Huntington's disease is an inherited neurodegenerative disease that is
caused by the expansion of a (CAG)n trinucleotide repeat sequence located
on chromosome 4. In the human population most alleles at the locus have
between 15 and 20 CAG repeats. Individuals with a copy containing 35 or
more repeats develop fatal neurological disorders. I discuss the
population genetics of Huntington's disease. I develop a model of array
size evolution that incorporates selection against alleles with many
repeats. I show that even with an upward bias, there is always an
equilibrium array size distribution. Most interestingly, for the
biologically relevant parameter values, the shape of the equilibrium is
almost independent of the degree of upward bias. Using empirical data froma
variety of sources, I show that array size distribution may be close
toequilibrium in the human population.

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