

This is often observed with tetraploids that may need only one good gene out of four for plant viability. This allows accumulation of a lot of bad genetical mater.

There is a real possibility that Sa genes have imperfect viability and hypotheticaly need a favourable genetical environment to be successfull.

Friendly yours

Pierre

Le mercredi, 28 jan 2004, à 16:39 Europe/Paris, Neil A Mogensen a écrit :

The purpose in making crosses with "cv's not producing SA's" is to uncover, one might hope, either or both of two possible interpretations of SA genetics:  1) The SA traits are a single weak dominant with results varying with dosage, and/or 2) SA traits, whether dominant or recessive, can be masked by a dominant "normalizing factor" analogous to the dominant white "I" factor in relation to blues.
 
Our other cross types *may* reveal, in large enough quantities and careful counts of progeny types, whether the distribution of forms and variation of SA traits conform to a Punnett square of 1) a single recessive, 2) a single weak dominant, 3) one one of several Punnett squares assuming a two factor condition of any one of four types (AB, Ab, aB or ab) or something even more complex.

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