system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come full siblings share only 50% genetic with each other and not 100% since they both have the same parents?

Someone told me that full siblings with the same parents only share 50% genetic material, but why wouldn't it be 100% since they both have the same parents and are both getting 50% from the same mom and 50% from the same dad? Isn't it true that they're getting 100% of the same amount of DNA from the same genetic source (their parents)? What makes it only 50%??
gpt
During conception you get 50% of your genes from your father and 50% from your mother. But you still have the same number of genes so there is 50% of the genes from each parent you do not get. Your siblings will have the same distribution of genes. So of the 50% of the genes you get from each parent your sibling will have on average 50% of them and 50% of the genes your parents have not given you. So you end up sharing 50% of your genes with your sibling. For instance your dad have a gene for developing testicles which makes you into a man, however there is still a 50% chance that this trait is the same in your sibling then in you.

To better visualize how this works. You take a deck of cards and divide it into red and black suits. This represents the genes of your parents. Now on random chose 13 cards at random from each of those decks numbered 2 to A. The new deck will have equally many black and red suits. This represents your genes. Now repeat the task with another deck of cards and you end up with a deck for your sibling. Since you picked the cards at random you now share half the cards in the decks.