A drawing that exemplifies the hypothesized mitotic homologue non-disjunction. The upper part of the figure shows two separate homologue pairs from a conventional mitosis, and in one chromosome a chromatid non-disjunction occurs. The lower part of the figure illustrates two pairs of homologues from a mitosis where pairing has occurred between all homologue chromosomes, and where all (four) chromatids of one such pair segregates to the same daughter cell. We suggest that this might have happened very early in development to two homologue pairs, made by chromosomes 8 and 18. We also illustrate how a detrimental mutation that has arisen during replication (marked by an asterix) may be eliminated by segregation to one daughter cell only after a mitotic cross-over that also generates a segmental (and terminal) uniparental isodisomy.