Re: Recessive amoenas


>The following excerpt is from a Region 13 newsletter article by Terry
>Aitken (Spring '94). I found this article to the helpful when trying to
>understand RECESSIVE and DOMINANT characteristics.
>
>Mendel's theory of genetics would have us believe that if you cross a
>square pea with a round pea, you should expect a crop which is one half
>square peas and one half round peas.  If you then cross two of these square
>peas together (line breeding,)  you should get three quarters square peas
>and one quarter round peas.

Not so, says the bio prof with 30 years experience teaching genetics.  If
the square allele were dominant, all the peas of the first generation would
be square; conversely, if the round allele were dominant,they would all be
round.   Assuming the square allele is the dominant one, crossing two of
the heterozygous offspring would result in a 25% chance of any one plant in
the next generation  having round peas.  The actual ratios would depend on
how many offspring of this second generation you raised; to see the 3:1
ratio you would have to have quite a few.  However, this would not hold for
modern tetraploid irises, in which each gene is present in 4 copies, not
two.

Everyone interested in this subject should read Kenneth Kidd's excellent
chapter on iris genetics in WORLD OF IRISES.
>
>Working with iris hybrids is not so simple. Characteristics of the parents,
>the grandparents, and the great-grandparents may show up in different
>intensities and at any time.  In line breeding, Dr. Ackerman reported it
>took five generation of selective line breeding to make specific RECESSIVE
>characteristic DOMINANT.  The beginning hybridizer should follow the theory
>of Mendel's law, but brace yourself for some really wild surprises along
>the way!
>

I do not know of a single  case in which a  recessive character was "made
dominant" and I suspect that what Dr. Ackerman did was to simply eliminate
from his breeding all plants that showed the undesired dominant.  By doing
this, you can eliminate the dominant not in five generations, but in one!
The evolution of dominance is one of the most vexing problems in genetics
and remains unsolved.  But you can't make a recessive character dominant.

>What makes iris breeding so difficult (and challenging,) is that almost any
>characteristic in a plant may have varying intensities of DOMINANCE OR
>RECESSIVENESS.  For example : wild bearded irises seem to have three
>primary colors, white, yellow, and purple.  These are DOMINANT over any of
>the other colors.  Oranges, pinks, reds, and plicatas are considered
>RECESSIVE; and you will lose them if you outcross to the primary colors.
>Line breeding is required to maintain these RESESSIVE colors.  Other
>characteristics to consider are contrasting beards colors (as opposed to
>self beards,) rebloom, bitones or bicolors, luminata patterns, haft marks,
>variegated or purple-based foliage, disease and insect resistance, plant
>vigor, bud count, fertility, fragrance, and cold, heat or drought
>resistance.

What's being talked about here are dominance heirarchies.  An allele can be
dominant to some of its other alleles, but recessive to others.  These can
be arranged in a dominance heirarchy.  But an allele that expresses itself
to varying degrees against different genetic backgrounds in fact shows
reduced penetrance, not 'degrees of dominance."


Best wishes, Bill
___________________
William A. Shear
Department of Biology
Hampden-Sydney College
Hampden-Sydney VA 23943 USA
phone (804) 223-6172
FAX (804) 223-6374
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