# Sure-fire way to identify an heir in a world without genetic testing

My question is about unusual hereditary traits in humans. I'm new here and this is my first question - apologies in advance if I've accidentally messed this up or covered existing ground.

I'm writing a post-apocalyptic story in which the protagonist realises partway through that a child he's rescued is biologically his own. This is a post-apocalyptic earth, relatively realistic, so genetics have to adhere to our current real-world rules. There is no genetic testing in this future, though the characters in question are educated and understand genetics and hereditary principles. My question is: in what way/s could I show that (child) is the daughter of (Male B) rather than (Male A)? In this scenario those two men are the only possible fathers.

I looked into birthmarks, but from what I can find, they're not actually hereditary, and this was just one of those little literary cheats - one I'd rather not use.

I've also considered having it become obvious that the little girl is colourblind, and so is (Male B), but not (Male A). A girl can only be colourblind if her father is (AND her mother carries the gene, but that's besides the point). This seems like a fiddly, talky way to handle this reveal though, so I'm wondering if there's anything like a birthmark - a medical condition? A distinct but subtle physical feature? - that can only be inherited from a father, can be identified visually, and provides incontrovertible proof of parentage.

• Note: such a feature would have been of immense value to royalty through the centuries. – Cort Ammon - Reinstate Monica Oct 9 '17 at 3:34
• In a lot of Chinese period dramas, the go-to test for this was to have the father and child each drip a drop of blood into a bowl of water. If the two drops mixed, they were related, and if the two drops stayed separate, they were not. I bring this up not because it is scientifically accurate, but because it was a neat plot device to create tension. Your answer should consider that too, if it is for a story. – Xenocacia Oct 9 '17 at 8:03
• I think you have misconstrued the birthmark here as a genetic device. It is not (in my reading at least) used to identify a parent or lineage, but to identify the individual that is already known to have it. That is, when born it was observed and that is why they search based on it later, it has nothing to do with the parents per se in the scenarios I have read. Despite that, they can change during growth. As far as I know what you are looking for doesn't exist. There is always a chance that mutation gave the same result, though you can get very high confidence, but not incontrovertible. – ttbek Oct 9 '17 at 14:28
• @ttbek Inheritable birthmarks are a decently common trope in Fantasy (eg Discworld, Belgariad, and see some examples at tvtropes.org/pmwiki/pmwiki.php/Main/BirthmarkOfDestiny), possibly due to things like en.wikipedia.org/wiki/Port-wine_stain and en.wikipedia.org/wiki/Mongolian_spot both of which are indicative of heredity and have inheritable positioning (though not usually shape) – Dewi Morgan Oct 9 '17 at 17:20
• Real life example touching on the topic of distinct but subtle physical features: The creepy Tamam Shud case involved a murder victim with a very unusual combination of traits for a white Caucasian male: Hypodontia of both incisors and a rare specific morphology of the ear. He had a poetry book on himself, The Rubaiyat of Omar Kayyam, with the phone number of a woman living 400m away whose son had the same rare ear and teeth features. The probability of this being coincidence was estimated at 1:10,000,000. – Iwillnotexist Idonotexist Oct 10 '17 at 7:15

While you point out there is no genetic testing, & I can certainly understand why that would be the case, Blood typing is a skill that is likely to not have been lost. Specifically because it is so useful to medicine, making possible blood transfusions.

It is very true that while it cannot conclusively prove paternal parentage, Blood Types can absolutely rule out one of the fathers, if types are not compatible. And that can be found out in the oddest of ways...

Here is a site that gives a pretty simple chart of parental types and possible resulting children's types: https://canadiancrc.com/Paternity_determination_blood_type.aspx

Here is a site that gives a pretty simple explanation of how to test for blood type: https://www.nobelprize.org/educational/medicine/bloodtypinggame/2.html

There is a movie that talks about something along these lines, and uses as a plot element some of the other responses about genetic characteristics: The Switch It was a pretty fun watch in a chick flick sort of a way, might be worth taking a look if you enjoy movies and don't mind Jennifer Aniston or Jason Bateman in a lead role.

