[ConfusedHermit (OP)]

This is an unrealistic question, but if everyone were completely healthy and disease was not a threat, would inbreeding still be an issue?

Also, is it possible for people inbreeding to have benefits OVER mating outside their family?

Such as if one person has genes for being physically advantageous and the other person has genes for being mentally advantageous, since they’re related their kid would be more likely to get both of those qualities instead of just one or the other?

Or if they both had just one of those things, would it make a ‘twice as physically’ or ‘twice as mentally advantageous’ kid? Maybe you can think of better examples; I’m not sure if mine are worded correctly.  I already know very little as it is... :{o~

I was just curious about this because I’ve read that this was one of the tricks used by some of the ancient dynasties to maintain leadership by mating siblings, and was in a way the forerunner of genetic engineering. The families even killed off their defective progeny so nothing undesired was passed on. Society looks down on incest now, but it’s interesting to know that cultures once saw it as a superior option.



_{Mod edit - formatted the subject as a question.  Please do this in future to help keep the forum tidy and easy to navigate.  Thanks!}

[CliffordK]

The problem is that there are many rare recessive asymptomatic mutations that are not a problem in society in general.  In fact, it is quite possible that every person has a couple of new mutations. 

In some cases, these mutations could produce desirable traits. 

It is possible that evolution may actually involve a restricted gene pool in which certain traits are amplified.  But, also at great risk to the individuals.

In the future, it may be possible to sequence a person's entire genome, as well as the genome of one's potential mate to look for recessive genes that could lead to homozygous genetic abnormalities.

[Don_1]

If you look at animals on small isolated islands, such as the Galapagos, you will realise that in some instances all the individuals within a species will be related. The giant tortoises are all one species with different sub-species on each island, or up to three sub-species on one island.

The Galapagos are volcanic islands which have never been connected to the mainland. How the tortoises got there in the first place is a mystery, but you can be sure that the entire population of Galapagos tortoises originate from just a few accidental migrants. The chances of what might be called a 'viable reproductive gene pool' arriving on the islands would be slim, to say the least. So it is probable that these tortoises have been inbreeding since their arrival on the islands.

The only thing which might have been in their favour is the fact that tortoises had had at least 200 million years of evolution prior to their arrival on the Galapagos. It may be that this was long enough to breed out any problems which inbreeding might of otherwise have thrown up.

[CliffordK]

Aren't the tortoises very good swimmers.  So, arriving at any particular island shouldn't be any mystery at all.  The issue is that they tend to return to ancestral breeding grounds.  So, on rare occasions they must choose new breeding grounds.  It is interesting then how isolated certain groups can become.

[Don_1]

No, tortoises are not good swimmers, though some can float, they have a problem breathing since their head is so low compared to where the water line would come. So whether they can float or not, they would drown. Even the Box Turtles of North America, which are more alike tortoises than turtles, are strictly terrestrial, not aquatic. A mistake so frequently made by so many Americans, who take them from the wild to keep as pets and proceed to house them in a water filled tank, with little or no dry landing stage. It is possible that, due to the long neck of the Galapagos tortoise, they may have been able survive prolonged floating in the ocean. Being some 800km from the nearest mainland, this would have been a long time at sea and at the mercy of the current.


Just a footnote on those poor Box Turtles.

Box Turtles, the Eastern Box, Three Toed Box and Ornate Box are highly terretorial creatures which DO NOT fair well when taken from the wild. Box turtles removed from their home ground are known to spend many years in an effort to return to their home. Few survive being taken into captivity. Despite this and the dwindling population, there are no federal laws to protect them. In some states they are protected, in others they are not. The state of Texas allows 36 animals to be taken into captivity in one year by each and every citizen. In the state of Illinois, over a 21 month period, a survey revealed the demise of an astonishing 36,000 Box turtles.

Time for federal laws to protect them. In the mean time, please America, if you see a Box turtle, leave it in peace.

[ConfusedHermit (OP)]

Alright then. I guess sometimes you learn about tortoises without expecting it! :{O~

You guys are very helpful in these forums! Thanks for that.

[CliffordK]

Thanks Don,
Somehow I was lumping Sea Turtles (which are good swimmers) and Tortoises in my mind.

[Don_1]

Indeed, the marine turtles are excelent swimmers. But did you know that not all aquatic fresh water turtles can swim?

The Mata mata (Chelus fimbriatus)  is a South American side neck turtle which is a very poor swimmer. Its diet is fish, which it catches by stealthily walking on the bed in the shallows of the Amazon. The North American Stinkpot (Sternotherus odoratus) is a good swimmer, but cannot float, so prefers shallow water, where it can easily walk and rest on the bottom. The Alligator Snapping Turtle, (Macroclemys temminckii) an endangered species of the southeastern US is a large bottom walker. Unlike some of its aquatic cousins, it does not have 'flippers', but rather has the clawed feet of a terrestrial tortoise.

