More censorship of science by political correctness: cousin marriage becomes a taboo topic in the UK

December 16, 2024 • 9:30 am

This new Times of London piece by writer (and former table tennis champion) Matthew Syed describes a new form of “taboo science” that, although normally a topic of discourse and treaching in genetics, has become verboten to discuss: cousin marriage. This is because of wokeness. Click on the screenshot below to see the piece (or find it archived here). I’ll quote it after I explain a bit of genetics.

I’m sure you’ve heard that marriage among relatives is not a good thing, and that the closer the relatedness, the worse it can be. The reason is that relatives carry identical gene forms, and the closer you’re related to your mate, the more identical genes you share.  For instance, my sister and I share half our genes, which means that if you single out one of the two gene copies I have at a given DNA site (“locus”), the chance that my sister has that identical copy is 1/2, or 50%.  This is expressed as a “coefficient of relatedness,” which in this case is 0.5. It’s the same for me and either of my parents, or any of my offspring (if I had any). Identical twins, being genetically the same at every locus, have a coefficient of relatedness of one.

Because of the dilution effect of marrying someone unrelated, my coefficient of relatedness to any of my grandparents is 0.25, and you can probably figure out that my coefficient of relatedness to any of my first cousins (say the daughter of my mother’s brother) is 1/8, or 0.125. That’s because I carry half the genes of my mother, a quarter of the genes of my mother’s brother, and thus an eighth of the genes of my mother’s brother’s offspring.

What this means for medical genetics is that because each human carries a certain proportion of deleterious recessive genes (that is, genes that have a very bad effect, but only when an individual carries two copies), there is a chance that such genes might come together in those two copies when relatives marry.  Because nearly all these genes are normally present in only one copy, we don’t get the genetic disease, but if two copies appear in a mating between relatives, that offspring will show the bad trait. (An example of such genes are sickle-cell anemia or Tay-Sachs disease, the former more common in West African blacks, the latter in Ashkenazi Jews. But there are many, many deleterious recessive genes, whose frequencies are kept low by natural selection against the few people unlucky enough to get two copies.)

Since most of us carry some really bad disease-causing genes, but in single copies (the average number is 1-2), if we marry a relative the chances that those genes can appear in two copies, producing a sick or dead offspring, increases. For example, if I carry a gene for Tay-Sachs disease, the chance that my sister would have the gene copy would be 50%, and if that were to be the case, if we mated the chance that the offspring would get both copies of the gene would be 1/4.  Thus for every bad recessive gene a person has (and most of us have one or two) incest with a sibling would give a probability of 12.5% that our offspring would get the genetic disease. This is why incest is prohibited: it produces an inordinately high proportion of offspring with genetic diseases or homozygosity for rare genes that have milder deleterious conditions.

The chances are lower for first-cousin marriages because first cousins share fewer gene copies in general.  Although it’s generally seen as bad to marry a first cousin, if the practice hasn’t been going on in your lineage for a long time, the harmfulness of such marriages isn’t so great. To quote Wikipedia:

Opinions vary widely as to the merits of the practice. Children of first-cousin marriages have a 4-6% risk of autosomal recessive genetic disorders compared to the 3% of the children of totally unrelated parents. A study indicated that between 1800 and 1965 in Iceland, more children and grandchildren were produced from marriages between third or fourth cousins (people with common great-great- or great-great-great-grandparents) than from other degrees of separation. [JAC: Thus not just child mortality can be lowered, but also fertility, perhaps because of early miscarriages.]

. . . . In April 2002, the Journal of Genetic Counseling released a report which estimated the average risk of birth defects in a child born of first cousins at 1.1–2.0 percentage points above the average base risk for non-cousin couples of 3%, or about the same as that of any woman over age 40. In terms of mortality, a 1994 study found a mean excess pre-reproductive mortality rate of 4.4%,  while another study published in 2009 suggests the rate may be closer to 3.5%.  Put differently, a single first-cousin marriage entails a similar increased risk of birth defects and mortality as a woman faces when she gives birth at age 41 rather than at 30.

This is still stuff you need to know if you’re contemplating marrying a cousin, especially if you’re in an ethnic group with a high frequency of certain genetic diseases. That’s why we have genetic counseling. First cousins still need to know that the risk of having a genetically diseased child could be 50-100% greater than for nonrelated couples.  It’s still low, but could affect one’s decision  And if you are, for example, an Ashkenazi Jew, it’s best for both partners to get genetically tested, for if neither carries the Tay-Sachs gene, they can go ahead and reproduce like rabbits. If only one carries the gene, they can still go ahead without worry, though that gene will still be passed on in single form to half their offspring, who will be “carriers.”

