eli5: if you inherit 50% of your genes from your father and 50% from your mother, what stops you from getting two of the same gene, and missing one? also why do siblings look different?

Question

eli5: if you inherit 50% of your genes from your father and 50% from your mother, what stops you from getting two of the same gene, and missing one? also why do siblings look different?

In: Biology

Answer 1

The 50% thing is a bit misleading: you have two complete* sets of genetic code, one from each parent. Your parents also have two copies of the code, one from each of *their* parents.

When sperm and egg cells are created, a process called [crossing over](https://www.genome.gov/genetics-glossary/Crossing-Over) occurs: your father’s cells line up both copies of his genetic code and randomly choose chunks from either the code they got from your grandmother or from your grandfather.

It’s a bit like lining up two copies of the Bible, cutting them up at the same random points, and reassembling them by picking each verse from one copy or the other. You might get the first half of Genesis from one copy and the second half from the other, but you won’t have two copies of anything.

Your father then creates sperm sells with this randomized genome, your mother creates egg cells with a different randomized genome selected from her parents’ genes, and those two copies are combined to make your genome.

Your siblings will get a different randomized sample from each parent, so they’ll be a different combination of your shared grandparents’ genetic code.