Should incest be decriminalised?

[Drifterwood]

Isn't the argument that it's OK if it doesn't lead to procreation, ironically inversely the same as is/was used to criminalise homosexuality by earlier morality?

 

[Tattooed Goddess]

This has actually been studied and is known as the Westermarck Effect. Children raised together undergo a sort of reverse psychological imprinting that makes them averse to being sexually attracted to each other later in life...it's like an evolved deterrent to our innate genetic sexual attractions.

Yes, i'd rather call it "He farted on your head Syndrome"

 

[Guy-jin]

I was asked by Rouge to look at this thread because I know about these things. :smile:

What is the evidence that in-breeding causes genetic defects? I know plenty of people with genetic defects born to unrelated people so how do i know that its not a myth and that specific examples have been cherry picked to back up the myth?
Curious.

To expand on what's been said in response to this question, inbreeding is imposing homozygosity on your offspring. Homozygosity means you have lost the presence of two unique copies of the genome in some regions and now have two identical copies. This is deleterious because your fail-safe--the second copy of your genome that typically protects you if your first copy has something wrong with it--is no longer there. You're basically concentrating deleterious variants.

You share 1/8th of your genome with your first cousin. If you and your first cousin have a child, 1/16th of that child's genome has lost heterozygosity and therefore lost its protection. This is why incest, particularly across generations in the same family, results in genetic disease. Great examples in human history can be seen in European royal lineages, where rare genetic diseases and traits are more common than in the population at large. You're going to see increases in miscarriages in these types of joinings because the offspring will often have fatal mutations that are not masked by the second, functional copy of the given genes.

So by this logic Azkenazi jews who may be carriers of Tay-Sachs disease Mount Sinai - Center for Jewish Genetic Diseases - Department of Human Genetics should all be screened and prevented from procreating by law, and people of African descent with sickle cell anemia, or indeed anyone who may be a carrier of a genetic disorder since by definition these people expose their offspring to the increased possibility that they will inherit these diseases.

Well, we do actually do prenatal screening on Ashkenazi Jews, and sometimes decisions regarding procreation are made with foreknowledge of the genetics, particularly in that population. We actually have a panel of genetic tests that are commonly performed on Ashkenazi Jews. We don't call it "eugenics" because of the unfortunate connotations the word carries thanks to the unethical ways it was performed in the distant past, but to be fair, I think the word "eugenics" would actually be an accurate description.

If a couple knows they're likely to have a child with, say, Tay-Sachs, they might have IVF and screen the fertilized offspring for one that doesn't have it.

Totally off topic... but...

Depending on how that fallopian tube was damaged... you have a point, if it was not by birth but say, trauma, then indeed it could be a perfectly healthy baby.
However, the low sperm count is exactly my point... It could be because of various reasons, a too high bodytemperature that kills off sperm cells or just a low production count or... Thing is, this is genetically coded for. And allowing someone to reproduce who, in nature, wouldn't have been able to get offspring, is messing with natural selection.
I'm not saying this is a black/white thing... someone with a low sperm count might just have had a lower chance of getting a child, and might have gotten lucky and naturally procreate. But then this kid has a big chance of being less fertile too, and his/her chance of reproducing might be much lower because of this. This *is* "weakening" the gene pool. (Although that's probably not correctly said, you get the gist of what I mean?)

My cousin works in an infertility clinic, and we've had many discussions about this. But still I am convinced that improving the chance of an egg to be fertilized by concentrating semen near it, or even injecting it in, is essentially on a very basic level wrong.
It's horrible, terrible for a couple not to be able to have kids, and when a solution like that comes along and works and makes them deliriously happy, of course I understand why people would do it.
It concerns me however that no one looks past that, and thinks about the fact that the child, although perfectly healthy, could possibly (a higher risk of) have problems with fertility itsself later on.

First of all, you have presented no evidence that fertility problems are even commonly genetic. My understanding is that fertility issues are very rarely genetic. If they are genetic, it's usually due to a very significant germ-line genetic defect in one of the parents, such as a Robertsonian translocation.

