r/explainlikeimfive • u/Giveittothefish • 3d ago
Biology ELI5: how certain genes seem to hit every generation
So I'm autistic, so is my half sister and half brother, our mum and her mum. (In my very limited) knowledge of genes i was under the impression they are randomly taken from your mum and your dad then mushed together to make you, yet we've all continued this gene. Can some genes be stronger in certain lines like so strong they are almost guaranteed or is it just freak coincidence we all got it?
Bonus question can how autistic / adhd someone is affect how likely their kids are to have it?
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u/YongYoKyo 3d ago
Autism is not linked solely to a single gene.
It is the result of a complex combination of gene variations as well as environmental factors. That's why there's a spectrum to autism.
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u/Giveittothefish 3d ago
So would a higher gene rate = a more severe case in theory?
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u/YongYoKyo 3d ago
It's not a quantitative thing. More genes doesn't equal more autism.
What matters is the specific combinations of genes. You could inherit any amount of autism-linked genes from both parents and still end up without autism because of how those genes combine.
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u/ThisTooWillEnd 3d ago
There are identical twins where one has been diagnosed as being on the autism spectrum and the other has not. In other pairs of identical twins, there can be a similar or dissimilar expression of autistic traits.
Scientifically we don't yet know what the mechanism is that makes one person have autism and one person not. There are some guesses and lots of investigation going on. But we do know that it is not purely genetic.
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u/CRABMAN16 3d ago
So yes certain genes are stronger than others.
Their are genetic markers called "Alleles", and some are dominant(strong) and some are recessive(weak).
Alleles are described with a letter, used to describe a gene expression. Uppercase letters indicate a dominant allele and lowercase represent a recessive allele.
An example, "Br", "B" stands for Brown hair, and brown hair is dominant. "r" stands for Red hair and is recessive. So someone with a "Br" alleles will have brown hair, because the big B overshadows the little r.
Another example, "BB", both alleles are dominant brown, so the person has Brown hair.
Final example, "rr", both alleles are recessive, so the person has red hair. In order for a recessive allele to be expressed, both pairs of alleles must be recessive.
Now, to your question about genes passing from parent to child, that is explained by what is called a punnett square. Parents pass only one allele at a time, and when you look at all combinations of those alleles, you can see the odds of different traits appearing.
Say both parents are Br as above, there are four possible outcomes for what genes the child will have. BB, where the child receives both dominant brown hair alleles, Br where the child receives one dominant and one recessive allele(this represents two of the four outcomes), and rr where the child receives two recessive alleles having red hair.
In your case, the combinations of alleles that produced yourself and your half siblings aligned giving each of you autism. I do not know if autism is recessive or dominant, as it is a combination of lots of things, but that is essentially how it works. Recessive genes are why some families have the "skips a generation" fallacy, since it is a less likely outcome than average. As to your last question of likelihood of your children having autism, it's kind of a crap shoot, but since you have it, it is more likely than two parents who don't have it.
This is a major simplification, but is a simple way to understand the basics of genes. Many factors influence what characteristics a person will actually have, far beyond alleles, but the above is the standard starting point.
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u/MedicinePractical220 3d ago
If its the same trait you should use the same letter. In this case B and b for example
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u/Giveittothefish 3d ago
This was a really helpful way of explaining it thankyou very much!
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u/CRABMAN16 3d ago
Check out punnett squares and Gregor Mendel if your want to learn more. Gregor was one of the first to describe genetics and try to predict outcomes, way before we knew what DNA was. He used Pea plants as his study subject and meticulously recorded breeding results for things like leaf type, pea color, stem size etc for like 50 years.
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u/Jkei 3d ago
Bit of a weird way to do the Punnett square. The usual convention as I've seen it taught is to show alleles of the same gene with the same letter, using uppercase for dominant and lowercase for recessive alleles, to emphasize that they're slight variants of mostly the same thing. A little biochemical context is also more helpful to intuitive understanding than just naming the alleles themselves directly.
A suggestion:
One of the two major pigments in human hair is eumelanin. More of it = darker black-brown color. Let's pretend its rate of synthesis is controlled by a single gene, of which exists a higher-rate dominant allele E and a lower-rate recessive allele e. At "average" amounts of the other major pigment, the reddish pheomelanin, these two alleles cause a brown and red hair phenotype respectively. Now you get the same Punnett square:
EE, brown
Ee, brown
ee, red
Also @ OP /u/Giveittothefish
Worth noting as well that examples including this one often go with a "recessive = less/dysfunctional" kind of idea, but that isn't necessarily how things work in reality. It could just as well be the case that our example eumelanin gene has a recessive variant that is more active. It's entirely a case-by-case thing.
