How do we get from a fertilized egg to 22 non-homologous chromosomes and a sex chromosome?

Question

From the classes I've taken, there is a heavy emphasis on meiosis, the nondisjunction of some steps in meiosis, and diversification of meiosis. So I know that the product of meiosis is a fertilized egg with diversified chromosomes from your parent in the egg (hopefully just one of each). But my question from here, is what is the exact way that we get to the other 22 somatic chromosomes from here? Do somatic chromosomes express diversity since one is from each parent? In other words, how is the gene expression for somatic chromosomes occur, if there is any gene selective expression? Thank you!

skeptik · Accepted Answer

First thing: The product of meiosis is not a fertilized egg.  Rather, the product is either an UN-fertilized egg, or a sperm cell - depending on where the meiosis is occurring.

One set of haploid chromosomes exists in the unfertilized egg cells and another exists in the sperm cell.  Fertilization combines those two haploid sets into a single diploid set of 23 pairs.

And yes, diversity is the product of one set of genes from one parent and another set from the other.

The combination of meiosis and fertilization inserts a randomization factor into the end genome. Which means that no offspring is a genetic duplicate of either parent, or of each other - except in the case of monozygotic twins.

How do we get from a fertilized egg to 22 non-homologous chromosomes and a sex chromosome?

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