Recessive and dominant alleles control traits because if your alleles are either both recessive or both dominant than you are homozygous. If your alleles are dominant and recessive then your trait would be dominant because the dominant allele would mask the recessive allele. Traits are controled by co-dominant alleles because they control your genes and if you are co-dominant then you are neither dominant nor recessisive so your trait will be mixed. The alleles of two parents combine to give its offspring its traits because one allele from each parent is given to the offspring randomly to give it its trait.