Sister chromatids have different alleles although carry same genes on same loci. So if it doesn't happen so, like if there are same alleles on sister chromatids then there is a problem in its formation. i.e., male and female alleles haven't contributed equally ( may be an error during their segregation process ). And obviously it doesn't happen in a real cell so we must understand this point. Because normally whenever it happens there's one half from the male and one half from the female. (i.e. maternal and paternal alleles contribute equally).
I hope you get the answer!!!
I believe the answer is The spindle fibers
Microtubule will form the spindle fibers that will grab the chromosome. The formation of microtubule will be start from prophase and it will have role in moving the chromosome through the metaphase and anaphase. If the substance disrupting the formation of microtubules, then spindle fibers will not be formed.
B that our early ancestors were tailed primates
