Talk:Nightblood/@comment-2A02:C7F:6E2F:2600:2C44:E931:684:7EF2-20180416181531/@comment-69.145.120.37-20180724000151

Basically, a DNA gene is made from 2 alleles. We inherit one allelle from our mother, the other from our father. And each of our parents also have 2 allelles. So we inherit one of two possibilities from each (ie, Father=F and f; Mother=M and m; so we can inherit FM, fM, Fm, or fm). Basic mendellian genetics tell us that any particular combination is considered one out of four, or 25%.

Sometimes an allelle is 'dominant' in that it only takes one to be expressed, and the other is surpressed. Other allelles are recessive in that it takes 2 of them to be expressed.

So as to your comment, if a Nightblood gave birth to a nightblood, that would demonstrate the allelle is Dominant because the baby nightblood would also have an allelle from it's father that is probably not a nightblood.

But there's clues in The 100, such as the flamekeepers 'searching' for nightbloods which seems to suggest it's not a Dominate allele. If it were, they would already know who and where all the nightbloods were...the children of other nightbloods. We also know that they are found as children, raised in seclusion in the enclave, then fight to the death for the chance to be the new commander. There doesn't seem to be any childbearing in the group.

I don't know if there's official (or fan theories) on the genetics of nightblood. It might be a new blood type making Grounders ABON. Or it might be some other gene that affects the blood (ie, sickle cell anemia). But in any case, it doesn't seem to be a Dominant trait.

But as a recessive trait, the child would need to inherit 2 nightblood alleles from parents that are carrying at least one nightblood allele each. If it's a new blood type in the ABO human group, then it could be expressed like this:  AN, BN, ON, NN (the first three would be carriers, but only the fourth NN would actually be a nightblood).

And if having at least one N allele conferred an advantage such as increasing your survivability to a radiation soaked earth (maybe the key difference between grounders and mountainmen?), then the N allele would spread very quickly through the gene pool.

For instance, let's say a newborn is a typical AO but is born with birth defects. The grounder culture developed that the child would be left out to be reclaimed by the elements. But if the child was born with AN, and didn't have birth defects, that child would live to grow up and have children of it's own, passing either it's A or it's N onto it's children. And if it's other parent was an ON, then the odds of a child inheriting 2 NNs would be 1:4 or 25% (AO, AN, NO, NN). So here again, if one out of four was born AO and had birth defects, it was unlikely to survive. But the AN and NO would survive to have children of their own. (But the NN would be the nightblood, and unlikely to survive to have children due to the enclave.)

Now it does seem that nightblood is much, much more rare than even standard recessive 25%. So perhaps we can speculate that it's both a recessive blood type (N) as well as a new recessive antigen like positive/negative. So +, -, ∞ (yes, that's the sacred symbol, lol). If this were the case, then the only way to have a Nightblood would be if their blood alleles were N∞N∞. This changes the inheritance odds to somewhere around 1 out of 144.