Figure 6. Non-disjunction during the second meiotic division of the oocyte with (I) or without (II) alleles interchanged. When the oocyte resumes the meiosis II, the gamete retains the two chromatids (Isodisomic) and the second polar body is nullisomic. On the left side (I), an allelic interchange has occurred (between alleles A and B), so the zygote will present an allelic pattern 1:1:1. On the right side (II), the alleles exchange doesn’t occur and the zygote allelic pattern observed will be 2:1.