After obtaining a transgenic
event, the genetic engineer hands the seeds over to the plant breeder.
Lines that survive transformation
and tissue culture well are typically lower yielding than current elite
lines. To make these transgenic lines marketable, plant breeders must
use breeding techniques to transfer the transgene into a high yielding
elite line.
First, the breeder obtains an inbred line by self-pollinating the
transgenic line. Each plant cell in the transgenic inbred line now
contains two copies of the gene.
The transgenic seeds produced
are planted along with seeds from an elite inbred.
Due to a lack of hybrid vigor,
both of the inbred plants that grow from these seeds will be smaller
than current hybrids.
When the plants reach the
proper stage, they are cross-pollinated.
The seed from this crop, the F1 seed, is harvested. All of the
offspring have one copy of the transgene, as well as 50% elite
and 50% non-elite genes.
The F1 seed is planted near
another elite inbred seed. The plants grow. Due to hybrid vigor, the
F1 plant is larger than the elite inbred, but it still contains many
undesirable genes.
The F1 plant is mated back
to the elite inbred. This process is known as backcrossing.
This seed, the backcross
1 (BC1) generation is harvested. The plants that grow from these
seeds will have 75% elite genes, and half will contain
the transgene.
Again, BC1 plants are grown
along with elite inbreds. Those that express the transgene are cross-pollinated
with the elite inbreds.
This process is continued until the plants contain at least 98% of
the elite genes and the transgene. This takes approximately
5 to 6 generations.
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