What better explains the generation of both of the recessive phenotypes in 15 mice in the F1 generation?

Want to examine the effects of X-irradiation on a wild type mouse that was zygous for the wild type alleles at two distinct genetic loci. So crossed a X- irradiated zygous wild type with non irradiated zygous recessive at both loci. F1 progeny were 1455 wild type at both loci and 15 wild type at both loci. What could be the cause of these recessive progeny besides mutation of wild type to recessive or non- disjunction in the irradiated wild type parent?