It isn't weakening the species to flex our scientific might. That said, if the fertility problems in question are genetic, once we understand the genetic etiology of the fertility issue, we'll be able to select offspring that do not have that genetic problem down the road. Just as we select offspring that don't have Tay-Sachs today. I imagine there are cases where one parent has a germ-line translocation (a genetic defect that results in infertility) where IVF would be able to solve the issue and not even pass it on to the offspring. That kind of kills your whole thesis.

Regardless of those techincal problems with your argument, if you're going to take the stance that we shouldn't use IVF to solve fertility issues, where do you stop? You could argue that we shouldn't solve any health issues with medicine or surgery because it's "weakening the species".

Look at it this way: In the evolutionary sense, what is our most highly evolved feature? I would argue it's our brain. We used our collective intelligence to come up with all these techniques to solve health issues, including fertility issues. Previously, if you couldn't have offspring due to a genetic defect, you didn't pass on your genes. But nature apparently does a poor job preventing fertility problems (if your theory that fertility issues are genetic is true, and I'm not sure it is) because fertility problems persist to this day. Now, we have ways of solving fertility problems through techniques such as IVF. This basically removes fertility problems from nature's genetic playbook.

You may be thinking, "But what if those fertility problems get worse as we allow those with poor fertility to have offspring?!" First of all, that's unlikely. But my other, more fun answer? We can create embryos from stem cells. We have the technology. In the near future, you won't even necessarily need to combine gametes directly to have offspring. Maybe it sounds like science fiction (a little too Gattaca?), but it's true.

 

[Tattooed Goddess]

Thanks Muchly Guy-Jin! I knew your expertise in this subject would be beneficial.

I would also like to add that in remote areas in Pennsylvania where the Amish are isolated and often marry within a small geneticly diverse pool, that the genetic metobolic disorder called MSUD (Maple Syrup Urine Disease) is an issue for them.

I would like to think that the Westermark Effect is something that is within us to help us pick someone genetically unsimilar for preservation and promotion of the strongest species.

Having been an infertility patient and also getting to know thousands of others on thet internet. There is rarely a genetic cause for infertility amongst the normal population. And there can be a defect within the reproductive tract that is more common in males than females. Because the urinary tract/path is utilized in men to carry sperm outward, there can be a defect in this area that has nothing to do with genetics that can prevent pregnancy from occuring. I have a friend who needed IVF to get pregnant because of retrograde ejaculation her husband had. Even though their chances of pregnancy was pretty much zero naturally, she did IVF and got pregnant and strangely she emailed me a couple of years ago to show me pictures of her new baby boy that was conceived without IVF or any other measures. I guess he did manage to ejaculate without retrograde atleast once.

I also have an Aunt (not related by blood) who married into our family with a genetic history in her family of having severely retarded children. Her sisters each had 2 and she had one. Her one child that is retarded (rumours in the family is that her mom tried to abort her by drinking poisons and thats why she is retarded) also married someone who is retarded and they have a child with many similar handicaps, all the way down to the exact same clubbed feet her mom had when she was born. So that rumor about poisons can't be true because it wouldnt change the genetic pattern on the infant. That infant wouldnt be born with the same congenital birth defects in her legs that her daughter would later be born with if it was a mere abortion gone wrong.

Genetics is simple and complex all at the same time.

 

[mitchymo]

Thank you Guy-Jin for the explanation. And thank you for explaining the big word that i had to read three times and still probably mis-pronounced lol

I will accept the explanation as understood even if i'm never gonna be able to explain it to anyone else. It will suffice that i can say inbreeding is bad with conviction when i do.

 

[Tattooed Goddess]

Mitchy, explain it this way:

A. You need 21 chromosomes from Mom
B. 21 from Dad
C. If Mom and Dad share chromosomal similarities from being related to one another you technically arent getting 42 different chromosomes at all. Some of these chromosomes are duplicates.
D. Which brings your overall amount of unique chromosomes down.
E. In a non-incestuous conception your 21 chromosomes from your Mom are diverse enough to protect against abnormalities from your Dad's 21 chromosomes.
F. If mom and dad shared genetic similarity, you aren't getting a flat out 21 against another 21. You are altering the number of cellular success, as each of these chromosomes have a job- such as creating your eye tissue, your heart tissue, skin tissue, etc.
E. So if your genetic makeup has too many chromosomal duplicates when they are joined together from Mom and Dad, you are more likely to have one of these chromosomal jobs fail. For example you might have a heart defect that goes back to the chromosomes responsible for the cardiovascular system. And that is an imprint in your DNA that will be a risk to your children. The risk will go up immensely when you have a child with someone from your family that will start the genetic/chromosomal duplication process all over again.

