Re: HYB: aphylla ancestry

From: Bill Shear <BILLS@hsc.edu>

 As Belle Meade is 1/16th aphylla, that would give JS almost
>one percent aphylla ancestry.

Perhaps we are missing the role played by selection.  It might be that
Belle Meade and especially JS might retain none of the genome of I. aphylla
unless certain parts of that genome were selected for.  To say that JS is
1% aphylla is probably statistically correct, but does it in fact retain
any aphylla genes?  That would depend on the selection regime that produced
it.  It could carry no aphylla genes or much more than 1%.

By putting an emphasis on pedigree and crosses (certainly very, very
important) we might tend to ignore or shortchange the selection process.
From a cross that may yield 500 seedlings, a breeder selects one or two to
continue to work with.  After several generations of this, all the genetic
information of one or the other parent may have been selected out, and to
expect characteristics of that parent to crop up in future generations is
wishful thinking.

Suppose a seedling gets one allele of each pair from each of its parents.
That would mean that a seedling of I. aphylla x TB would have, at each
locus, one TB allele and one aphylla allele.  Crossing these among
themselves, for each locus one half of the offspring would have lost either
the TB or the aphylla allele (that is they would have become homozygous for
one or the other).  Multiply this by all the alleles and add the factor of
selection (favoring the TB or aphylla homozygotes) and I think you can see
how the genome (at least that part of it discernable in the phenotype) of
one parent or another could be completely eliminated over several
generations.

So there is a question in my mind about the utility  of compiling long
pedigrees for irises.....Mike, what do you think?

Bill Shear
Department of Biology
Hampden-Sydney College
Hampden-Sydney VA 23943
(804)223-6172
FAX (804)223-6374
email<bills@mail.hsc.edu>

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