I would like some rare humans (1 in 10 million) to have a mutation that increases their lifetime by a factor of three. In other words, these rare humans could live for ~240 years on average.

But I also need to have one of these constraints:

  1. This mutation is not transferable to their children; or
  2. This mutation is transferable to their children, but on a 1/100 ratio, i.e., only 1 out of 100 children has the same mutation; or
  3. They could transfer their mutation, but they are almost sterile (rare chances of having one child during their lifetime).

Which one of these alternatives is more realistic, from a biologic point of view?

up vote 5 down vote accepted

Roughly 1% transfer chance is achievable.

Suppose you have a gene that is useful if you get one copy, but fatal if you get two (think sickle-cell). Now if you breed with a normal person, 1/2 the offspring gets the trait and 1/2 is normal. If you breed with another mutant, again 1/2 of the offspring gets the trait (from either parent), 1/4 is normal and 1/4 is dead.

Now suppose you have n such genes, in different chromosomes (independent, so the probabilities multiply), which are all required for the needed trait. Go out with normal person, 2^-n of the offspring are mutants and 2^-n normal, the rest are passive carriers of some of the required genes.

Go out with another mutant, the probability for the offspring to live is (3/4)^n=3^n*2^-2n and the probability to get a mutant is 2^-n again.

At n=7 you get 1/128 for 2^-n and 2187/16384 =~ 1/8 for (3/4)^n. So 7 in 8 children in a mutant couple are dead or disabled, no surprise they'll keep separated.

The hard part is making them 1 in 10M and not 1 in 128 though. Longevity and vigor are attractive. Maybe you should limit their fertile age somehow.

Fun is also likely to begin when longevity genetics are discovered and normal children of long-lived people start looking for a couple who has complement set of altered genes. The chances for longevity of the child are still 1 in 128 though.

Now if you tell that "longevity is acquired through getting gene1, gene2, gene3..." this is just boring. I would suggest thinking of actual traits, e.g.

Disclaimer here. I'm not a biologist, need one to refine the following.

1) Cancer resistance. 1x = no cancer, 2x = cell division is slowed down, no hair, no nails, deficient bone marrow leads to anemia and death.

2) Brain tissue regeneration. 1x = very nice, 2x = brain keeps growing, fills cranium, and dies from obstruction. One can show superior intelligence in the meantime.

3) Very potent immune system. 1x = excellent health, 2x = autoimmune disease of choice.

EDIT Maybe you should think of unpleasant but tolerable side-effects of the 7 genes that cancel out if one gets jackpot. This will explain 1/10M distribution. Or it won't, not sure here.

  • You could realistically assign any probability here, I think (see my comment on bowlturner's answer). Exactly how and why depends on how much detail you want to provide in the explanation. – goldilocks Nov 14 '15 at 20:52
  • 1
    Longevity and vigour are attractive, but perhaps they don't become apparent in a woman until after her menopause, which is one thing not affected by the mutation? (Or even caused earlier than normal by the mutation). – nigel222 Nov 16 '15 at 15:40

Actually not transferable to their children (other than the same odds as any other person) would likely be the best. If it can pass on to their children. Any gene of the parent has a %50 chance of being passed on to their children. A recessive gene will need to have another parent with the same trait for it to come to the fore. At this point 3/4 of their children will carry the gene and 1/4 will have it surface. If it was a recessive trait, then all of the long lived children will at least carry the gene, and this means that if two long lived have children their offspring will have (almost) %100 chance of being long lived.

So families with this gene that intermarry will have much higher chances of long lived children. If they have more children over that much longer life span there will become 'communities' of these people.

However, there are other things that cause different genetic issues other than normal gene inheritance. So I'd use either mostly sterile, or some environmental variable (mutation) that causes the change.

  • 2
    You might find this article interesting, about how two blue eyed parents can have a brown eyed child (a fact which contradicts traditional assumptions about how blue eyes are purely the product of a recessive gene). There could be very unusual characteristics that are 100% genetic, but rarely manifest, not necessarily when it manifests in both parents, but possibly even when it does not in either, if the characteristic is a combination of multiple recessive genes. – goldilocks Nov 14 '15 at 21:26

Option 1: This mutation is not transferable to their children

is probably not possible. DNA just doesn't work that way.

Option 2: This mutation is transferable to their children, but on a 1/100 ratio

What if their children also inherited DNA from a different long-lived human? The mutation would almost surely be passed down to the child. If it's recessive, as I'm assuming it is, the chance would be 1/4, and if it's dominant it would be 100% likely, which defeats the purpose.

So I believe that option 3 will be your best solution:

Option 3: They could transfer their mutation, but they are almost sterile

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