You might be getting one cardiac genetic disorder possibility from one parent and 2 times the risk from other now. Now the chances of the same thing happening to the next generation because that child, too, picked someone within their family that carried the gene multiplies. So on and so fourth each generation.

Your genetic possibilities, good and bad, get diluted by half for your kids when you have a child with someone who has an entirely different set of genetics than you do.

 

[mitchymo]

Thank you Rouge. Yes, that is easier to explain than hymogenysesis or whatever it was lol

 

[Tattooed Goddess]

You know, it makes sense in so many ways about how these things work to protect us against abnormalities....

Imagine what kind of children you'd have if you had a child from a set of identical twins. Each twin doesn't bring any unique DNA to the table for the child. But it's also impossible because identical twins are the same gender and unable to reproduce anyway.

 

[mitchymo]

Do you think that possibly a gay gene is in place to stop the birth of Beelzebub? :biggrin1: I did think that perhaps homosexuality was natures way of ending a redundant genetic line.

 

[galaxus]

If it is two consenting adults, where there is no power dynamic, mental illness, or pedopheilia (which falls into WAY inappropriate)...then I couldn't care what people do in their personal life.

I think you start getting into trouble when one person is forcing another to do something they wouldn't otherwise do.

Why do you ask Mitchy?

Very Correct!

 

[THEDUDEofDestiny]

Following on from another thread. Now we are in the 21st century do you think that the illegal status of incest should be removed?
If you think its not so simple as a yes or no then feel free to elaborate. I want to fuck my family members real bad.

hahaha i edited your post

 

[mitchymo]

hahaha i'm a dim joker that spins things to amuse myself

oh LOOK, this is fun.....

 

[D_Andreas Sukov]

oh LOOK, this is fun.....

HAHHAHA:biggrin1:


This Bitch has claws...:wink::smile:

 

[mitchymo]

HAHHAHA:biggrin1:


This Mitch has cause...:wink::smile:

We should start a thread in the funny stuff section with this editing technique, it could be real amusing. :smile:

 

[lefebvre254]

In a word: No.

Same reason as Mademoiselle Rouge listed, the chance of things going wrong, genetically, if they have a child, is high. Plus, I'd never think of a family member in that way, that's just weird.

 

[dandelion]

To expand on what's been said in response to this question, inbreeding is imposing homozygosity on your offspring. Homozygosity means you have lost the presence of two unique copies of the genome in some regions and now have two identical copies. This is deleterious because your fail-safe--the second copy of your genome that typically protects you if your first copy has something wrong with it--is no longer there. You're basically concentrating deleterious variants.

You ignore the possibility that having two identical copies of one gene may be beneficial. An example I vaguely recall was hearing of a gene which grants immunity to HIV...just so long as you have two copies. Otherwise you only have enhanced resistance. In the event of an epidemic, those with two correct identical copies will be the only survivors. Yes, genetic diversity is a guarantee against unexpected changes affecting a population, but that diversity includes having individuals with double copies of genese. It is diversity within the population as a whole which is important and needs to be maintained. The great issue with using scientific techniques to choose not to have children with certain genomes is that our knowledge is imperfect and we do not know what advantages might be assiciated with apparent disadvantages. For example, sickle cell anaemia gives resistance to malaria.

You share 1/8th of your genome with your first cousin. If you and your first cousin have a child, 1/16th of that child's genome has lost heterozygosity and therefore lost its protection. This is why incest, particularly across generations in the same family, results in genetic disease.