Another movie that gives some insight into the whole Blood Type can eliminate potential paternal situations is: Made in America

• Getting the appropriate antigens could be tricky, but it’s easier than checking paternal dna. – Joe Bloggs Oct 9 '17 at 6:35
• @joebloggs cant you just mix A blood and the heir's? Or B or O or AB – Vylix Oct 9 '17 at 7:23
• @Vylix: if you’re certain of the test blood’s type, I guess so? Not sure how well it’d work. – Joe Bloggs Oct 9 '17 at 7:34
• Blood typing can't prove paternity, it can only disprove it, except in cases where the pool of potential fathers is extremely small. – Mark Oct 9 '17 at 22:30
• @Mark Given that the pool size is specified in the question as 2, that shouldn't be a problem. – Reinstate Monica Oct 10 '17 at 12:30

Since you are creating a post-apocalyptic (and if that apocalypse was either nuclear or biological in nature), the genetic proof you are looking for might be a mutation which is not currently present in the human gene pool.

Look around the animal kingdom for examples of biologically possible traits. Perhaps something from our genetic neighborhood, like a prehensile tail, or maybe something from a little more distance, like reflective cats eyes.

When you throw a little nuclear fallout or a few gene jokers into the mix, almost anything becomes possible.

• This actually seems like a great idea. Worldbuilding and plot arc in one. Good call! – Daniel B Oct 9 '17 at 8:39
• Cool but wouldn't work if potential parent was born before the apocalypse(not clear whether this is the case for OP or not). Changes to DNA after you are an adult will present differently than those present in a child. The changes relevant to the child will be in the gametes anyway, and these could all be different. All in all, it's like blood type, there are a few cases with pretty much perfect accuracy (dad only person alive with purple hair, until son born with purple hair) but no certain falsify-ability, ie dad has purple hair, child doesn't, can't prove anything, as oppose to blood-type - – wedstrom Oct 9 '17 at 20:24
• - where you can conclusively disprove, but not prove, lineage. – wedstrom Oct 9 '17 at 20:25
• @HenryTaylor "When you throw a little nuclear fallout or a few gene jokers into the mix, almost anything becomes possible." No, that is not how it works. That dumb trope is so overused, and so damned wrong, that is is comparable to the myth that everyone else thought the Earth was flat while only the visionary Christopher Columbus thought the Earth was round (in fact: everyone knew it was round, and Columbus was way wrong about the size, everyone else was righbt). You have to differ between DNA damage and genetic mutation. Radiation does the former, not the latter. – MichaelK Oct 10 '17 at 8:59
• We have more than 10 different factors that break up our DNA all the time. Each cell in your body suffers between 1,000 and 1,000,000 DNA strand breaks per day. The only thing that a heavy dose of radiation does is to cause more damage. A mutation is a much more complex process, and while DNA damage can be a part of that process, it is just wrong to say "Oh if you are subject to radiation your genes will mutate and you will develop a new trait". – MichaelK Oct 10 '17 at 9:01

Recessive alleles only show if two copies of the recessive allele are inherited - you couldn't get blue eyes unless both parents had the recessive allele for blue eyes. These pages have some suggestions about dominant/recessive characteristics: http://www.blinn.edu/socialscience/LDThomas/Feldman/Handouts/0203hand.htm and http://faculty.southwest.tn.edu/jiwilliams/human_traits.htm.

If the child knows the mother (and/or both of the men do), they might be able to rule out one of the men if the child exhibits traits that only come from inheriting two recessive genes.

Some initial thoughts are:

• Having red hair
• Having earlobes that are attached to the head
• Being susceptible to poison ivy (just imagining the child falling into a ditch, both of the men getting the kid out, and finding out that one guy and the child come out in a huge rash...)
• Not being able to roll their tongue
• Being able to fold their tongue

And there are several more in the links above.

