Question: In genetics, we have 46 chromosones in each egg and the sperm cell and egg cell both have 23 chromosones in each cell so therefore when the join together they make up 46 chromosones for the embryo, but when there is twins there is either 2 eggs which means 46 chromosones and 1 sperm which is 23 chromosone and when they join together they only makes up 89 chromosone for the two embryos and that means one of the embryos gets 46 chromosones and the other only gets 23 chromosones and the embryo wont be able to live then. Unless the egg divides? My question is how does both embryos have 46 chromosones in each cell when there is 2 eggs and 1sperm or when the egg divides?

Tim Downing answered on 10 Nov 2012:

Hi Cliodhna,
Interesting question! Yes counting chromosomes and getting them all matched up is a major challenge for cells and they have lots of different proteins and enzymes to let them do this properly.
In any sperm or egg or any cell, the cell itself asks “do I have the right number of chromosomes?”. The difference here is that for general cells like ones in our blood or liver or brain they have 46 chromosomes – 2 copies for all 23. But cells that can produce another individual like sperm/eggs generally called “gametes” they only have 1 copy for all 23.
So when an egg and sperm fuse the cell organises the chromosomes together and asks this question: 23 + 23 = 46? If it has 46 and they all match up, it quickly alters it’s cell membrane (the coating of the cell) so that nothing else can enter. So the chances of having multiple sperm/egg combinations is very low in humans.
But in plants and bacteria, sometimes these weird fusions of different cell types can occur.
Tim