You study eye formation using Mexican cave-dwelling blind fish. You know that blindness is a trait controlled by multiple genes and inherited in a recessive manner. A blind fish from a true-breeding line in one cave was crossed to a blind fish from a true-breeding line in anothercave. If the mutation that causes blindness is in two different genes in the two fish, you should see:

The whole progeny will be not-blinded. All the individuals in the F1 will be able to see.          

Explanation:  

Multiple genes can affect the same trait. Two blind fishes can produce not-blinded offspring because parents have mutations in two different genes. This is a case of complementation. When parents have a mutation in the same gene, this process is known as non-complementation, and in these cases, their progeny will exhibit the mutated trait.

We know that blindness is a trait controlled by multiple genes and inherited in a recessive manner. So we need at least two genes to have not-blinded eyes, and in the exposed example the mutation that causes blindness is in fact in two different genes.

So let´s say that genes B and E are two genes involved in eye formation and that their respective recessive alleles will express blindness. Then:

Cross: A blind fish from a true-breeding line in B gene crossed to a blind fish from a true-breeding line in E gene.

Parental) BB ee   x   bb EE

Gametes) Be   Be   Be   Be

                 bE   bE   bE  bE

Punnet square)    Be      Be      Be      Be

                   bE   BbEe  BbEe  BbEe  BbEe

                   bE   BbEe  BbEe  BbEe  BbEe

                   bE   BbEe  BbEe  BbEe  BbEe

                   bE   BbEe  BbEe  BbEe  BbEe

F1) 100 %  BbEe, heterozygote for both genes. None of the F1 individuals are blind.                

carbon atoms, hydrogen, oxygen, and nitrogen.