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Re: Iris Breeding


Linda Mann wrote:

:   I was assuming that the sorting of genetic traits would be random,

For autoploids with homologous chromosomes, that's a fairly apt generalization
of chromosome conjugation itself -- though it doesn't hold when you get down to
the level of individual genes.
  
:  so the probabiltiy of having much in the way of aril genes would be small,
:  but that it could happen. 

Yes, it COULD happen.  But the chances of it happening are not the same as the
chances of retaining a similar number of genes during conjugation of homologous
chromosomes.  And for the case under discussion, there were many generations in
which hybridizers had been selecting for nonaril characteristics.

:   Are you saying the aril genes have a higher
:   probability of being dropped or that there is some active selective process
:   in the sorting of heterozygous chromosomes that makes aril genes be dropped
:   preferentially?

Neither.  Just that non-homologous chromosomes -- in this case the aril ones --
do tend to be dropped or transmitted as a set.  This oversimplifies the process,
but you can think of BLUE OX as an unbalanced ABBB-type tetraploid that can
produce both AB and BB gametes.  When one of these is crossed with a BBBB-type
TB, which produces only BB gametes, the result can be both ABBB-type offspring
(chromosome-set quarterbreds) and BBBB-type offspring (chromosome-set TBs).  

If the hybridizer happens to select an ABBB-type and crosses it back to another
TB -- same song, second verse.   "Nine-ty-nine bot-tles of beer on the wall,
nine-ty-nine bot-tles of beer...."  This holds, generation after generation,
until the hybridizer selects a BBBB-type seedling.  Then it's TB hybridizing as
usual.

THEORETICALLY, about half of the seedlings produced by crossing an ABBB and a
BBBB will be ABBB and half will be BBBB.  But the ABBBs tend to be infertile and
the BBBBs tend to be fertile, so that the chances of an advanced generation
seedling from such a series asymptotically approaches zero.

And that's how many aril genes I'd expect EARL OF ESSEX to possess.  Unless you
want to discuss crossing over....

Sharon McAllister
73372.1745@compuserve.com





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