Eli5 what decides whether genes are dominant or recessive
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As with almost everything in genetics, dominant/recessive is an oversimplification of things. But within that model, a variant of a gene is usually dominant if it produces something with an effect, and a variant of a gene is recessive if it *fails* to produce something with an effect.
For example, [Creutzfeldt-Jakob disease](https://en.wikipedia.org/wiki/Creutzfeldt%E2%80%93Jakob_disease) is dominant (when it’s genetic at all) because all you need is to start producing the misfolded protein that causes the disease – once you’ve got the bad protein, it multiplies until it eventually kills you. On the other hand, [sickle-cell disease](https://en.wikipedia.org/wiki/Sickle_cell_disease) is recessive because you only have it if you *fail* to produce normal hemoglobin – half normal and half variant hemoglobin mostly doesn’t affect your cells.
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