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

Question

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.

Answer 1

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

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

I would suggest webbed toes.

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

http://luken.us/wmluken2/syndactyly/syndactyly6.htm

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

Answer 6

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.

Answer 7

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.

Answer 8

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.

Answer 9

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.

Answer 10

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

Answer 11

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.

Answer 12

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.

Answer 13

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.

Answer 14

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".

Answer 15

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

Answer 16

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. .

Answer 17

... , 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.

Answer 18

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.

Answer 19

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.

Answer 20

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

Answer 21

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.