Actually, you dont. A child inherits half its genes from each parent. Thus a second child might inherit the exact opposite half from each parent, and be 100% different genetically from its sibling. I thing I recall that in fact most genes are identical in all pairs, or come from a very small selection, so the actual genetic similarity will be greater than this, even though in fact the two have inherited completely different sets of genes. Only on average do siblings, or cousins for that matter, share thiese proportions of genes. In the case of cousins, 50% of their genetic material is absolutely guaranteed to have come from the non-related parent and will not be shared except by its general frequency in the population (which as I said, is quite high). As to the remaining 50% which came from their related parent, even if the parents were identical twins, their children could inherit entirely separate halves of the genome. Or identical halves. So the possible range is from 100% identical inheritance to 0% identical, of that 50%. The expected value of 1/16 is simply a probability subject to variation within the possible range.

As to the harm resulting from this, I refer you back to the last answer: there is genetic advantage in doubling up useful recessives as well as harm in doubling up bad ones. The real isssue is probably not the overall genetic inheritance, but just a relatively few rare recessive genes which can cause a problem if someone inherits two. In such a case, I think there is a 1/4 chance of inheriting a pair of the bad genes if two of the offspring mate. But if the occurence in the natural population of the bad gene is 1/50,000, say, then the chance of a badly affected child is 1/200,000.

I would also point out that evolution works by the random creation of new gene variations. A brand new recessive only has an effect if it gets into the wider population. Mating children makes it much more likely that a new recessive will exist as a pair and have a chance to work and spread. Mating children therefore has a long term benefit in building up new good traits.

Great examples in human history can be seen in European royal lineages, where rare genetic diseases and traits are more common than in the population at large.

But this is deliberate inbreeding generation after generation, not the odd sibling marriage which is all that is remotely likely if people are left to choose.

First of all, you have presented no evidence that fertility problems are even commonly genetic. My understanding is that fertility issues are very rarely genetic. If they are genetic, it's usually due to a very significant germ-line genetic defect in one of the parents, such as a Robertsonian translocation.

I find this a difficult assertion. Ok, maybe it is unusual that, say someone inherits a gene causing sperm ducts to be twisted. But what are the other causes of infertitlity? If you are more susceptible to mumps, you may end up infertile? Where do you draw the line? Maybe it was only because that male happened to get mumps he has a very low sperm count and any technology assisted children wont inherit the infertility, but they will still inherit the disadvantage if being susceptible to mumps.

It isn't weakening the species to flex our scientific might.

In fact it is an evolutionary adaptation to a changed environment. Differnt things matter. But if for some reason conditions changed back (the oil runs out?), then it will matter. Are we certain our technological gains are permanent?

 

[Gecko4lif]

It should only be allowed in porn. Otherwise it's disgusting.

I find myself agreeing

 

[dandelion]

Mitchy, explain it this way:

A. You need 21 chromosomes from Mom
B. 21 from Dad
C. If Mom and Dad share chromosomal similarities from being related to one another you technically arent getting 42 different chromosomes at all. Some of these chromosomes are duplicates.
D. Which brings your overall amount of unique chromosomes down.
E. In a non-incestuous conception your 21 chromosomes from your Mom are diverse enough to protect against abnormalities from your Dad's 21 chromosomes.
F. If mom and dad shared genetic similarity, you aren't getting a flat out 21 against another 21. You are altering the number of cellular success, as each of these chromosomes have a job- such as creating your eye tissue, your heart tissue, skin tissue, etc.
E. So if your genetic makeup has too many chromosomal duplicates when they are joined together from Mom and Dad, you are more likely to have one of these chromosomal jobs fail. For example you might have a heart defect that goes back to the chromosomes responsible for the cardiovascular system. And that is an imprint in your DNA that will be a risk to your children. The risk will go up immensely when you have a child with someone from your family that will start the genetic/chromosomal duplication process all over again.

Current estimates are that chimpanzees have 95% the same dna as humans. Also, quite possibly most DNA is junk and does nothing, so we dont much care if its the same or not. Wikipedia says two random humans share identical 99.5% DNA.

You inherit 21 chromosomes, but each chromosome is a collection of genes which may differ one by one. In fact although each parent has two copies of each chromosome, it is possible for the two chains to swap parts with each other so you inherit part of one and part of the other. Parts of chromosomes are responsible for different things and more than one part may play some part in the same thing. I presume that parts of different chromosomes similarly interact.

