public class CodominantPopulation<O extends CodominantOrganism> extends SexualPopulation<O> {

public CodominantPopulation(List<O> organisms) {
        super(organisms);
    }
    ...
}

public class SexualPopulation<O extends SexualOrganism> extends Population<O> {
    public SexualPopulation(List<O> organisms){
        super(organisms);
    }
    ...
}

// in CodominantPopulation
for (O organism : population){
    // organism must be a CodominantOrganism
    CodominantGenotype.Dominance dominance = organism.getTraitDominance(trait);
    ...
}

Exception in thread "main" java.lang.ClassCastException: SexualOrganism cannot be cast to util.CodominantOrganism
    at CodominantPopulation.countDominance(CodominantPopulation.java:xx)
        ...

public class Population<O extends Organism> {
    List<O> population;

    public Population(List<O> organisms){
        this.population = new ArrayList<>();
        this.population.addAll(organisms);
    }
    ...
}

class A
    A (X y) { f(y); }
    protected void f(X x) { }

class B
    B (X y) { super(y); f(y); }
    @Override
    protected void f(X x) { }