Dominant and recessive traits are regulated by combinations of genes received from one or both parents. We have pairs of genes for each trait, one gene contributed by each parent. If a trait is dominant, it is because that gene that controls it's expression will always be expressed if the gene is present. So for one trait you can have two genes both dominant for that trait, or one dominant and one recessive. Only if you have two recessive genes for a trait will it be expressed. So- brown eyes are dominant, and blue eyes are recessive. For brown eye color, it wouldn't matter if there was a recessive gene for blue eyes, because as long as there is a brown eye gene you will have brown eyes. You need two recessive genes for blues eyes to have blue eye. Co-dominance may mean that genes are linked, so that if there is dominance for one characteristic, another characteristic will go along and also be expressed.
