In science, "trans configuration" can mean several different things:

  1. In chemistry, it can refer to the configuration of an organic molecule containing a double bond between two carbon atoms, where the largest, most complex R groups attached to each carbon are on opposite sides:
     H3C     H
        \   /
         C=C
        /   \
       H     CH3
    
    
  2. It can alternately mean the configuration of an organic molecule containing a ring, where the largest, most complex R groups attached to each carbon are on opposite sides:
    
     H3C                     H     |       ___
        \                   /      |      // \\
         ring viewed edge-on       |      \___/
        /                   \      |      /---\
       H                     CH3   |   H3C     CH3
    
    

    These configurations are the opposite of cis configuration.

  3. In genetics, "trans configuration" is a term meaning that the changes in gene expression are due to something acting on the gene from a distance, or from an unrelated location, rather than on the same strand of DNA.

  4. Alternately, it can refer to two mutations in different genes coding for the same phenotype which are on opposite homologous chromosomes (as opposed to the cis configuration where both of the mutations are on the same homologue).

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.

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