Answer: Chiasma
Explanation:
Chiasma occurs during the 1st stage of Meiosis, a cell division that halves the number of parent cell.
During the 1st stage of meiosis, Chiasma is the contact point between the two chromatids of a chromosome during meiosis. Thus, proper disjunction of a chromosome, which leads to segregation of alleles, requires the link Chiasma.
Note that any error in the Chiasma would lead to chromosomal deficiency like Down syndrome.
