I guess it might be partly out of morbid curiosity, but I have wondered for some time now, how do children of incest usually turn out? Are they hideously deformed usually? When I was smaller people used to say children of incest were typically retarded. Is that true?
I’m talking about people very closely related having children together. Redneck jokes notwithstanding, I am not talking about cousins here either. Queen Victoria married her first cousin Albert. And all of their children came out alright.
So what is the usual outcome of very close relations?
They look like the boy in The Ewok Adventure. At least, that’s what my mom said when I was a kid and I was watching that movie on TV in a hotel room. No joke.
If the two parents didn’t have “hideously deformed” genes, why would the kids? The only unusual occurrence would be if both parents have the same recessive gene (which may be a “positive” trait or “negative”, which is more likely the closer they are related. If there are no undesirable genes to begin with, no problem.
Except that all humans have some number of rare recessive genes that almost never manifest without incest, since they’re so rare. And when one gene is significantly better or worse than another, it’s almost always the recessive that’s the bad one, because defective dominant genes get selected out of the gene pool very quickly.
Inbreeding and linebreeding is a great way to set desirable inheritable traits. It’s also a dandy method to set undesirable traits. In nonhuman animals, this can be a good thing. You breed the various animals that are healthy, and you cull, or at least don’t breed, the ones that have the undesirable traits. With humans, well, most people are uncomfortable with the notion that people with undesirable traits shouldn’t be allowed to breed.
For further reading, I suggest Googling the words hemophiliac royal or inbreeding linebreeding and the phrase “popular sire syndrome”.
Brother and sister, separated at birth, meet, fall in love, have children, find out they are siblings. Not only in the movies. In Spain. Adult children are normal and have no psycological problems or otherwise. The family is not ashamed or shy about the case. - EL MUNDO | Suplemento cronica 495 - HERMANOS, PAREJA Y PADRES
A simple way to understand it is by analogy with cards: Every family has a deck of cards, representing their gene pool, and there are complex rules determining what comes out of any particular hand. A good rule to remember is that mostly pairs or other combinations actually have an observable outcome (expression in the phenotype).
Every child gets a hand created out of shuffling their parents’ cards. Outbreeding, the usual way to select mates in our culture, ensures that the number of pairs is kept fairly low: Mom’s hand isn’t especially similar to dad’s hand, so junior won’t end up with a lot of repeats. Additionally, cards combinations that result in hands that never get reshuffled (people who never breed) will be destroyed and their component cards will be so lost in the shuffle they’re essentially nonexistent. (This is why so few babies are born without lungs, for example.)
Inbreeding, however, keeps all the same cards circulating from one generation to the next. This results in pairs, flushes, and even inside straights, or, in genetic terms, a larger amount of phenotypes that are rare in the general population.
This doesn’t always mean diseases. However, it results in diseases often enough that inbreeding is mostly a bad idea. Mainly, this is because we don’t (and, to some extent, can’t) do detailed genetic testing of our prospective mates. Hemophilia in European royalty is a famous example of what can result from a particularly intensive long-term human inbreeding project. One of the most dramatic examples was Charles II of Spain, also known as Charles the Hexed, who was descended from Joanna the Mad 14 different ways.
I knew a woman a few years ago whose son had married her younger sister(his maternal aunt). They had three children, who were apparently slow but not deformed. The worst part was the taboo - it really broke her heart that her sister and son could do such a thing.
Norway’s current king, Harald V, is the son of a first cousin marriage. His father was also the son of a first cousin marriage. Okay, he has a big nose and a rather pronounced case of male pattern baldness, but I don’t think you can call him deformed. He’s also somewhere around average on the intelligence scale.
Intermarriage among the royal families of Europe was the normal thing up until the early 20th century. You can certainly find examples of children born with serious problems because of that, and even today certain traits (like dyslexia) seem to be more widespread among the royals than the general population, likely because of the pedigree collapse issues. But most of the babies of these first cousin marriages were born quite normal. Cousin marriages make it more likely that the bad stuff hidden in the genes will come out in the babies - but they do not create new mutations.
Take Queen Cleopatra (VII), who had love affairs with two of the most powerful men in the world, Julius Caesar and Mark Antony.
Her parents were uncle and niece, and were also first cousins. Her paternal grandparents were brother and sister. Her maternal grandparents were uncle and niece. Five (or six, depending on which way you ascend) generations up, she only had two ancestors, Ptolemy VIII and Cleopatra III – and those two were uncle and niece. http://en.wikipedia.org/wiki/Ptolemaic_dynasty. All that inbreeding didn’t seem to hurt her.
As far as I can tell in my near-total uneducation on the subject, most families can handle one incident of inbreeding. But if it continues, decline sets in fairly quickly. A couple of years ago I was reading about the Egyptian pharaohs, who were in the habit of marrying their own siblings. You could kind of see it in the mummies illustrated–the first generation would look OK, the second not so much, the third would be pretty strange-looking, and then they would have to find someone else to be Pharaoh, because by that time it was hard to produce a living child from the union. Each successive generation would have more miscarriages and fewer live births.
Everyone carries a few spontaneous deleterious recessive mutations. These only manifest if two people carrying the same mutation have children, and even then they only show up in 1/4 of all children. In the general population, out-breeding means that any two individuals won’t carry the same mutations the vast majority of the time.
Let’s start with a non-related pair. Each parent will have a handful of these recessive alleles - let’s go with four each* (eight total), to make the math simpler. In their children, each will inherit (on average) half of these parental mutations, so they’ll carry four. Any two of their children, again on average, will be carrying two of the same mutations. If these siblings reproduce, then the next generation (grandchildren of the original non-related pair) will have a 1/4 chance of expressing either individual mutation, which works out to be a 3/16 chance of expressing at least one. Also, they’ll have a 12/16 chance of not expressing anything, but still being carriers.
This is a vastly higher risk of genetic disease compared to the general population, but it’s hardly a guarantee. A lot of these mutations will also lead to infertility for the inbreeding couple, or very minor problems for their children. Hemophilia isn’t easy to deal with, but it’s not a death sentence either. So the products of incest won’t be hideously deformed monsters, but they’ll have a much higher risk of genetic disease. However, if you keep inbreeding for generations, the genetic problems will accumulate and become fixed, so that they become very prevalent. This is what happened with the European royal family, and similar things have been documented in other cases.
*perhaps too small, even. I’ve heard estimates of around 5-10 new deleterious recessives in every individual, but I can’t find a cite at the moment… it’s something that’s been mentioned repeatedly in many of my classes and textbooks on genetics.
My mom used to work as a social worker and she said that there was a higher incidence of kids with physical and mental handicaps in the areas that she worked in than in other areas in the Netherlands. This, she argued persuasively, was due to the social structure in those areas (fishing villages), where there’s a lot of different churches. The people there don’t want to marry outside the church but their communities are very small so they’re faced with generations and generations of first-cousin marriages and the like. While it is not possible to tell whether individual instances of handicaps and disease are caused by this, it’s apparently pretty clear that on the aggregate level, there’s a strong difference between these villages and other parts of the Netherlands.
This is a big problem in Amish communities with closely related marriages over several generations. Much higher incidences of genetic diseases than in the normal population.