> Are they really doing PCR analyses on every plant to separate the ones that have the antibiotic from the ones that don't????  The original question was how to sift through a planting for those individuals that carry the gene....right?

Right. I was specifically thinking of transgenic animals where the success rate is much higher. However, the efficiency of gene modification is being raised to an entirely new level with the technology called CRISPR. Some experiments have found nearly transgenes in 10% of plants raised using this technique. This level is high enough to make PCR effective in identifying the transgenic plants

The CRISPR/Cas9 technology was
successfully applied in model plants (Nicotiana benthamiana,
Arabidopsis thaliana) and crops (rice, wheat). The Cas9 nuclease
and the sgRNA matching the gene of interest are co-expressed
using Agrobacterium tumefaciens as a vector in N. benthamiana
leaves or transfected into protoplasts from Arabidopsis, wheat or
rice. Then, the genomic DNA is extracted from the leaf tissues or
protoplasts and subject to PCR-amplification with primers flanking
the target site.

Plant genome editing made easy: targeted mutagenesis in model and crop plants using the CRISPR/Cas system
Belhaj et al. Plant Methods 2013, 9:39
http://www.plantmethods.com/content/9/1/39

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