No Two Sperm or Eggs are Alike!

chromosomes and sperm cell

Hopefully you figured out possible combinations of chromosomes for this sperm by calculating 2 to the power of the number of chromosome pairs (4).  24 = 16 possible sperm from just these four pairs of chromosomes.  But in reality, Aaron has 23 chromosome pairs.  So 223 = 8,388,608 possible sperm cells just from Aaron's relatively limited amount of DNA!  This is the amazing thing about independent assortment.  It means that each reproductive cell randomly receives one chromosome from each pair, and every time reproductive cells are created, that randomization happens all over again.

Screen Shot 2018-10-04 at 11.49.08 AM.pngAnd this doesn't apply only to Aaron's body!  Each of the four individuals in this scenario is capable of producing over 8 million genetically different egg or sperm cells.  When you add crossing over to the mix, the possibilities go up even further.  Crossing creates brand new combinations of genetic material on chromosomes that might never have existed before on Earth!  After those new chromosomes go through independent assortment, the resulting sperm and eggs are likely 100% unique and never before encountered.  Altogether, this means it is basically impossible for these twins to create identical babies.  Even if the parents' DNA is the same, crossing over and independent assortment create huge amounts of variation in the combinations of DNA they might pass on.  This is why, as much as siblings might share certain traits, they will never actually be genetically identical, except in the case of identical twins.  (Identical twins are formed by a single embryo splitting in two, so they actually come from the exact same sperm and egg.)