The  11  cloned  loci  contained  43  nucleotide  variations  from the
   consensus  Adh  sequence  of  2721  base  pairs.  These variations are
   distributed  throughout the gene: 14 in the exon coding regions, 18 in
   the  introns,  and 11 in the untranslated and flanking regions. Of the
   14  variations  in  coding  regions,  only  one leads to an amino acid
   replacement  -  the  one  in  codon  192,  producing  the two observed
   electrophoretic  variants.  The  other  13  coding  region  nucleotide
   substitutions  do  not  lead  to  amino  acid  replacements.  Are  the
   differences  in  the  number  of  allozyme and nucleotide variants the
   result of natural selection? If so, of what significance is this fact?
   Let's examine these questions.

   Among  the most intensively studied loci to date is the locus encoding
   the   cystic  fibrosis  transmembrane  conductance  regulator  (CFTR).
   Recessive  loss-of-function  mutations  in the CFTR locus cause cystic
   fibrosis,  a  disease  with  symptoms  including  salty  skin  and the
   production  of  an  excess of thick mucus in the lungs, which leads to
   susceptibility  to bacterial infections. Geneticists have examined the