Meiotic crossover and chromosome segregation are responsible for the formation of segmental aneuploidy. The hypothetical model of meiotic drive viable and inviable ascospores. (A) In the absence of crossover (interhomolog recombination), the parental ditype (PD) hexadecad asci and the non-parental ditype (NPD) hexadecad asci can be generated by random segregation of homologous chromosomes during the first meiotic nuclear division (MI). (B) A single crossover between scaffold M and scaffold F or scaffold X and scaffold 33 results in the tetratype (TT) hexadecad asci. (C) The NPD hexadecad asci can also be generated if two crossovers exist between scaffold M and F or scaffold X and 33.