In humans, without considering the variation provided by crossing over, how much will two siblings from one set of parents vary from each other?

Random segregation of homologous chromosomes makes the two siblings differ from each other for 0-23 chromosomes.

Explanation:

Sexual reproduction adds genetic variations in the progeny by crossing over, independent segregation of homologous chromosomes and random fusion of gametes. Without crossing over, independent segregation of homologous chromosomes towards opposite poles during anaphase I of meiosis may result in two siblings to vary from each other for 0-23 chromosomes.

There is an equal probability of each of the two siblings to get a chromosome from mother or father. Hence, irrespective of the variations provided by crossing over, random segregation of homologous chromosomes makes the two siblings differ from each other for 0-23 chromosomes.

Siblings inherit essentially 50% of their genes from each parent, but two sibling offspring may share with each other from zero to 23 chromosomes in common from each parent and therefore very widely from each other.

Explanation:

It is not a surprise that when two brothers do a DNA test the results are very similar. However, it may be a surprise to many people that two brothers (not twins) with exactly the same parents and ancestors may have a different genetic makeup. After all, identical ancestors should give identical genetic compositions, right?

Well, it's not that simple. In fact, it is even common for siblings to have different genetic compositions. There are several factors that can affect genealogical ethnicities and result in different chromosomes for siblings of the same parents.

The siblings inherit essentially 50% of their genes from each parent, but two children can share from zero to 23 common chromosomes from each parent and thus very widely from each other.