It is a well-known fact that genetic sequences may
contain sections with repeated units that differ 
in length over a population, with a length distribution
of geometric type. A simple class of recombination
models with single crossovers is analyzed that
result in equilibrium distributions of this type.
Due to the nonlinear and infinite-dimensional
nature of these models, their analysis requires
some nontrivial tools. This makes them interesting
also from a mathematical point of view.