Fig. 1.

Graphical representation of the model. (Legend) C describes the number of chromatids with respect to a haploid genome (N). Shortly before ovulation, a dictyate stage oocyte [2N(4C)] will complete MI, giving rise to a euploid egg [N(2C)] and the first polar body, PB1 [also N(2C)]. When fertilized, the egg will complete second stage of meiosis, MII, giving rise to a euploid zygote [2N(2C)] and a secondary polar body, PB2 [N(1C)]. Parental nuclei are separate entities at this stage [N(1C) each]. Possible outcomes of meiosis are enumerated for either error-free MI (A) or MI errors NDJ (B), PSSC (C), or RS (D). For zygotic maternal pronuclei (blue circles), only a deviation from normal haploid status N(1C) is noted. (A) Following error-free MI, a euploid egg [N(2C)] is fertilized and gives rise to either a euploid zygote (with probability 1 − 23q₁, where q₁ stands for the probability of an MII-NDJ error on a single chromosome; Table 1) or to an aneuploid zygote (with a probability 23q₁; Table 1). (B) There is a probability d of an MI-NDJ error. An affected dictyate oocyte will then give rise to an egg that either carries an extra chromosome copy (i.e., two more chromatids, 2C + 2) or lacks it entirely (2C − 2). (C) There is a probability p of an MI-PSSC error in MI. An affected dictyate oocyte will then give rise to an egg that either carries an extra chromatid (2C + 1) or lacks one (2C − 1). (B and C) In MII, the sister chromatids can fail to separate due to NDJ. MII-NDJ can either affect that same chromosome (depicted in red, with a probability q₂ in B and q₃ in C) or a different one (depicted in blue, with a probability 22q₂ in B and 22q₃ in C). (D) There is a probability r of an RS error in MI, but the affected dictyate oocyte will give rise to an egg that carries normal chromosome content (2C). Following MII, the zygote will either carry normal chromosome number (77% of the times) or not (23% of the times) (18). The pairs of homologous chromosomes are colored in black/red and gray/blue. The dots represent centromeres. Complex events affecting multiple chromosomes during meiosis occur with an overall probability c, which is included in model equations in Table 1.