Shuffled at Conception

Each parent gives you 23 chromosomes, but which copy of each one you get is pure chance. Counting only that coin-flip, two parents can produce more than 70 trillion different chromosome combinations. And that is before crossover, where chromosomes swap chunks of DNA and reshuffle the deck again.

This is why siblings can look so different even though they share the same two parents. Each child gets a fresh draw from the same gene pool. Full siblings share about 50 percent of their DNA on average, but which 50 percent differs wildly from one child to the next. One kid pulls Mom's eyes and Dad's height; the next pulls the opposite.