<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_fitness(self, chromosome):
 """ Get the fitness score for a chromosome, using the cached value if available. """

        fitness = self.fitness_cache.get(chromosome.dna)

        if fitness is None:
            fitness = self.eval_fitness(chromosome)
            self.fitness_cache[chromosome.dna] = fitness

        return fitness