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HYB: Punnett Square Question


Message text written by  Cindy Rivera:

>Ok, here is my uneducated question. As to fertility, do the gametes have
to
be balanced to be fertile, ie, AABB? Does that mean the seed won't
germinate
or does that mean the results will be sterile? Or does that mean that
combination won't even form a seed? I do understand there are no absolutes,
but absolutely what is the general rule?

Please help me out of the darkness I with my lone cross of White Lightening
x Red at Night. None of my aril/MTB crosses took this year.
<

Remember that a gamete carries only HALF of an individual's genetic
material, which must be combined with another gamete to produce a new
individual.  As a rule, a gamete must contain complete chromosome sets to
be viable but they may be of different types.     

If the plant's chromosome sets are balanced, as in an AABB, it is capable
of forming many viable AB-type gametes so tends to be fully fertile.

If its chromosome sets are unbalanced, as in AAAB or ABBB, it may be
capable of forming some viable AA, AB, or BB gametes and therefore be
partially fertile -- but a lot depends on finding the right mate. 

To form a seed, a viable gamete must combine with a compatible one from the
other parent -- that's why partially fertile cultivars produce pods with
more chaff than seeds.  Sometimes their seeds germinate naturally,
sometimes they have defective endosperm and require a bit of help.  

Once you get a seedling from such a cross however, whether or not it is
fertile depends on its own chromosomal makeup.

Sharon McAllister
73372.1745@compuserve.com

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