• Would this imply that both of the father's parents had the recessive gene (for the father to have the trait) and both parents of the daughter would also be required to have the recessive gene (ie, the male B and the mother?) If so, this requirement would certainly cut down the odds. Red hair seems really viable. The child could have her hair died, or perhaps lost it completely from radiation, but the true color was revealed, well, from other areas of the body under circumspect conditions. Tending to an injury, perhaps. – Justin Thyme Oct 9 '17 at 14:44
• Poison ivy susceptibility is a recessive inherited trait?? I’m learning all the awesome plant based facts today! – Joe Bloggs Oct 9 '17 at 17:12
• For this the simplest situation would be if the Mother had the recessive trait and the child had the dominant trait. Then the father has to also have the dominate trait and if one fathers also has the recessive trait then they are eliminated as the possible father. This would work particularly well if the recessive trait is common in that particular population and the other father is a foreigner. Red hair would work well with that. – Evan Steinbrenner Oct 9 '17 at 17:26
• @ChrisStratton - the child could definitely be a carrier for the recessive trait, even if they didn't display it. A child could be ginger even if neither parent was (if both parents were carriers of the recessive gene), but couldn't not be ginger if both of their parents were visibly so (as both parents would have the double-recessive gene). I suppose the best way to confirm identity would be for a child to have a dominant trait, when the mother and one of the men displayed the recessive one. – K. Price Oct 9 '17 at 21:10

Answering outside the box: consider a matrilinear society.

In these societies, the heir is the child of the queen/princess/[women whose status grants power]. Thus heir identification becomes only a matter of witnessing the royal birth (which was apparently usual), and there is no bastard problems.

Note: matrilinearity does not necessarily imply matriarchy. If patriarchy is required in your story, you can for example say that the queen is a descendant of god (chief of religious power), and her husband is the executive ruler.

I would suggest webbed toes.

Not immediately visible, but still hereditary, and it doesn't have to be a hindrance.

If the hero has webbed toes and the kid does too...

Simplest answer I can think of: eye color.

You need to get a recessive gene from both parents to express light colored eyes. If the child and Male B both have dark eyes, but Male A and the mother both have light eyes, that is undeniable proof that Male B is the father.

Keep in mind, eye color can change when the child is very young. So if the kid goes from having light eyes to having dark eyes, that'll cause a lot of drama.

• Huh, no. It's a recessive gene. (Actually it's more complicated.) Both parents may have dark eyes, while the child has light eyes inherited from one or more grand-grand-parents. – AlexP Oct 9 '17 at 2:53
• @AlexP I realize that it's possible for Male A to be heterozygous for that trait, but for OPs purposes I thought this scenario would work better. Also I know that it's a recessive gene, that's why you need to inherit 2 copies of the gene for it to be expressed. If the mother and Male A in this scenario had a kid they would only have light eyed children, whereas if the mother had the kid with Heterozygous Male B there would only be a 1/4 chance of having a light eyed child. – Lot-Of-Malarkey Oct 9 '17 at 3:06
• @AlexP You are correct in raw knowledge, but you are clearly wrong in understanding the principle. Blue eyes being recessive is a proof that brown eyed child can not be conceived by blue eyed parents. In this example, it conclusively rules out "male A". – M i ech Oct 9 '17 at 8:04

Here's a pair of genetic conditions that could become obvious in an 'epiphany' (this would /have/ to be an X-linked disorder, since she doesn't get a y-chromosome from him)
1) Hemophilia. His daughter has a nose bleed that just won't stop, and is very careful about getting hurt. This would also mean they have similar mannerisms for avoiding injuries (subdermal hematomas can be fatal in the case of hemophilia.)
2) Fabry disease, which causes skin-speckling that could be mistaken for freckling, and a whorl-like pattern in the corneas. That would be the sort of thing that could be a fire-light reveal.

