Hi Lido and welcome. Hope you decide to stick around.
Siblings aren’t identical because al sperm and ova (eggs) are not genetically identical. To understand why we need to go back to basics.
Human genes aren’t all located in the one place physically within a cell. Instead they are spread out over a number of packages called chromosomes. You could if you wished to think of chromosomes as ‘boxes’ containing various genes although in reality they are more like strings of genes joined together in a chain. However you think of it it’s important to realise that in humans the genes are located in 46 discrete packages called chromosomes.
Not only are the genes located in discrete chromosomes but there are two very similar versions of each chromosome. Each chromosome in a pair contains the same basic genes but the exact form of each gene can vary. So for example in the chromosomes containing the gene that codes for hair one chromosome might contain a gene that codes for code for red hair while the other may contain a gene that codes for black hair. Of course both could code for red or both for black but for the moment we will concentrate on chromosomes with different versions of a gene.
Sperm and ova (collectively called gametes) are made by a special type of cell division in which the cell splits the chromosome pairs apart and donates one of each pair to the daughter cells. In other words gametes only have half the number of chromosomes of the rest of the cells in the body. What this means is that in the example above for every two sperm produced one will receive the chromosome that codes for red hair and one will receive the chromosome that codes for black. And of course the same is true of ova. As you can see the gametes are not all exactly the same as each other.
Because they all differ from one another, when the gametes from two individuals combine there is a huge potential variation. Just to concentrate on hair colour for example assume that both parents carry one gene for red and one for black hair. The offspring could be formed from both gametes that both carry red genes and thus have red hair despite the parents being black haired. That occurs because the black pigment literally colours over the red. Or the offspring could inherit just the black genes form each parent and have black hair. Or they could inherit one black and one red and be dark haired like their parents.
Of course there are 23 pairs of chromosomes and gametes can receive any copy of any papir to give an incredible number of chromosome combinations. You can work out the permutations your self but there are thousands if not millions of genetically different types of gametes produced by every individual. And of course each chromosome contains numerous genes that code for all sorts of unrelated things.
The diversity is added to even more because:
There may be more than one copy of each gene on each chromosome
There may be copies of the same gene on different pairs pf chromosomes
Genes on one chromosome pair are affected by genes on other unrelated pairs.
Things like crossing over where chromosomes swap genes and differential gene expression depending on which parent the gene is inherited from.
In all it allows for an incredible diversity of human beings even form just two individuals.
So as you can see even parents that are genetically very similar to one another can have children with wildly varying characteristics because the sperm and ova are genetically dissimilar.
Because there are some pedantic people on these boards (yes you know who you are :wally ) I will add the disclaimer that hair colour is not quite as simple as I described it and is governed by several genes. But that is roughly how it works and it makes a handy example.
This is simple. A human, or any mammal for that matter, has 2n(diploid) genes (n representing an integer)
The sperm and ova only have n (haploid), or half of the genes of the parents.
Siblings are different because the gametes do not split into the same half every time. This process is called meiosis.
This might help a lot more than I have … http://www.accessexcellence.org/AB/GG/meiosis.html
cheers.
engine.