title: How to transform column with lists of strings into a new column of unique identifiers
body: <p>Apologies if a similar question has been asked before, I was unable to find one, possibly because of wording of the question. </p>
  
  <p>Some current sample data looks like this, where the first column is a list of identifiers (genes) and the second column is a set of descriptors (gene ontology IDs):</p>
  
  <pre><code>Gene    Gene_Ontology_ID
  Gene1   GO1, GO2, GO4, GO6
  Gene2   GO2, GO3, GO4
  Gene3   GO5, GO7
  </code></pre>
  
  <p>I am wondering if there is an efficient way to transform a large table formatted similarly so that the "Gene_Ontology_ID" column now serves as the indentifying column, and the "Gene" column is now a list of genes with that Gene_Ontology_ID, like this:</p>
  
  <pre><code>Gene_Ontology_ID    Gene
  GO1                 Gene1
  GO2                 Gene1,Gene2
  GO3                 Gene2
  GO4                 Gene1,Gene2
  GO5                 Gene3
  GO6                 Gene1
  GO7                 Gene3
  </code></pre>
  
  <p>Is there a solution to this, preferably using Unix, Python, or R? Any help would be greatly appreciated, thank you. </p>