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.
As a rough generalisation dominant genes are because they produce something that causes an effect whereas recessive genes are genes that fail to produce something.
So eg Albinism (lack of melanin) is recessive because if you only had one copy of the gene causing albinism then your other normal gene could still make melanin.
Whereas dominant huntingdons diesease is because the mutated form of the gene basically makes a toxic protein. So even if you had one normal copy the mutayed foem would still be affecting you.
Its basically which one has the stronger effect – they dont directly suppress the other gene normally, they just do a lot more, so they are expressed dominantly.
So someone with brown eyes could have the genes for blue and brown, but the pigment expression for brown is the much stronger effect, rendering the other gene irrelevant.
This also explains things like incomplete dominance- someone with genes for brown and green eyes often ends up with a mix, because neither gene out competes the other.
A lot of genetic illnesses are because one version of the gene is faulty – cystic fibrosis is a classic example (caused by a faulty chloride channel). In this case one version of the gene is enough to get by, so the fact that half your channels don’t work is fine, and the gene is therefore recessive.
Huntingtons disease is a dominant condition, as the single broken gene codes for a protein that ends up having toxic effects, so having one normal working copy doesn’t really help.
One example: recessive allele could be that a specific protein isnt made, so if you have at least one dominant allele then the protein will be made.
Let’s say you’re Rachel from Friends and you’re about to make dinner for girls night. You have two identical recipe books, one is fine, and the other has pages that stick together. You know you’re not a great cook so you decide to make two of each dish just in case one turns out bad.
The first book has the normal recipe. No problems there, your trifle is perfect. The second book with the sticky pages leads to you adding meat and peas to your trifle. Yuck.
Whether a gene is dominant or recessive basically depends on the effect of the mistake.
If your dinner guest is Monica, the sight of your meat trifle will make her freak out so bad she won’t even look at your other, perfectly fine trifle. The mistake had an effect on the receptor. That’s a dominant mistake: the bad trifle was so bad it put Monica off the rest of the food.
If your dinner guest is Phoebe, she won’t mind that you made a mistake, she’ll politely take a serving of the normal trifle and ignore the meat one. The mistake had no effect on the receptor. That’s a recessive mistake: the good trifle made up for the bad one, and all is forgiven.
In your body, a mistake in DNA is only a problem if it translates into a faulty protein, and if that faulty protein has an effect on how the cells function. Most of the time, having one “good” copy of a gene is sufficient to make you function normally, as long as the bad protein doesn’t wreak havoc on its own
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