cant we prove the converse? ie we prove that alleles for eye colour is not carried on Y chromosome so must be carried on X chromosomes. And this is easy, just breed a homogenous female with a male, and all male offsprings will guarrantee to obtain the phenotype of the pure breed females, this shows that Y chromosomes has no potenial to mask the allele present on the X chromomes. Therefore does not carry any alleles associated with eye colour.