The variation in your chromosomes is not simply a question of having the same parents. Most humans historically have lived in very small communities with little interchange, so there is enormous inbreeding. The example given earlier was a particular jewish sect. My mothers family comes from a small town in England where we have traced generation upon generation of big families with the children all marrying each other. The effect of interbreeding is to throw the genetic dice. Individuals with bad traits do not have offspring, so those combinations tend to die out of the population and become rare. Thus it is unlikely that any random pairing even within an inbred population will contain a bad trait, so even if the immediate offspring interbreed the chance of a bad combination is small.

You might be getting one cardiac genetic disorder possibility from one parent and 2 times the risk from other now. Now the chances of the same thing happening to the next generation because that child, too, picked someone within their family that carried the gene multiplies. So on and so fourth each generation.

Only in the presence of modern medicine! Without it you either have a good heart and have children, or a bad heart and die. The chance of inheriting two recessive bad copies of a gene is 1/4. So 1/4 children is sick but just as many are completely free of the defect, while 2/4 have a gene as did their parent but are unaffected. The 1/4 sick dies and has no more children. that leaves 1/3 completely healthy and 2/3 carrying a gene. In the parents it was 2/2 carrying the gene.ie the situation has improved in 1 generation!

Thats how evolution works. bad traits disappear. It works. Seee, there are people around you: we survived and did not get cumulative death from bad hearts. The most clear place you see inherited genetic disorders is where people have inbred animals for particular traits. The reason you see the bad traits is because we favour particular traits we want and accept bad ones which come along for the ride. This is still evolution in action: dogs with geneticly deformed legs survive because we keep feeding them and they dont have to hunt for themselves. They are adapted to an environment where hunting is no longer necessary.

 

[Guy-jin]

First of all, to be accurate, you inherit 23 chromosomes from each parent. It's a detail, but don't forget the sex chromosomes. (To be completely accurate, actually, you get 24 from mom--the mitochondrial DNA counts!)

Thank you Guy-Jin for the explanation. And thank you for explaining the big word that i had to read three times and still probably mis-pronounced lol

I will accept the explanation as understood even if i'm never gonna be able to explain it to anyone else. It will suffice that i can say inbreeding is bad with conviction when i do.

No problemo, dude. Sorry if I used "big words", haha. Hope you learned something (and Rouge did a nice job explaining too... just make sure you say 23 chromosomes!). But yeah, basically there is a genetic reason that inbreeding is bad.

You ignore the possibility that having two identical copies of one gene may be beneficial. An example I vaguely recall was hearing of a gene which grants immunity to HIV...just so long as you have two copies. Otherwise you only have enhanced resistance. In the event of an epidemic, those with two correct identical copies will be the only survivors. Yes, genetic diversity is a guarantee against unexpected changes affecting a population, but that diversity includes having individuals with double copies of genese. It is diversity within the population as a whole which is important and needs to be maintained. The great issue with using scientific techniques to choose not to have children with certain genomes is that our knowledge is imperfect and we do not know what advantages might be assiciated with apparent disadvantages. For example, sickle cell anaemia gives resistance to malaria.

I certainly do not ignore that. As I described, royal families often have rare traits that aren't necessarily deleterious because of inbreeding. However, the chances of losing a protective variant increase as you lose heterozygosity in your genome due to inbreeding. It's basically Russian roulette every generation that you inbreed. To mention an example from earlier, the many positive features that we enjoy in specific purebreed dog breeds almost all come with negative features as well--breed-specific diseases, blindness, deafness, arthritis, et cetera. Meanwhile, mixed breeds are often immune to the problems associated with the breeds they were derived from. They also end up looking different from their parents thanks to losing the same homozygosity that would have given them said diseases. in the end, mixed breeds are on the whole less typically affected by genetic diseases, but may not be as "pretty" as purebreeds. However, in this case, God does indeed play dice. But they're loaded: Eventually too many deleterious mutations will stack up if inbreeding occurs that will outweigh any positive effects in favor of disease, infertility or mortality.

