Two siblings will have acquired their genes from their parents in specialized sex cells called gametes, which are sperm or eggs. The combinations of these genes are very unique which explains the differences noticed between siblings. Therefore, offspring vary from each other as well as from each parent.
Meiosis is a special type of cell division resulting in haploid gametes, or sex cells. These will fuse to form the fertilized egg which is diploid. Since a person has diploid cells, which contain chromosomes from both their mother and father, when they produce haploid gametes, there are many million possibilities of how their genes are organized into sex cells.
During meiosis which is a reduction division, a process known as crossing over occurs in prophase I of meiosis which helps to exchange genes between the maternal and paternal chromosomes in a corresponding pair known as nonsister chromatids. When this occurs, individuals chromosomes that will be placed in a gamete contain genes that originated from each parent. It shuffles the DNA in unique combinations. Crossing over results in recombinant chromosomes which helps to make unique combinations of traits in gametes.
Another reason siblings are unique is due to a stage of meiosis in metaphase II where nonidentical sister chromatids sort independently from each other, which increases the possible gene combinations in the gametes even more.
When fertilization occurs, because it is a random event it adds to more genetic variation in the possible offspring because any sperm can fuse with any egg with each containing several million possibilities of traits due to crossing over and independent assortment.
It is the recombination that occurs during meiosis after crossing over that shuffles the DNA into millions of unique combinations that has the greatest impact on why siblings are unique from each other with independent assortment and later fertilization adding to the genetic variability of an individual.
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