At what stage of fetal development does twinning cease to be biologically possible?

whar are the meiotic events that could lead to the birth of an individual with either Patau syndrome or Turner syndrome?

What causes most of the phenotypic abnormalities in someone with Down's Syndrome? A) They have many gene mutations that produce nonfunctional proteins. B) They express too much of some proteins during development. C) Their cells cannot go through mitosis properly during development because they have an odd number of chromosomes. D) Their chromosomes cannot pair during meiosis.

During which phase of meiosis do alleles of a gene separate? Does it occur in meiosis I or II?