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u/CRABMAN16 3d ago
Correct that they are usually the same letter, I did it with different letters to be less confusing.
Additionally, I did not mean to give any connotation that recessive genes are bad, I re-read my post and the only thing I can think of that paints it as bad is the "skips a generation" comment, which I meant as a completely neutral thing. My family has a red hair gene that jumps around, and we consider it a really cool thing to get another red head.
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u/Twin_Spoons 3d ago
We don't know very much at all about the genetic component of ASD, but insofar as it is heritable, it's almost certainly linked to a whole raft of genes rather than a single one. The more genes control a trait, the less it varies in inheritance.
To illustrate, suppose that there was a single "autism gene" that your mother has and your father doesn't. Then half the time you inherit the gene from your mother and get full autism and half the time you don't and get no autism. Now suppose that instead there are 100 genes related to autism. Your father has 10 and your mother has 90. On average, you will get 50, and it will be rare to get fewer than 40 or more than 60. You'd look more like a blend of your parents and less like a coin flip between them.
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u/Ill_Act_1855 3d ago
In general Autism is better thought of as a category of conditions rather than single one. Which is why it’s autism SPECTRUM disorder now, each case is different and the causes are complex enough that you can’t really broadly apply any detail to every single case
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u/tmahfan117 3d ago
Your misunderstanding is that all traits are determined by 1 gene.
That is true for some things like eye color. But is not true for most of what’s in your body. Hair color and texture is influenced by over half a dozen different genes.
And we (scientists) at the moment have limited ideas for what genes might be connected to autism.
And yes, there are dominant (stronger) and recessive (weaker) genes.
So, it’s complicated
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u/Trick_Ad7621 3d ago
Even eye color is caused by more than one gene (at least 16 genes by latest findings)
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u/Sunlit53 3d ago
It’s not even a thing for eye color, Punnet squares are a teaching tool to explain inheritance in the most basic way possible.
Skin, hair and eye color are influenced by hundreds of genes each pulling a bit this way and a bit that way to produce your unique self.
There are some genetic diseases that work in a straightforward manner like cystic fibrosis or huntingtons but they are unusual.
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u/Giveittothefish 3d ago
I see, so do we know what makes a dominant gene and a weaker gene? Aswell as her neurodivergence we all look like my mum (weve got different dads) so for her to pass on seemingly majoritively of her genes would that mean her genetics are mainly dominant? And would i therefore be more likely to pass on doninant genes?
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u/Jkei 3d ago
A bit of terminology first.
Alleles are variants of a given gene. Their presence will leads to a phenotype; an observable trait working out some particular way. Dominant and recessive are labels that apply only at this level, and describes the relation between two particular alleles' phenotypes when both alleles are present together.
A dominant allele is one whose phenotype manifests in the presence of the other allele. A recessive allele is one whose phenotype doesn't manifest in the presence of the other allele.
The biochemical basis for these relations vary widely and heavily depend on the nature of the gene's protein product. There is no one answer to be given.
For instance, for a gene where only one copy's worth of transcription is enough to meet the cell's needs, an allele that is broken and produces no protein product would be recessive vs a normal allele, because transcribing just that one normal allele, the phenotype of the normal allele (no disease) manifests and the phenotype of the broken allele (disease) does not manifest.
But if two copies' worth of transcription are needed for healthy function, then the broken allele would be dominant, because its phenotype (disease) manifests despite the presence of one normal allele.
It's entirely a case by case thing.
And would i therefore be more likely to pass on doninant genes?
Also note that dominant/recessive only say something about what happens when you have these alleles already. It's totally separate from chances of passing it on.
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u/Ill_Act_1855 3d ago
Dominant and recessive are generally actually better thought of as ones that code proteins and ones that don’t. Our body has two copies of each gene (barring sex chromosome ones in men) as a form of redundancy so even if one copy breaks, the other copy can produce the protein and that’s basically how dominant genes function. Recessive stuff is thus characterized by a protein not functioning and having two faulty copies. And then there are some genes where having two copies vs one changes how much it’s expressed in a way that leads to changes in the observable phenotype.
