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.