• My dad had hemophilia. It would be hard to survive to post-apopolytic, but possible. Finally, most hemophiliacs don't live to late adulthood. He might live long enough to observe the birth of a hemophiliac son from the questionably fathered daughter. Since 1/2 of the daughter's male children will be affected, a prior birth might have dispelled doubts, but a secondary birth will confirm them (without deniability). Also, a hemophiliac will know about the genetics, even if the ability to test was destroyed, and it is a famous disease due to prior royalty being affected. – Edwin Buck Oct 9 '17 at 23:58

Are you after a general, non-specific answer, or one that only applies in this case? A coincidental confluence of genes? And do you want a sudden, immediate, obvious recognition, or do you want it to occur over time?

I have seen fathers and daughters whose facial appearance is so unusual, and so identical, that there can be no doubt about parentage. For example, all of the following: buck teeth, incisors, wide brows, long nose, cauliflower ears, button chin. Not just one criteria, but the entire facial image. If it absolutely matches one male, but bears absolutely no resemblance to the other male, it would be conclusive.

Add quirks of behavior (a particular cough, a particular way of snuffling up the nose, a quirky way to fold the legs while sitting) that are similar would be added proof.

Not all fathers bear a close resemblance to their daughter, so NOT looking or behaving alike is not conclusive proof they are NOT father and daughter, but when they do look and behave alike, parentage is striking and unavoidable. The daughter got ALL the 'image' genes from the father, not just a random sample.

This would be the exceptional, unusual case, not the 'bell curve' standard expectation. But it IS credible, and does happen. Sort of like the idea that twins are not always identical, but when they are, it is striking. The vagarities of genetic inheritance being random. Sometimes, you roll a Yahtzee on the first throw.

• I definitely want the recognition to occur suddenly, but after they've known each other a few weeks. A flash of recognition works better for the story than a slow build. I'd want something equivalent to seeing a birthmark previously hidden by clothing and recognising it as near-identical to your own. But failing something like that, I could definitely approach it as a slow suspicion, gradually confirmed by shared mannerisms and features. – Tania Walker Oct 9 '17 at 6:12
• Like a shared dislike of some food? "No, I've always hated x, can't stand it, it tastes bitter to me." – RedSonja Oct 9 '17 at 11:17
• @RedSonja Not sure if this counts, but people who are supertasters typically taste more bitterness in foods. There is also this classic test where someone tastes strips of phenophaline (I think) and it only tastes bitter to some people. – Lot-Of-Malarkey Oct 9 '17 at 14:07

Maybe he's the only white, or black, or Asian man in town; the child would be mixed race, easy to see. Or maybe he has one of the numerous anatomical variations and again he's the only one in town; for example, a few people have a dedicated extensor muscle for the middle finger, or an accessory nail on the little toe; or lack a palmaris longus.

Or maybe the inherited characteristic is not anatomic. He may be very good with numbers, or have an exquisite sense of equilibrium, or perfect pitch; or an absolute sense of time, like Jack Reacher.

Anyway, he's a man. In the absence of genetic testing the old Roman rule applies, mater semper certa est, pater incertus.

Everyone in my maternal side of family has a mole that looks exactly the same (a little bit like Australia's map). Each one of us has it in a diferent place in our body, but we all do have it.

I think this is used too proving Jace is not Valentine's son in Shadowhunters by Cassie Clare

• Moles are an interesting avenue to pursue 'Dysplastic naevi are moles that are larger than average and irregular in shape. They tend to have uneven colour with dark brown centres and lighter, uneven edges. These moles tend to be hereditary (passed on from parent to child through genes). People with dysplastic naevi may have more than 100 moles and have a greater chance of developing melanoma, a potentially fatal form of skin cancer.' – Justin Thyme Oct 9 '17 at 14:55

As you said you want a sudden reveal like finding out a matching birthmark under clothes... From your comment:

I definitely want the recognition to occur suddenly, but after they've known each other a few weeks. A flash of recognition works better for the story than a slow build. I'd want something equivalent to seeing a birthmark previously hidden by clothing and recognising it as near-identical to your own.

