Figure 4. Premature separation of sister chromatids with alleles exchanged during prophase I.
The diagram illustrates the four possibilities of segregation during meiosis I and then six possibilities of segregation during the second meiosis for every primary oocyte. Three of the ova correspond to an euploidy rescue, which originate a normal euploid zygotes; two disomic secondary oocytes with an allelic pattern 1: 1, which lead a trisomic zygotes with a 1:1:1 allelic pattern; and finally a disomic ovum, but with a double dose of only one allele, which originates a trisomic egg with a 2:1 allelic pattern.