Does trisomy 21 happen from anaphase I or anaphase II?
A nondisjunction of chromosome 21 that occurs in either anaphase I or anaphase II can result in the formation of a gamete that is diploid for 21. When that diploid gamete fuses with another that’s haploid for 21, then the trisomy emerges. Now, If nondisjunction were to occur during anaphse I, and only in anaphase I, then two of the four produced gametes would be diploid. If nondisjunction were to occur during anaphse II, and only in anaphase II, then one of the four produced gametes would be diploid. So there is a greater chance (by 25%) that a nondisjunction in anaphase I would produce a fetus that is trisomic for 21. That is, assuming, that the other gamete is normal and did not undergo nondisjunction. To answer the question, a trisomy 21 can be the result of nondisjunction in EITHER anaphase I OR anphase II, but there is a greater chance of getting trisomy 21 when the nondisjunction occurs in anaphase I. Hope that made sense.
