Why don't siblings with the same parents look exactly the same?

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Alf Larcher

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Alf Larcher  asked the Naked Scientists:
   Hi Chris  It's Alf from Perth, we met some time ago not sure whether you remember.  We must catch up next time you're in fair WA.  My question is for siblings from the same parents, why don't they look exactly the same as each other since they are a combination from the same DNA?  Also, why do some siblings look more similar to each other, while others not?  Regards  Alf  

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« Last Edit: 20/07/2016 11:57:34 by _system »


Offline evan_au

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Humans genes are spread out along 23 chromosomes (2 different versions of each chromosome).
A person's appearance is affected by both copies of each gene.

One copy of each gene comes from your father, and one from your mother.
So the pair of genes you have will be different from the pair your mother had, and different from the pair your father had. You will look similar to your parents, but not identical to either of them.

Your brother or sister will get a different mix of genes from your father, and a different mix from your mother; on average you will have half of your genes in common.
If you happen to share 75% of your genes with your sibling you will look more alike than if you share only 25% of genes with your sibling.

A special case is identical twins, who form from the same fertilized egg, and share 100% of their genes; you have to know both twins fairly well to tell them apart.

But even identical twins will have different nutrition in the womb, and suffer different accidents as children, and so environmental effects can make them look slightly different.