Several unique phenotypes have physical characteristics that can be seen and readily identified clinically. The key to observational identification is the age of onset of the distinctive characteristic that is the tip-off to transgenic zygosity, says Ruth Hoyt, manager of one of Taconic's transgenic animal production operations. This is because we generally set up the animals for production at 6-8 weeks of age. If the clinical symptoms, or transgenic phenotype, don't develop until after the animals must be set up as breeders, or shipped to researchers for their studies, the phenotype of the transgene is not of value to us for genotyping. For example, the HLA-B27 rat develops clinical symptoms such as inflammatory bowel, arthritis and psoriasis, but not until it is about 5 or 6 months of age.
Symptoms become more pronounced and severe as age increases.
But we cannot wait for the symptoms, we must set up or ship the animals, based on DNA testing, weeks or months before the transgene carriers can be visually identified. In other transgenic animals, the onset of tell-tale symptoms occurs much earlier. These early-onset phenotypes can be used to identify the animals at about 4 weeks of age. But we must know that the tell-tale symptoms occur in animals that are transgene carriers and that the phenotype occurs in a large percentage of the carrier population and is fairly distinctive or easily recognized. In other transgenic animals the phenotype may not be clinically observable but can be detected using various non-DNA assays.