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Old 05-22-2019, 08:38 PM
Wesley Clark is offline
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The theory that parents dump all their genetic defects on one child who dies, is there a name for it


I read about this theory 15 plus years ago. I do not remember the details beyond the idea that all of the defective genes are somehow dumped on one child who is expected to die since they carry all these shoddy genetics. Almost like a sacrificial lamb to purge the defective genes we all carry.

I'm not sure if the body has that ability to somehow select poor genes to pass on, and the process reminded me of that scene in Twins when the doctor said 'all the leftover shit went into you' to Danny DeVito.
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Old 05-22-2019, 08:42 PM
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That sounds ridiculous to me. Genetic expression in one individual has no effect on the genetic expression in a sibling.
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Old 05-22-2019, 08:58 PM
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It is essentially the Gambler’s Fallacy in a different guise.
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Old 05-22-2019, 10:19 PM
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It sounds like an old wives' tale, perhaps derived from a misconception that this is what causes runts in animal litters?

When a parent "passes on" half its genes to an offspring, that means half the genes are copied, they are not extracted and removed from the parent. All the genes are still available to be passed on to any subsequent offspring. The statistical term is sampling with replacement. It's not like dealing hands from a pack of cards, where a card dealt to one hand is not available to be dealt to other hands. Non-identical siblings get independent random samples from their parents' genes.

In any event, our individual bodies/cells do not "know" which alleles (gene variants) are defective. Knowledge about the fitness of alleles accrues only at the population level in the total frequency of alleles in the population.

The only thing I can think of that's remotely similar to what you're describing is genetic purging due to inbreeding. We carry two copies of most genes (one from each parent), and for many genes a single functional copy is sufficient. So, under normal circumstances, uncommon deleterious alleles may be "hidden" from purifying selection, since they will usually occur in the presence of another good copy. But extensive inbreeding means greater homozygosity, two identical alleles present in the same individual. Inbreeding means that the two copies are more likely to be identical by descent, i.e. two copies of an allele that originated in a single ancestor may be passed through two lines via both mother and father to come together in one afflicted individual. That does not mean that viable inbred offspring are themselves likely to be healthier. What it means is that inbred offspring are more likely to be non-viable, or to die without offspring, removing deleterious alleles from the population. So the frequency of deleterious alleles decreases in the population as a whole.

Last edited by Riemann; 05-22-2019 at 10:22 PM.
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Old 05-22-2019, 11:10 PM
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Not directly related but there is a term in genetics called aniticipation with genetic disorders that manifest an an earlier age with each successive generation:

https://en.m.wikipedia.org/wiki/Anticipation_(genetics)
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Old 05-23-2019, 12:22 AM
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The theory is stupid, of course because the combination process is governed by chance and not by a master plan, but children with serious birth defects can have several or many defects.

I don’t know the reason, although I guess that if something has caused some damage to the chromosomes, it can affect multiple things.
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Old 05-23-2019, 12:44 AM
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There is not a hint of science in that kind of theory. Genes are not things we give out and then they are gone from our body. All the gametes you ever produce are drawing their chromosomes from a common source (your DNA).

Last edited by Isamu; 05-23-2019 at 12:44 AM.
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Old 05-23-2019, 04:39 AM
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Oops

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Old 05-23-2019, 10:00 AM
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The OP may be vaguely remembering the mutational deterministic hypothesis, which has been put forth as an explanation of why sexual reproduction might have arisen from asexually reproducing ancestors. The theory is that most deleterious mutations are only slighty deleterious, but that when multiple deleterious mutations are present, they act together to lower fitness to a greater degree than simply additive or multiplicative effects ("synergistic epistasis"). As a bad example: there are four mutations, each of which lowers overall fitness by 1%. Under one model (multiplicative effect), the result of having all three mutations would be to lower fitness to (0.99x0.99x0.99x0.99) = 0.96 that of the ancestor. Under the synergistic hypothesis, fitness falls off much faster, so (making up numbers) one mutation gives 0.99 fitness, while two give 0.90, three give 0.75, and four give 0.3.

The point of this is that it acts like a filter to aggressively cull deleterious mutations from the gene pool. All offspring will carry a couple of deleterious mutations, but if one happens to catch more than some threshhold number, that offspring will have a massive drop in fitness and won't survive. Only the offspring with the smallest number of bad mutations will live to reproduce. Of course, they may still pass their 2 or 3 bad mutations on to the next generation, but nobody lives to pass on 7 or 8. It's not that the bad genes are all shunted to one child by some supernatural mechanism; it's that the children that by bad luck get 2 or 3 more mutations than their siblings drop like flies.

The problem with this theory is that empirical support is lacking. Experiments with bacteria suggest that multiple mutations combine multiplicatively like my first example, or even that deleterious mutations actually mask one another, so that fitness falls off less and less with each additional mutation (antagonistic epistasis). Richard Lenski, of the longterm E. coli evolution experiment fame, tested this in E. coli: Test of synergistic interactions among deleterious mutations in bacteria. Another good article here:Evolution Can Favor Antagonistic Epistasis.