As far as your example of the sickle cell trait, it is certainly not a good thing to have sickle cell anemia! Having two copies of the sickle cell gene is highly deleterious due to the severity of that form of anemia. Sickle cell is the classic case of what we call "heterozygous advantage". People who have one copy of the sickle cell gene are both immune to malaria and do not have sickle cell anemia. Half of their blood sickles while the other half maintains the usual doughnut shape. In the case of traits like sickle cell, there is a selection pressure for people to be heterozygous (that is, half one copy of the sickle cell gene). This results in a higher proportion of the population having two copies and ending up with sickle cell anemia, of course, but there is also the selection pressure on those without the sickle cell trait in the form of malaria. Bottom line is that the selection is not for homozygosity, but for heterozygosity.

Actually, you dont. A child inherits half its genes from each parent. Thus a second child might inherit the exact opposite half from each parent, and be 100% different genetically from its sibling. I thing I recall that in fact most genes are identical in all pairs, or come from a very small selection, so the actual genetic similarity will be greater than this, even though in fact the two have inherited completely different sets of genes. Only on average do siblings, or cousins for that matter, share thiese proportions of genes. In the case of cousins, 50% of their genetic material is absolutely guaranteed to have come from the non-related parent and will not be shared except by its general frequency in the population (which as I said, is quite high). As to the remaining 50% which came from their related parent, even if the parents were identical twins, their children could inherit entirely separate halves of the genome. Or identical halves. So the possible range is from 100% identical inheritance to 0% identical, of that 50%. The expected value of 1/16 is simply a probability subject to variation within the possible range.

I'm sorry, but you're wrong. While it may seem logical to you that it's possible you won't share around 50% of your DNA with your sibling, you do. You get enough genomic material randomly distributed from each parent that it centers right at around 50%. The only type of siblings that share 100% of their DNA are identical twins. To share much greater or much less than 50% of your DNA with your sibling is extremely rare. So rare, in fact, that for the purposes of this discussion, they're not worth mentioning.

As to the harm resulting from this, I refer you back to the last answer: there is genetic advantage in doubling up useful recessives as well as harm in doubling up bad ones. The real isssue is probably not the overall genetic inheritance, but just a relatively few rare recessive genes which can cause a problem if someone inherits two. In such a case, I think there is a 1/4 chance of inheriting a pair of the bad genes if two of the offspring mate. But if the occurence in the natural population of the bad gene is 1/50,000, say, then the chance of a badly affected child is 1/200,000.

You're highly overstating the occurrence of these "useful recessives".

I'm going to just refer you to a textbook or two to read up on these things because you're making a lot of uninformed statements and you might want to brush up: Thompson & Thompson Genetics in Medicine. Human Molecular Genetics 3.

I would also point out that evolution works by the random creation of new gene variations. A brand new recessive only has an effect if it gets into the wider population. Mating children makes it much more likely that a new recessive will exist as a pair and have a chance to work and spread. Mating children therefore has a long term benefit in building up new good traits.

Actually, no. The vast majority of evolution is caused by variations that are already present in the genome being selected for or against, not by random mutation. New alleles are created by random mutation, but very rarely do they result in a survival advantage. You're thinking of speciation in particular, but even that it appears the majority of variation is due to founder effects and genetic drift, not random mutation events. Check out the textbook Evolution by Futuyma for more on that (seriously).

But this is deliberate inbreeding generation after generation, not the odd sibling marriage which is all that is remotely likely if people are left to choose.

It could happen after one generation. Very possible. The offspring of a sibling-sibling mating has 25% homozygosity. In other words, it has lost 25% of its protection against deleterious inherited alleles. Russian roulette. Have four children and one of them is very likely to die or have a genetic disease. In fact, there's a good chance you'll end up with a number of miscarriages before you even have a child in that joining.

I find this a difficult assertion. Ok, maybe it is unusual that, say someone inherits a gene causing sperm ducts to be twisted. But what are the other causes of infertitlity? If you are more susceptible to mumps, you may end up infertile? Where do you draw the line? Maybe it was only because that male happened to get mumps he has a very low sperm count and any technology assisted children wont inherit the infertility, but they will still inherit the disadvantage if being susceptible to mumps.

My understanding, which is fairly extensive given my field, is that the vast majority are environmental: STDs, other infections, dietary, physical abnormalities of a non-genetic origin.