Gone rather off topic here. Tch tch, me and my chelonians eh!

[Don_1]

Another example of extreme inbreeding are the thoroughbred horses, all of which come from just 3 original Arabian horses. Of course, that was instigated and continues today by the hand of Man.

[evan_au]

One estimate I saw (many years ago) was that the average person has about 7 lethal mutations. The vast majority of these are recessive genes, and will only be lethal if you inherit 2 copies. This would reduce fertilisation rates, increase stillbirth rates and increase disabilities with closely-related parents.

If we assume that each mutation will be independently inherited with 50% probability, and that cousins share 25% of their genes, so they will have about 2 lethal mutations in common. A child has a 25% chance of inheriting two copies of each of these genes, or roughly 50% chance of inheriting two copies of some lethal gene. If true, this would suggest about a 50% reduction in fertility for cousins. This ignores non-lethal genes in common.

Some fairly isolated populations such as Orthodox Jews have had their population decimated by multiple persecutions, and suffer inbreeding. There is a voluntary programme of genetic testing and counseling for prospective couples; by adopting this scheme, it has effectively eliminated genetic diseases like Tay-Sachs disease. In this case, you could say that inbreeding is no longer a disadvantage. http://www.jewishgenetics.org/?q=content/dor-yeshorim

On the other hand, there are some villages with a large proportion of very old citizens. One could assume that this is an isolated population who all have a favourable set of genes (and live away from pollution, get good quality food and plenty of exercise and are free from persecution).

[atnjoly]

The short simple answer is yes, inbreeding has numerous benefits, despite all of its well known negative consequences. But this is actually a very complex question, which requires an elaborate answer.
First, inbreeding should not be confused with incest. Incest is the mating of individuals related in a direct line, and thus sharing half of their genomes ( brother-sister, parent-child ), or a quarter (child with grand-parent, or with uncle or aunt). Inbreeding is simply the mating of individuals that are more closely related than the average overall population. Hence, even in a theoretical completely panmictic population (i.e. mating completely at random ), a large proportion of matings will still be inbred.
The first advantage of inbreeding is that it reduces the cost of sex: when you breed with someone who is completely unrelated to you, each of your offspring only carries half of your genes. But if you share say 1/10 of your partners’ genes because you have common ancestors, your offspring will carry 55% of your genes.
The second advantage of inbreeding is that it cleanses the genome of recessive deleterious mutations, which relates to the answer provided by evan_au: because DNA can mutate, and particularly because DNA replication is not a completely faithful process, new mutations will occur in the germ line every generation. In humans, recent estimates obtained through whole genome sequencing are of about 100 new mutations per generation, occuring more frequently in sperm, and increasing with the age of the father. The vast majority of those mutations will occur in silent DNA, however, but one or two may affect a gene’s functions. And of those, most will inactivate the gene and thus be deleterious rather than result in an improvement, but they will mostly be recessive because having just one functional copy of a gene suffices for most of them. Inbreeding will indeed bring those recessive mutations to light, and result in inbreeding depression, in other words in unfit offspring or even in lethal phenotypes. Somewhat counter intuitively, however, the overall result is that the mutation load ( i.e. the average number of mutations per genome ) will be reduced in the surviving offspring after inbreeding. This is why inbreeding depression is reduced in subsequent generations of inbred stocks, and how purebred stocks and varieties can actually be obtained and maintained by plant and animal breeders.
The third advantage is directly related to the second in that inbreeding allows the expression of advantageous but recessive phenotypes. This is particularly relevant for mutations that allow to become insensitive to pathogens such as viruses, or to evade predators because those usually correspond to the loss of a function or character, and those are usually recessive. In an essay published last year, I have actually advocated that this type of mutations was likely acting as a main driver of speciation, i.e. the preferential breeding  of individuals within small groups rather than with the ancestral stock ( newbielink:http://www.biology-direct.com/content/6/1/62 [nonactive]).
The fourth advantage of inbreeding is that it allows to keep useful combinations of genes together rather than separating them every generation, and thus reduces what is called the recombination load
A fifth advantage of inbreeding is that it promotes cooperative behaviours and altruism. Indeed, in populations that are structured in small groups, the fitness of individuals will also depend on the fitness of the groups to which these individuals belong. This is called group-level selection, and is a more contentious concept which is not accepted by all population geneticists. What is clear is that, in panmictic populations, most models predict that selfishness will be promoted. In this essay of mine ( newbielink:http://www.biology-direct.com/content/6/1/62 [nonactive]), I actually go a step further and suggest that extensive panmixia will eventually promote species extinction via the accumulation of recessive mutations, but this is too complicated to explain here, and you will have to tackle reading it if you want to understand how.