What is the best possible explanation better than the two provided to explain the 15 recessive mice @ both loc?

Effects of X-irradiation on a wild type mouse that was zygous for the wild type alleles at two distinct genetic loci: d+/d+ and se+/se+. These two loci are very close together on the same chromosome (.16 map units.) After the x-irradiated mouse was crossed with a non-irradiated zygous recessive mouse they produced progeny (1455 wild type at both loci) and 15 recessive at both loci. There are several explanations including mutation of d+ to d and se+ to se, or non-disjunction in the irradiated wild type parent producing (d se// ). Neither of these is the best possible explanation. What is?