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When two parents (regardless of species) reproduce in sexual reproduction each offspring is generally a random combination of genetic traits from each parent. Their developed traits then depend on the expression of each of their genes per established genetic combinatory science. All well and good.

Now my question - when does the actual randomization of genes for a given offspring happen? Does it happen when the individual sperm and egg cells are formed, or does it happen when they combine?

So does every one of a female's eggs have the same genetics, and every one of a male's sperm have the same genetics, and when the two combine it is randomized to form the zygote?

Or is each egg unique and each sperm unique, and already "pre-randomized", and the combination to form the zygote is then a straight-forward addition?

Or is it some variation or combination of those two options? Maybe the answer actually even differs across species, or across plants vs animals, or fish vs mammals, or something?

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  • $\begingroup$ Randomization is the act of making random. I have corrected one clear misuse. While an enquiring mind is to be commended, it's owner should learn how to find the answers to its questions himself using the sources available on the internet if he does not have access to a University library. And, indeed, that is what SE Biology expects of posters (see the Help). Google for NCBI Bookshelf, where you can find older editions of standard textbooks in cellular and molecular biology. If these do not answer your question, then post citing the information provided and how it is deficient. Have fun! $\endgroup$ – David Feb 10 '18 at 20:24
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Randomisation happens in the forming of the the egg and the sperm. In a first approximation, the egg and the sperm receive a random single chromosome of each of the chromosome pairs during meiosis.

More randomisation happens due to the possibility of crossing over when homologous chromosomes exchange chromosome parts.

EDIT: In terms of the question, bothe the egg and the sperm are pre-randomised.

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Your view of the randomization of the parent traits seem a bit wrong and it makes the question a little unclear. But still, I think we can answer it...

The randomization happens at the moment of the creation of sperms and ovules. The two "randomizing processes" are segregation and recombination.

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