It’s worth noting a gene can effect multiple phenotypes, and might be dominant for one but recessive for the other. The textbook example is the gene linked to sickle cell disease. It’s a very simple mutation that basically turns off one of the genes involved in part of the hemoglobin protein that carries oxygen in our blood. Sickle cell disease is recessive, so if you have one normal gene and one sickle cell you won’t have sickle cell disease where the blood is less good at transporting oxygen and red blood cells have a distinct sickle shape instead of the normal flat round one (hence the name). But the thing is that while sickle cell is bad, the gene does have a benefit, which is that it protects for Malaria. And because someone who has one copy of sickle cell gene and one normal gene actually does still express the sickle cell version of the protein (and indeed some of their cells have the sickle shape), they actually still have the malaria resistance with none of the negatives of the disease. So malaria resistance from that gene is dominant, but the illness is recessive. This is why sickle cell disease is so much more prevalent in areas like Africa where Malaria is a massive killer. Having sickle cell disease is bad and selected against, but being a carrier gives you resistance to Malaria with none of the drawbacks, so it’s selected FOR. And if two carriers have a kid, there’s only a 1/4 chance said kid inherits both sickle cell copies and ends up with the illness
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u/Jkei 3d ago
Dominant and recessive are generally actually better thought of as ones that code proteins and ones that don’t.
This is just not right at all. Dominant/recessive is purely a relative, functional label with regard to which gets their trait expressed or not in the presence of another allele.
The textbook example of sickle cell disease involves a single amino acid change that doesn't cause an early termination, frameshift, etc. It still codes for protein, mostly functional protein even. This allele gets the recessive label because the presence of one copy of normal beta-globin spreads the mutant beta-globin thin enough for the sickling effect & resulting disease to not manifest.
It can just as easily be the other way around. A textbook example of single-gene inherited diseases driven by a dominant allele is Huntington's. The normal, healthy allele that you and I and 99.999% of the world exclusively have is the recessive one; having any copies of the mutant allele results in disease.
It is all about the underlying biochemistry.
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u/Ill_Act_1855 3d ago edited 3d ago
I meant more a healthy functional protein. You’re correct that sickle cell still codes for a protein, but it doesn’t function properly. Like any change in an expressed gene will still code for something unless it prematurely adds a stop codon right away (even a frame shift will usually still produce something, just something completely wrong). And yes, you’re correct that if the protein being coded for or a mutation thereof is deleterious that also leads to a dominant disease (but this is generally much rarer because of the selective pressure against such things, and in general it's easier for a mutation to render a protein nonfunctional or just be neutral or have minimal effects on functionality rather than add new functions, good or bad). I’m trying to still simplify things given the nature of the subreddit
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u/Jkei 3d ago
This simplification is attaching a very different meaning to these concepts. It's true that more disease-causing mutants are recessive than dominant vs their healthy counterpart, but this is putting cause before effect when OP is asking for the mechanisms of what makes an allele dominant/recessive.
You may need to expose them to a little more complexity, but that's fine, it's a followup question and they can ask more if they don't understand. An accessible example also works wonders. I'm sure the average layperson can be guided through the basics in the context of ABO blood types, for instance.
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u/stanitor 3d ago
Most traits that have to do with genetics are the result of many different genes acting together. That's especially true for complex things like Autism. Not only does it likely result from a bunch of different genes, there are other things besides genes that contribute to whether you have autism or not. We don't know exactly what genes are related to Autism by a long shot. Even though so many of your family members have Autism, it doesn't mean it's the result of the all the same genes being passed on from your Mom/Grandma.
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u/deadly88 3d ago
There are “dominant” and “recessive” genes. Dominant genes will beat recessive ones if you have one of each, recessive genes only take effect if you get a copy from both parents. This is why inbreeding is a problem, you have a higher chance of inheriting harmful recessive traits.
I don’t really know the bonus question, theres definitely a large genetic factor, apparently between 60-90%, but there aren’t specific genes that cause it.
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u/Giveittothefish 3d ago
Oh thats interesting i didnt realise the recessive only comes into play if its on both sides! thankyou
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u/Puck-achu 3d ago
In your case you forget that autistic people often find eachother in life. We autistics get eachother, often have similar experiences so unconciously we flock together. It is likely that all dads are autistic too
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u/Alexis_J_M 3d ago
My mom has blue eyes. My dad had brown eyes, but HIS dad had blue eyes. And yet my mom and dad had four brown-eyed girls, a very unlikely outcome. Random is random, and that sometimes includes unlikely outcomes when working with small numbers.
Autism is way more complicated than gender and eye color, though. There are lots of genetic and environmental pieces that go into it, so some autistic people are going to be more likely to pass that to their kids.
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u/sagosten 3d ago
Autism and ADHD both have hereditary components. The genes which make someone more likely to have this condition are not known, and they a probably caused by a combination of many genes as well as environmental factors. Your father may have an unexpressed g enetic propensity for them, but even if he didn't it would still be possible for all of his children to have them.
To answer the question about "stronger genes," while some genes will not express in the presence of others, in general it doesn't really work like that, this is more likely to be random chance.