Date: Mon, 26 Jan 1998 11:13:00 -0500 From: "Mellard, David" <dam7@cdc.gov> To: cp@opus.hpl.hp.com Message-Id: <aabcdefg345$foo@default> Subject: FW: question
Which brings us back to genetics.....
>Talking of which, how do polyploids fit into the genetic story? For
>instance, if a triploid has three sets of chromosomes when the sex
cells
>split do you get one gamete with two chromosomes and one with one set,
>or am I way off course?
Hi Phil,
I think you're on the right track. Here's my best shot.
Let's see if I can do this visually:
triploid = XXX
all other chromosomes (GG, SS, TT, etc) are the appropriate double set
GG represents a double set of chromosomes, all the other double sets of
chromosomes behave in the same manner as GG during reductive division to
form the gamete cell.
Diploid germ cell divides reductively to form a haploid gamete.
G1G2 + X1X2
division to gametes
G1X1 or G1X2 or G2X1 or G2X2
You can count the chromosomes here (diploid germ cell is showing 4
chromosome; haploid gamete is showing 2 chromosomes). I think the
chromosome number for many of the cp's are around 10. In the example
above, the 4 haploid gametes will be found in the ovary. Another set of
haploid gametes will form the pollen.
Each of the above gametes has the appropriate number of chromosomes and
upon pollination (the fusion of an ovary with a pollen) will form a
diploid germ cell that becomes the seed and later the plant.
Now for the plant with triploid chromosomes.
Triploid germ cell divides reductively to try to form a haploid (?)
gamete.
G1G2 + X1X2X3
division to gametes
G1X1X2 or G1X2X3 or G1X1X3 or G2X1X3 or G2X1X3 or G2X1X3
Either the above gametes are not live or they cannot fuse with a normal
gamete (G1X1 or G1X2 etc) to form a live embryo (seed). I'm not sure
which is the case. Someone else know?
However, it does work whenever the chromosome number is even, for
instance in tetrapoloid plants. Duplicating the chromosomes so that
there is a double set of the same chromosome X1X1X2X2 results in live
diploid cells.
I certainly don't mind someone else adding to this to correct it or to
make it more clear.
The bottom line is that a triploid plant is not fertile while plants
with even numbered (tetraploid, octaploid) chromosomes are fertile.
That begs the question, how do you get triploid plants? I think I know
the answer but will hold off for fun to see if any of you can figure it
out. Now, you Ph.D geneticts types hold onto your answer to see if
someone else gets it.
David
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