Figure 3. Non-disjunction during meiosis I of the oocyte with (I) and without (II) alleles interchanged The oocyte is disomic (retains the two chromosomes), while the first polar body is nullisomic. To the left (I) is shown an allelic exchange. When the oocyte is penetrated by sperm and resumes the meiosis II, three possibilities of segregation can occur: A: B (or B: A), A: A and B: B, which originate an allelic pattern 1:1:1 or 2:1 in the zygote, respectively. To the right (II) meiosis I nondisjunction without allelic exchange is observed. When the oocyte resumes the meiosis II generates a disomic oocyte (A:B), which leads a 1:1:1 trisomic zygote.