The genetic resources and recombination of the DNA of the parent or parents of an organism determine whether the organism is heterozygous or homozygous dominant. In many cases, an organism expresses a trait more strongly when it comes from homozygous dominant genes than from heterozygous genes for the same trait. Not all traits have gene options which are strongly dominant, and organisms can be homozygous recessive as well.
During the production of gametes for sexual reproduction, half the DNA of the parent organism is copied into the gamete cell. However, the chromosomes of the gamete are not identical to either of the chromosome pairs of the parent organism. This is because of genetic recombination, where the DNA from the parent's paired chromosomes are mixed together. This results in a unique combination of genetic traits on the gamete's chromosomes, but no genes are new to the gamete without mutation. If the parent is homozygous, then the gamete carries a copy of the homozygous gene despite recombination. If the parent is heterozygous, the gamete could receive either gene.
Sexual reproduction allows offspring to have different traits than either of their parents. For instance, two heterozygous parents could produce homozygous recessive offspring, while two homozygous parents with different genes would produce heterozygous offspring.