How do egg and sperm form a zygote?

Question

When sperm and egg come together, the resulting zygote has genetic information from both. For each alele Is there a roll of the dice as to which information comes from which parent, or in each case is the information from one parent dominant? Would a given sperm and a given egg always form exactly the same zygote? Imagine, for instance, two cases, one in whch a sperm and egg come together naturally, or another case in which the same sperm and egg are extracted and artificially inseminated at another time...would the zygote be the same either way?

Answer 1

There's a lot in that question.  First of all, a specific sperm and a specific egg that combine will form genetically the same zygote no matter how the fertilization occurred, in vivo or in vitro.

Each parent's genes, in most cases, are equally expressed.  However, as we learn more about the field of epigenetics, we are discovering that there are some genes expressed during embryonic development that are imprinted.  In other words, the offspring "knows" whether they came from mom or dad and treats them differently.  An example of this is in Prader-Willi Syndrome vs. Angelman Syndrome.  The same allele will produce a different effect depending on whether it came from mom or dad. (For a brief intro to genomic imprinting, see: http://www.geneimprint.com/site/what-is-...

As to which alleles from mom end up in her egg and which alleles end up from dad end up in the sperm, that is primarily a roll of the dice, as far as we know.  The law of independent assortment guarantees new combinations each time meiosis occurs.