An octad is formed when
Neurospora ascus developmental pathway: diploid zygote nucleus immediately enters meiosis producing four nuclei linearly ordered along developing ascus axis, then each nucleus undergoes one mitotic division to produce sister spores adjacent to each other, yielding eight spores arranged as four identical pairs. Octad structure maintains tetrad genetics because sisters identical genetically barring conversion; collapsing neighboring pairs recovers original tetrad genotypes. Occurrence mitosis increases spore yield without altering segregation ratios, facilitating scoring and providing internal c
Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 5: Octad Formation via Meiosis Plus Mitosis