Given the low relative risk of having kids with your first cousin, it’s still curious that such marriages are widely prohibited. Again from Wikipedia:

In some jurisdictions, cousin marriage is legally prohibited: for example, first-cousin marriage in China, North Korea, South Korea, the Philippines, for Hindus in some jurisdictions of India, some countries in the Balkans, and 30 out of the 50 U.S. states. It is criminalized in 8 states in the US, the only jurisdictions in the world to do so. The laws of many jurisdictions set out the degree of consanguinity prohibited among sexual relations and marriage parties. Supporters of cousin marriage where it is banned may view the prohibition as discrimination, while opponents may appeal to moral or other arguments.

There’s a reason to prohibit first-cousin marriages in some groups, though, and I’ll explain this below.

This kind of stuff has been taught for decades in genetics classes.  There should be no taboo about talking about why brother-sister or first-cousin matings are frowned upon. But it has recently become taboo to discuss, as you can see from the article above. The reason is that some communities keep mating with their relatives generation after generation, and that considerably increases the chances that two relatives who mate within that community will have a genetically diseased offspring (this “inbreeding” raises the frequency of some genes that cause genetic disease, which is why the Amish and Ashlenazi Jews happen to have high frequencies of different genetic diseases.  Here’s one more paragraph from the Wikipedia article on “cousin marriage”:

After repeated generations of cousin marriage the actual genetic relationship between two people is closer than the most immediate relationship would suggest. In Pakistan, where there has been cousin marriage for generations and the current rate may exceed 50%, one study estimated infant mortality at 12.7 percent for married double first cousins, 7.9 percent for first cousins, 9.2 percent for first cousins once removed/double second cousins, 6.9 percent for second cousins, and 5.1 percent among nonconsanguineous progeny. Among double first cousin progeny, 41.2 percent of prereproductive deaths were associated with the expression of detrimental recessive genes, with equivalent values of 26.0, 14.9, and 8.1 percent for first cousins, first cousins once removed/double second cousins, and second cousins respectively.

The higher the frequency of deleterious recessive genes, the greater the relative risk of first-cousin matings compared to matings between unrelated people, and in the Pakistani community the frequencyu of deleterious recessive genes have been increased by inbreeding (“consanguineous mating”). Sometimes it can be hundreds of times higher!

Thus it becomes even more imperative to discuss the dangers of “consanguineous” matings (matings with relatives) with members of such communities. Sadly, the opposite has occurred. Because one of the communities that do this is in fact Pakistani (I’m not sure if other Muslim national groups do the same), the taboo with dissing people of color has now made it off limits to talk about cousin marriages. I quote from the Times article. Note that Syed’s father was a Pakistani immigrant to the UK.

Let me start by telling you about Dr Patrick Nash, a somewhat shy legal academic who in 2017 came across an intriguing finding. He noticed that much of the “extremism” emanating from Pakistani communities seemed to have a “clan” component. The perpetrators were linked not just through ideology or religion but by family ties stretching through generations. He noticed something else too: these communities were cemented together by cousin marriage, a common practice in Pakistani culture. By marrying within small, tightknit groups, they ensure everything is kept within the baradari, or brotherhood — property, secrets, loyalty — binding them closer together while sequestering them from wider society.

At this point Dr Nash hadn’t come to understand the genetic risks, the patriarchal oppression and the bloc voting, nor the growing evidence that rates of cousin marriage strongly correlate with corruption and poverty, but — like any good scholar — he thought he’d do a bit more digging.

But then something odd happened: several academics invited him to the pub for a “drink and chat”. He thought nothing of it, but it turned out to be an informal tribunal. “It was put to me that I might consider another line of inquiry that would be more ‘culturally sensitive’, less likely to provide ‘ammo for the right’ and less likely to ‘make life more difficult for myself’ as a junior, untenured academic,” he told me. “It was sinister.”

It was not just sinister, but woke, with the wokeness working to silence scientific discourse. And, as Syed discovered, the wokeness spread quickly. I’ve bolded the most arrant example of scientific censorship.