Well, you can still do that, as long as the birthmark is melaninic (brown) instead of vascular (red / blood).

Only the vasculars aren't inherited. The melaninic type is.

You are looking for a trait, that satisfies the following condition:

If the child has the trait,
then the father must have it.


There are a few traits like the colorblindness you mentioned that give you precisely that. However, if it is known whether the mother had the trait, the above is implied by:

If the child has the trait, and the mother has not,
then the father must have it.


Now, the beauty of this formulation is, that it fits any dominant trait: You cannot inherit a dominant trait unless either of your parents had it. Now, the wikipedia has a list of traits that could be useful to you: https://en.wikipedia.org/wiki/Mendelian_traits_in_humans

Among these is the lactase persistence trait which can easily said that the mother did not have it due to her origin. This is a nice common trait, that can easily be tested, and that's a tell-tale trait if looked-at from the right angle. So it's something that can easily be noted early on without anybody being any the wiser. Later on, when you need to reveal the inheritance prove, you can let a biologist, doctor, or other knowledgeable person make the connection between the lactase persistence and the parentage of the child.

But, of course, the colorblindness trait would definitely work due to being inherited via the X chromosome (which makes it useful for exactly your case, even though it's recessive). The only problem with that is that it's so rare that it looks odd for a character to have without a reason.

• Funnily enough, the colourblindness trait came to mind because I actually have it - I'm a colourblind woman. My father is also colourblind and I can only assume my mother carries the recessive gene. My brother, ironically, has normal colour vision. – Tania Walker Oct 11 '17 at 0:17
• Not quite. If the child has it, ONLY the father could have it. – Justin Thyme Oct 11 '17 at 16:09

Something like a Mallen streak... with both parent and child initially being so thoroughly dirty that they can't see each other's hair colour oddity... or both wearing headgear that covers it... or it being considered a mark of something bad so it's normally masked by dye and gradually grows out in the child.

Or as in GoT... a blond child produced from a family that only ever produced black-haired children.

Most of the really interesting genetic traits are "X-masked" so they don't usually show up in female children because they have a healthy copy of the gene from their mother. There are a couple of unusual traits that could be diagnostic, for example my wife is a carrier for an odd mutation for blue eyes which instead of being pale blue are extremely dark, this blue mutation is recessive to brown like the normal pale blue/grey eye mutation but dominant when mixed with the normal mutation, it is also very rare constituting only 2% of all blue-eyed individuals, there are similar "minority recessive" mutations for green and hazel eyes. Any child that inherited the gene for such a mutation would get the pheno- as well as geno-type, provided mum is a carrier of the "normal" mutation, and there would be a very small donor pool to choose from when playing "who's the daddy".

The father and daughter could share a trait such as heterochromia, a visible (but not debilitating or obvious), rare condition which can be inherited.

A genetic mutation on a dominant autosomal gene, that started with the father, and was passed on to the daughter. NO (as in zero) chance the other male would have it.

However, in some cases an autosomal dominant disorder results from a new (de novo) mutation that occurs during the formation of egg or sperm cells or early in embryonic development. In these cases, the child's parents are unaffected, but the child may pass on the condition to his or her own children (illustration).

If a genetic disorder runs in my family, what are the chances that my children will have the condition?

Another trait would be six fingers

So why if there are many dominant versions of genes that make six fingers is having six fingers rare? Well, those versions of genes are rare. You don’t meet many people with extra fingers or toes.

NOT having six fingers if the father has six fingers is not a good indicator, but having six fingers when the father has six fingers is a very good indicator. .

