code: public G getGene() {
  		assert _chromosomes != null;
  		assert _chromosomes.get(0) != null;
  
  		return _chromosomes.get(0).getGene();
  	}
docs: Return the first {@link Gene} of the first {@link Chromosome} of this
  {@code Genotype}. This is a shortcut for
  <pre>{@code
  final Genotype<DoubleGene> gt = ...
  final DoubleGene gene = gt.getChromosome(0).getGene(0);
  }</pre>
  
  @return the first {@link Gene} of the first {@link Chromosome} of this
  {@code Genotype}.