I quickly discovered that researchers wouldn’t return emails or calls. When I got through to one geneticist, he said: “I can’t go there.” It was like hitting a succession of ever-higher brick walls. I then came across evidence that scientists examining the UK Biobank had found that levels of incest (father-daughter, siblings etc) were significantly higher in the British Pakistani community than the wider population. This was a disturbing finding, possibly indicating abuse of a shocking kind. But the paper was never published, disappearing into what I can only describe as an Orwellian memory hole. When I approached the researchers, they were not prepared to talk on the record. One said that he feared he might not be able to bring up his children if he whispered the truth and lost his job as a result. It was like something out of Kafka.

What I hope you are gleaning from all this is how scientific inquiry is being distorted and suppressed out of an almost crippling fear of offending cultural sensitivities; how information vital to the public interest is being censored out of concern that it might be prejudicial to the “customs” of immigrant communities. It is a phenomenon that directly parallels the Rotherham scandal, in which young girls suffering horrific abuse at the hands of mainly Pakistani gangs were betrayed by the police and social services, which refused to investigate for fear of appearing racist.

Finally, there’s this:

But the other striking aspect of the debate was the sinister influence of scientific malpractice. MPs on all sides kept referring to the genetic risks of cousin marriage as “double” those of relationships between unrelated couples. This “fact” is endemic throughout the media, from the BBC to The Telegraph, and for good reason: journalists trust what scientists tell them. But the stat isn’t true — indeed, it’s absurd. When inbreeding persists through generations (when cousins get married who are themselves the children of cousins), the risks are far higher, which is why British Pakistanis account for 3.4 per cent of births nationwide but 30 per cent of recessive gene disorders, consanguineous relationships are the cause of one in five child deaths in Redbridge and the NHS hires staff specifically to deal with these afflictions.

And that, as Syed says, raises the chances of genetic defects in offspring of Pakistani-Pakistani matings far above those of matings of two people from non-Pakistani communities who aren’t part of a group that has mated consanguineously for generations.

What we see here is one more instance of scientific truth (like the sex binary) being suppressed because it supposedly demonizes a minority group.  But no good can come of such taboos. For one thing, the data don’t show that Pakistani people are morally worse than other people; they simply have a cultural custom that results in more child mortality.  Isn’t that worth spreading to both them and to genetic counselors? For another thing, it takes the whole topic of consanguineous matings off the table for discussion, which is both socially and scientifically harmful.  This kind of censorship is not new to me, as there are lots of examples, but Syed just grasped it:

I realised something else too. I was a victim of racism growing up, ostracised for long periods at school for the colour of my skin, which is why I’ve spent my life fighting the bigotry of the hard right. But I now believe the soft bigotry of the left is more insidious. After all, you can see and challenge a thug using the P-word, but how to combat the subtler bias that has seeped into our institutions? Ponder the scientists who, with the terrible certainty of their own virtue, played down the risks of cousin marriage, thereby denying the very community they presume to help crucial information; the researchers who concealed information on incest to “protect” minorities, thereby condemning the most vulnerable to sickening abuse.

The way to combat the bias is to tell the scientific truth and then add that the scientific truth doesn’t carry any moral or ideological implications. The taboo we see comes from bigots: in this case, the bigots of the left who don’t think that people, including Pakistanis, can handle the truth.

In this case there are lives that can be saved by telling the truth, but the “soft bigotry” of the left prevents that. It is, in effect, killing babies.

********

A few relevant tweets from Luana Maroja. Note that the first one uses a picture of a family that is NOT Pakistani!

This must be the paper that wasn’t allowed to be published (see above). The tweet has apparently been deleted.

47 thoughts on “More censorship of science by political correctness: cousin marriage becomes a taboo topic in the UK”

    1. I haven’t pondered the wisdom of banning the practice, so I have no opinion to express yet. It certainly cannot be banned only for one group, though!

      My point here was just to show how ideology is squelching scientific discussion.

      1. It was banned by the Catholic Church in the Middle Ages. Even after the Protestant Reformation the practice never came back.

  2. I remember the Ceballos story and assumed it would all have come out before now. I guess not. If writing true reports about cousin marriage (and father-daughter incest) gives license to some racists to abuse anyone from a particular ethnic group then that’s terrible. Is it worse than ignoring a preventable cause of physical suffering among thousands of children just in the UK? That question doesn’t seem so hard to answer, but then I’m no moral philosopher.