• This is believed to be the way blue eyes appeared in humans: a one-time mutation that happened somewhere near the Black Sea that was eventually spread. A unique eye colour is something that's unlikely to cause a Eureka Moment of sudden realization after they've known each other for a while since people would tend to notice that sort of thing right away. On the other hand, a trait like something that makes Blaschko's Lines visible on some part of the body normally covered by clothing would work. – Keith Morrison Oct 11 '17 at 15:50

... , so I'm wondering if there's anything like a birthmark - a medical condition? A distinct but subtle physical feature?

Other answers already suggested a number of genetically inherited conditions/mutations/diseases. My suggestion is to go into the opposite direction and use a hereditary resistance to a particular infectious disease.

Pros:

This trait is:

• realistic (Disease Resistance May Be Genetic);
• incredibly subtle ( there is no way to tell if someone is resistant to infection before the next outbreak);
• doesn't require any particular technology to check it.

Cons:

• can't be identified visually;
• to successfully and undoubtedly identify someone this trait should be incredibly rare, to the point of being unique.

... the protagonist realizes partway through that a child he's rescued is biologically his own.

Here is the scenario. The protagonist has an unusually strong resistance to a highly contagious infection. During the last outbreak, he was the only one who survived in his town/city/village/cave/commune. As the only one who could safely travel to plagued cities, he was sent to pass messages/resources to and from quarantined safe-zones.

The devastating epidemic occasionally ran out of resources, stopped and didn't occur again for 15-20 years. Now, if during the next outbreak the protagonist's daughter is the only other person who displays increased resistance, then this is a good reason to assume paternity.

Even if you can't directly measure DNA, you still can observe some characteristics (like hair color, eye color, some types of colourblindness... etc). These are the phenotype.

How this works is quite complicated, but I will use a simplified version to explain it. Keep in mind it is only vulgarisation.

For colourblindness there are two possible genes. One more prone to colorblindness (let's note it CB) or one that makes people able to see colours (let's note it NCB). People have two genes (one from their mother, one from their father), so basically one person can have either (CB,CB) , (NCB,NCB) or (CB,NCB) as a genome. A person will be able to see color with (NCB,NCB) or (CB,NCB), but people with (CB,CB) won't.

So with this example one can test if someone has a (CB,CB) genome or not. With that known you can build situations where it is impossible for the other man (male A) to be the father. For example Male A and the mother could be both colourblind (so with a (CB,CB) genome) and the child not colourblind (which would be impossible if they were his parents since none of his parents has a NCB in their genome).

You can replace "colourblind" with any recessive trait to get the same scenario (@KPrice gave a list in his answer), and make the surprise come from anywhere (Male B could discover that the mother has the trait, that Male A has the trait, or that the child doesn't have the trait).

Of course this won't prove that Male B is the real father by its own, but it proves that Male A isn't, which is what you were looking for.

Consider a society where some people are constantly monitored, and physical encounters can be looked up in people's logs.In a post-apocalyptic world records could be logged manually for some key people.

If the mother has interacted with only one man during impregnating ovulation, then there can only be one possible father.

Also consider preterm birth, which may offset the period to search through.

• Welcome to the site, MrMartin. Please note that the OP's world is post-apocalyptic, so constant monitoring is unlikely to be available. Additionally, answers are expected to answer the question that was asked, rather than introduce new aspects of the world. As a result, this may be deleted unless an edit is made to make this post more fitting to the question. Feel free to take the tour to get a better understanding of the site. – Frostfyre Oct 9 '17 at 13:03

You could find some way to reveal something about the girl's past to make it obvious that he is her father, but other than that idk

Humans are actually in the process of developing a 4th color gene. In the post apocalyptic world this genetic mutation has been accelerated causing the gene to be fully developed. With the fourth color gene only people with this can see a certain color beyond normal ranges of 3 color genes. A test of showing the color to the father and the offspring would prove its his if the other father cannot see.