As for the mutations being shunted to one offspring: no, that doesn't happen. The Law of Independent Assortment came from Gregor Mendel and his pea experiments, and it says that different genes get sorted into gametes independently of one another. In the real world this isn't strictly true, because the closer two alleles lie on a chromosome the more likely they are to both end up in the same gamete - this is genetic linkage. So, if by some remote chance all of one parent's deleterious mutations were clustered right next to each other on a single chromosome, that parent would tend to make gametes that either had most of those mutants or almost none of them. If you have mutations on separate chromosomes, though, they will certainly sort independently.
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Old 05-23-2019, 06:45 PM
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I thought that the reason for sexual reproduction (or something like it) is to enable beneficial mutations to spread more easily through a population.

Imagine that you've got some species that's well-adapted for its environment... and then the climate changes, and its environment is now hotter and drier. Well, out of the many offspring of the many individuals of that species, one might mutate a trait that lets it shed heat more efficiently, and that's good. And out of that many, another one might mutate a trait that lets it retain water more efficiently, and that's good. The descendants of both of those mutants will tend to outcompete their unmutated brethren, but if there's no sexual reproduction, then eventually those two clans will be competing against each other, and one will inevitably lose out, and you'll have a whole bunch of potentially-useful evolution going to waste. But if the two clans can mate with each other, then their offspring can have both of the useful new traits, and outcompete both the non-mutants and the single-mutation individuals.
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Old 05-23-2019, 07:36 PM
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My sister just finished a round of genetic testing, and I'm getting ready for a round of my own. There are plenty of bad genes to pass around the entire family.
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Old 05-23-2019, 09:21 PM
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You can't do it that way. To make it work you have to separate a large group into small groups and use inbreeding. That leads to offspring in which the recessives match up and kill the organism before it has a chance to reproduce. Once a few generations have gone by and the non-survivable traits have been removed you combine the separated groups and take advantage of hybrid vigor. It's why in a test of survival mutts win out over AKC registered dogs.
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Old 05-23-2019, 10:05 PM
Wesley Clark is offline
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Quote:
Originally Posted by brossa View Post
The OP may be vaguely remembering the mutational deterministic hypothesis, which has been put forth as an explanation of why sexual reproduction might have arisen from asexually reproducing ancestors. The theory is that most deleterious mutations are only slighty deleterious, but that when multiple deleterious mutations are present, they act together to lower fitness to a greater degree than simply additive or multiplicative effects ("synergistic epistasis"). As a bad example: there are four mutations, each of which lowers overall fitness by 1%. Under one model (multiplicative effect), the result of having all three mutations would be to lower fitness to (0.99x0.99x0.99x0.99) = 0.96 that of the ancestor. Under the synergistic hypothesis, fitness falls off much faster, so (making up numbers) one mutation gives 0.99 fitness, while two give 0.90, three give 0.75, and four give 0.3.

The point of this is that it acts like a filter to aggressively cull deleterious mutations from the gene pool. All offspring will carry a couple of deleterious mutations, but if one happens to catch more than some threshhold number, that offspring will have a massive drop in fitness and won't survive. Only the offspring with the smallest number of bad mutations will live to reproduce. Of course, they may still pass their 2 or 3 bad mutations on to the next generation, but nobody lives to pass on 7 or 8. It's not that the bad genes are all shunted to one child by some supernatural mechanism; it's that the children that by bad luck get 2 or 3 more mutations than their siblings drop like flies.

The problem with this theory is that empirical support is lacking. Experiments with bacteria suggest that multiple mutations combine multiplicatively like my first example, or even that deleterious mutations actually mask one another, so that fitness falls off less and less with each additional mutation (antagonistic epistasis). Richard Lenski, of the longterm E. coli evolution experiment fame, tested this in E. coli: Test of synergistic interactions among deleterious mutations in bacteria. Another good article here:Evolution Can Favor Antagonistic Epistasis.

As for the mutations being shunted to one offspring: no, that doesn't happen. The Law of Independent Assortment came from Gregor Mendel and his pea experiments, and it says that different genes get sorted into gametes independently of one another. In the real world this isn't strictly true, because the closer two alleles lie on a chromosome the more likely they are to both end up in the same gamete - this is genetic linkage. So, if by some remote chance all of one parent's deleterious mutations were clustered right next to each other on a single chromosome, that parent would tend to make gametes that either had most of those mutants or almost none of them. If you have mutations on separate chromosomes, though, they will certainly sort independently.
You might be right. All I remember with clarity was skimming a scientific paper 15-20 years ago but I don't remember the details of what I read. However what you're writing sounds closer to what I remember.

It may have been about why sexual reproduction evolved in the first place. Thank you for the reply.
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Last edited by Wesley Clark; 05-23-2019 at 10:05 PM.
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Old 05-24-2019, 04:53 PM
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The other old wive's tale may be the "first pancake" analogy. You always screw up your first kid, and the succeeding ones come out better. But that isn't genetic.
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Old 05-24-2019, 07:35 PM
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Think of the coordination! The testicles have to choose a sacrificial sperm to load up with all the bad character traits plus a turbo tail. The ovaries have to set up a sacrificial egg that all the other eggs give their bad genes to, and teach it to recognize the sacrificial sperm. Then they gotta tell the parents when to get it on, such that sacrificial sperm gets a chance to meet sacrificial egg. Somehow from within the respective bodies of the parents they must arrange all this.

Is Bluetooth involved?

Last edited by Left Hand of Dorkness; 05-24-2019 at 07:35 PM.
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