Genetic issues and pitfalls in transgenic plant breeding

Transgene technology provides a powerful tool for developing traits that ar e otherwise difficult to achieve through conventional breeding. In order to effectively apply the technology to breeding, we need to understand how tr ansgenes behave in plants. Transgenes may or may not follow Mendelian segre gation; their expression can be significantly affected by integration posit ions and structures of the transgenic DNA in host genomes; transgenes may b ecome unstable over generations, genetic backgrounds and environmental cond itions; and they may have significantly negative impact on expression of en dogenous genes. If not well understood, these hurdles could become signific ant barriers in transgenic breeding. This paper reviews some genetic issues and pitfalls that are often encountered in transgenic breeding. Because of the necessity of being brief, transgene expression, silencing, and breedin g are the three areas of focuses in this discussion. While molecular mechan isms underlying many of the transgenic phenomena have not been completely u nderstood, some practical `rules' are now available for creating, evaluatin g and selecting desirable transgenic transformants. It can be certain that with more transgenic plants generated and characterized our knowledge of tr ansgene genetics at both molecular and plant levels will continue to accumu late.