In fact it is an evolutionary adaptation to a changed environment. Differnt things matter. But if for some reason conditions changed back (the oil runs out?), then it will matter. Are we certain our technological gains are permanent?

Does it matter? It's all natural selection. If we fail as a species technologically, we're going to have a lot more problems than a couple of infertile people walking around. Selection is currently not pressuring those with many deleterious variations in their genome--in a post-apocalyptic world, the pendulum swings back and selection will benefit the strong again. I'd be more concerned with where you're going to get fresh water in that world if I were you. :tongue:

 

[dandelion]

I certainly do not ignore that. As I described, royal families often have rare traits that aren't necessarily deleterious because of inbreeding. However, the chances of losing a protective variant increase as you lose heterozygosity in your genome due to inbreeding.

In the context of a discussion about incest we are really only talking about the inbred children, not the population as a whole. In the context of an individual, there can only be two possible versions of a gene, not the very many in a wider population. I presume that law or no law, incest would remain rare. It doesnt really matter whether in one set of children amongst the entire population we bias the resulting genome one way or another. The only medical objection raised to the cross is whether they are more likely to inherit something which will immediately affect their own health. But I would also suggest that if one pairing is chosen of people with particularly close genetics, the remainder of the population still contains all the other partners these two might have chosen. No genetic variability has been lost in the overall population. Indeed, a close cross may lead to the next generation containing an unusual pairing of recessives, thus increasing the variability in that generation.

the many positive features that we enjoy in specific purebreed dog breeds almost all come with negative features as well--breed-specific diseases, blindness, deafness, arthritis, et cetera. Meanwhile, mixed breeds are often immune to the problems associated with the breeds they were derived from. They also end up looking different from their parents thanks to losing the same homozygosity that would have given them said diseases. in the end, mixed breeds are on the whole less typically affected by genetic diseases, but may not be as "pretty" as purebreeds. However, in this case, God does indeed play dice. But they're loaded: Eventually too many deleterious mutations will stack up if inbreeding occurs that will outweigh any positive effects in favor of disease, infertility or mortality.

No.

If you want examples of good inbreeding look at most animals we breed. Those with two many deleterious mutations are not represented in the population: theyre dead. You will find far fewer bad genes amongst farm animals than pets: Farm animals purpose is basically to be eaten, so the cull rate compared to the breeding rate is much greater. The problem with pets is that there is a financial benefit to their owners to keep breeding them despite genetic defects. The owners are actually selecting bad traits to breed, because they have a financial investment in the particular animals they own.

I refer you to the example of inbreeding I outlined, which you did not comment on. 1/4 offspring has a fatal pairing of recessives, and dies. 2/4 has a mix. 1/4 has homozygous good genes. The result is whereas the initial population was entirely 1 bad gene paired with 1 good, the next generation has 2/3 with 1 bad gene and 1/3 with 2 good genes. In other words, making a cross between parents with 1 bad and 1 good gene actually improves the position in the surviving offspring. The overall situation improves in the offspring if you cross recessive bad genes.

As far as your example of the sickle cell trait, it is certainly not a good thing to have sickle cell anemia!

Unless you live in a malaria infested area, in which case you live whereas your unaffected compatriots die.

Having two copies of the sickle cell gene is highly deleterious due to the severity of that form of anemia. Sickle cell is the classic case of what we call "heterozygous advantage". People who have one copy of the sickle cell gene are both immune to malaria and do not have sickle cell anemia. Half of their blood sickles while the other half maintains the usual doughnut shape. In the case of traits like sickle cell, there is a selection pressure for people to be heterozygous (that is, half one copy of the sickle cell gene). This results in a higher proportion of the population having two copies and ending up with sickle cell anemia, of course, but there is also the selection pressure on those without the sickle cell trait in the form of malaria. Bottom line is that the selection is not for homozygosity, but for heterozygosity.

