Figure 2

Cartoon illustrating the different types of meiosis I segregation that may take place in a normal disomy 21 oocyte. a) Normal chromosome pairing and crossing-over, attachment of the movement centres (kinetochores) at metaphase 1 and separation at anaphase 1. b) Lack of crossing-over and chiasma formation may lead to primary non-disjunction at anaphase 1. c) Lack of a chiasma can also lead to the same type of segregation at anaphase 1 as during mitosis (precocious meiotic disjunction).