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I'm not biologist and I have just a basic knowledge. I've been thinking for a long time about the following question: How does the body choose which 23 chromosomes should be active in human sperm and female eggs?

Thanks in advance for helping me with this!

Edit: Each egg cell or sperm contains 23 chromosome whereas a normal cell has 46 chromosomes. So How is it that those specific 23 chromosomes are the chosen ones. Are a random collection of chromosomes selected at the end of meiosis or is it more specific than that?

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    $\begingroup$ What do you mean by "decides, which chromosomes to choose to be active in human sperm and female egg"? The human body has either sperm or egg, not both. Or do you mean the developing embryo? $\endgroup$ – Chris Feb 27 '15 at 16:43
  • $\begingroup$ @chris I think that this question means the OP doesn't know of diploidy and assumes that a more or less random half of the chromosomes present in a normal cell end up in a gamete. $\endgroup$ – rumtscho Feb 27 '15 at 17:10
  • $\begingroup$ Amir, welcome to the site! Your question is very basic, and we have trouble finding out what exactly is unclear to you. Please edit the question to explain which situation you mean exactly (when are they "chosen"? Do you know that human chromosomes are paired? etc.) I will vote to put the question on hold until you have done so, else you will get the wildest guesses as answers, and they won't be very helpful. You can also try reading en.wikipedia.org/wiki/Ploidy, but I admit the language will be hard for somebody who is not a native speaker of English and does not know much biology. $\endgroup$ – rumtscho Feb 27 '15 at 17:13
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    $\begingroup$ This should clear up some of your confusion: Wikipedia: meiosis. Meiosis is the process that halves the genetic content and produce haploid eggs and sperm. $\endgroup$ – fileunderwater Feb 27 '15 at 20:04
  • $\begingroup$ I understand your question, without you having to rephrase it. I found a helpful link for me. Maybe it helps you too. sciencelearn.org.nz/Contexts/Uniquely-Me/… $\endgroup$ – user21217 Jan 15 '16 at 21:05
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They are not chosen, they represent an even half, a complete set of chromosomes.

The entirity of a cells genes (found on the chromosomes) is called a genome.

Somatic cells are the cells in our bodies that exclude sperm and egg. Sperm and egg are called germline cells.

The billions of somatic cells in a homo sapien have the same 46 chromosomes. 23 ( half) of them came from dad and 23 from mom. Thus we say somatic cells have two copies of each chromosome and it is abbreviated 2n. The germline cells are 1n or just n.

So your sperm has 23 chromosomes and after sexy time one of them can combine with an egg (which also has 23) to produce a cell(called a zygote) that has an entire complete genome (46). Then that zygote splits, and splits and splits.... And each time before it splits, all 46 chromosomes are copied (the copies are called sisters, or sister chromatids, respective to each other). The cells split into two new cells called daughter cell and each daughter has an entire genome. This form of cell splitting is called mitosis.

Probably the most important event in the cell cycle is the duplication of every DNA stand. This is called a Synthesis (S) phase. This is not a component of mitosis! It's a required event that happens before mitosis. So all 46 chromosomes get copied and then during mitosis each copy (an identical copy) is attached to each other at a region called the kinetachor. A kinetachor attaches sister chromatids (each sister is a copy of each other).

At the beginning of mitosis a web forms in the cell and you can think of the strands that make the web as sliding chains that can pull stuff from the center of the cell to 1 of two perimeter locations (poles) of the cell.

The chain like web attaches to the kinetachor and it rips apart the pair of chromosomes, rips apart the sisters. So that one sister goes to one side of the cell and the other sister chromosome goes to the other side of the cell. So now at opposite sides of the cell you have a complete genome (which will become the new daughter cells).

An identical process occurs in meiosis (specifically meiosis II) which is part of the process that forms the sperm and the egg. During meiosis however there's additional separation (separation of homologs which occurs in meiosis I) that occurs but it's really not pertinent to your question. enter image description here

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    $\begingroup$ In biological terms, I believe the original question is asking if the separation of homologous chromosomes is random or not. Perhaps you could touch on that briefly. $\endgroup$ – canadianer Feb 27 '15 at 23:02
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    $\begingroup$ Good point @canadianer $\endgroup$ – rhill45 Feb 27 '15 at 23:10
  • $\begingroup$ Thanks @rhill45 for your answer. I know the steps of the process, I just wondered, how are those initial 23 chromosomes chosen (@canadianer - separation of homologous chromosomes). So the answer is random as I deducted from your reply? Thanks! :) $\endgroup$ – ememem Sep 22 '16 at 11:01
  • $\begingroup$ @ememem It is random, and of course it would have to be: don't forget every person has hundreds of eggs or millions of sperm! The fact the assortment is random means every egg and sperm within a same person has a different combination of half of that person's chromosomes. If that weren't the case then brothers and sisters of same parents would be identical! $\endgroup$ – Oosaka May 2 '17 at 16:47
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    $\begingroup$ How specifically is this randomness achieved? Computers e.g. often have problems generating random numbers, how does the body achieve it? $\endgroup$ – Harald Nordgren Dec 29 '18 at 4:50

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