Eli5 what decides whether genes are dominant or recessive
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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.
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