But that is because the population as a whole is more healthy if the sickle cell trait persists in the population. If the stimulus disappears, ie we cure malaria some other way, then the result of continuing crosses will be that the sickle cell trait dies out from the population. The result of breeding is always to produce more of the most successful offspring in each generation...by the mechanism that the disadvantaged ones fail to breed. In the case of sickle cell anaemia, we very probably are interrupting the natural mechanism which would get rid of the trait from the population, because we treat and keep healthy those with the disease, in an analogous way to which we have used medicine to get rid of the malaria. This is not an example of genetic selection in action, but of medical intervention in action. If we chose to treat malaria but not sickle cell, then sickle cell would eventually cure itself by becoming increasingly rare. That may not be considered ethical now, but it is the way the genetics works.

I'm sorry, but you're wrong. While it may seem logical to you that it's possible you won't share around 50% of your DNA with your sibling, you do. You get enough genomic material randomly distributed from each parent that it centers right at around 50%. The only type of siblings that share 100% of their DNA are identical twins.

of course, I have forgotten most of my statistics. The two statements are correct, it centres at 50% and only identical twins will share all dna. However, I reckon 1/30000 siblings will share 10% or less of their DNA, 1/800 share less than 20%, 1/20 share less than 33%. Thats is, assuming 23 chromosome pairs of unique chromosomes. Should the chromosomes in the pairs already be identical, then naturally the individuals would be more alike, but to the same degree it would make no difference to the outcome whether this was a mating of siblings or of the random population. If the random population all have identical chromosomes then it makes no difference if we breed incestuously or not.

To share much greater or much less than 50% of your DNA with your sibling is extremely rare. So rare, in fact, that for the purposes of this discussion, they're not worth mentioning.

Some fumbling with a statistics book suggests otherwise? 1/20 less than 33% seems worth mentioning to me.

You're highly overstating the occurrence of these "useful recessives".

I have no way to judge that. Do you? What proportion of the population has a double gene for immunity to cholesterol? Tobacco smoke? a thinness gene? A doubled beneficial gene would likely result in extended lifespan. Has anyone done any research on this, how could it be measured? What proportion of the population is indeed genetically immune to tobacco smoke? Or HIV? By comparison it is very easy to measure the number who die from sickle cell, or a specific heart deformity, because we can specifically identify the disease which they did suffer from. It is quite hard to identify people who successfully fought off a disease and never come to medical attention.

I'm going to just refer you to a textbook or two to read up on these things because you're making a lot of uninformed statements and you might want to brush up:

Maybe I am not quite so uninformed as you think?

Actually, no. The vast majority of evolution is caused by variations that are already present in the genome being selected for or against, not by random mutation.

Actually, new variations can only come into existence by random changes. How else? Did a mad scientist perform experiments on the subjects and change them? Then these random variation spread through the population, depending on whether they were beneficial or neutral, or died out if they were entirely harmfull. The vast majority of slective development of a breed, as for example breeds of dogs, is done by making use of gentic variation already existing in the wild population. Thus we can push dogs from 6 in high to 4 ft high, but not 6 ft high, which is beyond the range achievable from the existing variation within the populaion. To get more we must either become the mad scientist, or wait for random changes to come along.

It could happen after one generation. Very possible. The offspring of a sibling-sibling mating has 25% homozygosity. In other words, it has lost 25% of its protection against deleterious inherited alleles. Russian roulette. Have four children and one of them is very likely to die or have a genetic disease. In fact, there's a good chance you'll end up with a number of miscarriages before you even have a child in that joining.

We all die from a genetic disease. faulty hearts, faulty livers, cancerous cells, faulty immune system, something fails and few survive more than 100 years. Show me the genotype for living 200 years and I want to inbreed it at once. The numbers I have heard discussed suggest you way over estimate the effect.

My understanding, which is fairly extensive given my field, is that the vast majority are environmental: STDs, other infections, dietary, physical abnormalities of a non-genetic origin.

STDs causing infertility? Isn't that a gentically inherited immune system incapable of dealing with the disease before it causes damage? I dont know anything much about the nature of reproductive difficulties, but an environmental response to lead, so youre poisoned, would be fixed by a genetic mutation to allow the body to deal with lead. A hypersensitivity to steroids in the diet would be fixed by a genetic ability to prevent their absorption in the gut. If the body in any way cannot reproduce because of an environmental factor, then some genetic adaptation would help. People are bad at reproducing underwater - we drown-, despite life evolving in the sea. Somehow we went wrong genetically and can no longer breed in an excessively wet environment.