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The term AB/Tübingen (also called Tübingen/AB, Tüb/AB, or Tu/AB) has been used to refer to a variety of situations. Originally the term was established to refer to two lines of fish generated by Nancy Hopkins and Adam Amsterdam from crosses between individual AB (designated AB* at that time) and Tübingen fish. The particular Tübingen fish used in the Hopkins lines had been obtained from a German pet store, had been kept in the Hopkins lab for about 6 years prior to use for this purpose, and harbored recessive embryonic lethal mutations. About 25 Tübingen by AB (AB*) crosses were made. The progeny of two crosses that lacked recessive embryonic phenotypes through 5 or 6 days of development (as determined by examining progeny from sibling matings) were selected to found the "lethal free" lines that were designated TAB-5 and TAB-14. Thus the lines' limited genetic diversity is based upon 4 chromosomal sets each (total of 8 chromosomal sets when combined). This should define the limit of their genetic variability. The line was in its fourth generation in September 2001. About 20 fish (40 chromosomal sets) are used to generate each new generation. Line advantages include: good fecundity, no embryonic lethal mutations (good for screening for induced mutations), a fairly even balance between the two sexes (Tübingen tends toward males, AB tends toward females). A disadvantage of this line is that females, particularly young females, have a tendency to produce eggs that yield edemic embryos. In a haploid screen using the TAB lines, Charline Walker and Hazel Sive observed a notochord defect in the background that made identification of some phenotypes more difficult. In addition to this official line, the AB/Tübingen or Tübingen/AB terminology has been applied to situations were the two parental lines (AB and Tübingen) are maintained separately and then crossed together to produce a "standard" AB/Tübingen hybrid fish. The goal is to maintain a reproducible genetic background that will not change from generation to generation, as the Hopkins line might. Such changes could be due to genetic drift (random loss of alleles, fixation of particular alleles in the breeding population, or creation of new alleles by random mutations), selection (inadvertent or intentional), or the accidental introduction of genes into the population (stock contamination by stray fish). The reproducibility and quality of hybrids created from different individuals of these two lines will depend upon the genetic variability among the fish of the two lines, the genetic stability of the lines over time, and their genetic quality.  (1)