Why is there genetic variation among siblings beyond gender?

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My impression is that genetic information in eggs and sperm are consistent per individual. If so, what causes the combination of those two sources to vary so greatly? Some brothers or sisters are total opposites of each other despite the same “ingredients”. How does that happen genetically? Why would only one of a couples children inherit an expressive genetic disease? Etc.

In: Biology

>My impression is that genetic information in eggs and sperm are consistent per individual.

This is the part that’s wrong. Each egg and each sperm has half of the parent’s DNA, but which half is actually present in a given cell is random. Whether the sperm carries an X or Y chromosome is only one of many factors that randomly varies between sex cells.

because you have a mother *and* a father.
You and your sibling will pick their genetic information from that pool, but this leaves a lot of freeroom and the individual pick will be different.

Imagine your DNA is a bag of marbles, each marble has all sorts of unique colours.

When a sperm/egg is formed, it is made using half of the bag of marbles. This way when the 2 are combined you have a full bag of marbles.

When the time comes for a sperm/egg to be created, half of the marbles in your DNA are used to create it. There is no way to control which exact marbles are used, so each sperm/egg will have its own half-set of marbles.

Parents, siblings and child share 50% of their DNA with you. Grandparents, aunts, uncles share 25% w/ you. First cousins share 12.5%. For each generation back and forward from you, your DNA is halved. You have 1.56% of your great-great-great grandparents’ DNA. The ELI5 is that the DNA from all your ancestors is shuffled and halved each generation. The halving is consistent, the shuffling is a free-for-all.