    “The character of Chidi, the moral philosopher, reveals the nature of the problem [in the Netflix series “The Good Place”]. Chidi…has a vast knowledge of ethics theory but finds it almost impossible to make an ethical choice.”
    https://hipporeads.com/the-good-place-good-show-bad-ethics

    I think it’s ok to say that cousin marriage is bad, and “trans women” are male.

  3. Ironically, cross-cousin marriage was one of the most common forms in the world in the not-too-distant past, and was largely banned by colonial regimes. It’s even encoded in Dravidian kinship terminologies, which are found around the world, in which the father’s sister’s children and the mother’s bother’s children (the cross cousins) are consider ‘potential spouses’ terminologically, while parallel cousins are termed brother and sister.

    The interesting aspect of incest taboos and prescriptive marriage systems is that they arose before the discovery of genetics, and probably had nothing to do with the potential biological effects of ‘consanguineous’ reproduction.

    1. There does seem to be an “ick” factor that prevents siblings from becoming sexually attracted to each other in healthy families or, less robustly, fathers to their daughters. You may be right that incest, even not involving force — it often does involve force, of course — is traditionally punished severely because breaking a taboo shows the perpetrator(s) to be ungovernable beyond redemption. Further reproduction must be prevented reliably.

      Still, I can’t help think that knowledge of practical genetics goes back a long way into our agricultural and pastoral history. In deciding which animals and seed crops to allow to mate, and which to keep apart, our ancestors weren’t entirely groping in the dark.

      1. Anthropologist Paul Roscoe published an interesting — and award-winning — article 30 years ago in which he developed Westermarch’s 19th century argument that close childhood association is critical for the development of incest avoidance and aversion. He pointed out there it appears that there are even neurophysiological grounds for associating sex with aggression, which would run counter to family ‘amity.’

        Levi-Strauss in 1949 developed the point that horticulturalists had some sense of heredity, as you mention, but he used that fact to suggest that encouraging incest could be used to eliminate unwanted traits, as happens in selective breeding of plants and animals, so rather than primitive genetics leading to taboos, primitive genetics might have led to prescriptive incest, sort of on the order of Egyptian monarchs marrying their siblings, since who else could marry a god except another god? or Inca rulers marrying sisters to keep hereditary power close to the family.

        1. The biblical authors/editors of Genesis and Exodus chose the same path. The pedigree they gave Moses was suspect, to say the least:
          Abraham was a halfbrother of Sara, his wife.
          Isaac, their son, and Rebeka, his wife, were related in the third degree: she was the granddaughter of an uncle of Isaac.
          Their son Jacob and his wife Leah were cousins.
          Their son Levi married an unrelated woman, for a change.
          Their daughter Jochebed married Amran, the son of one of her brothers. She was his aunt.
          Their son was Moses.
          And if you follow the Masoretic text of the Bible, Jochebed must have been at least 250 years old when she gave birth to Moses. That is a lot more than 40…

      2. our ancestors weren’t entirely groping in the dark.

        Given the topic, this made me laugh, whether you intended it or not.

        Yes, I’m a horrible person.

      3. The “ick” in sibling relations is called the Westermark Effect I think. There’s quite a lot of work on it about.

        Cousin marriage is s/t I’ve read a lot about but haven’t gotten around to writing about. I believe it is a big part of the Islamosphere’s dysfunction. There is no prohibition of it in Islam and (from memory) the prophet even endorsed it.
        Its frequency maps with Islam almost one for one.

        Aside the medical problems there are difficulties in state formation and respect for government when one’s tribal affiliations are also inbred family affiliations.
        Bad for nation building and a secular leviathan state.

        Cousin marriage is what I call a second order effect of Islam which the rest of the world has to deal with. Unfortunately.

        D.A.
        NYC

        1. If people choose their partners freely, it is unlikely that they would choose a cousin. To me, widespread cousin marriage implies widespread arranged and even forced marriage, and I find it even more troubling than the increased risk of disorders.

          1. Mayamarkov: Why do you say that people wouldn’t freely choose a cousin as a partner? Do you have evidence?

      4. Interestingly, commercial animal husbandry (and presumably cropping) relies heavily on inbreeding, but carefully and strategically managed (and often called “line breeding” to distinguish it from unmanaged forms of inbreeding). The same effect that magnifies the likelihood of recessive genetic diseases also magnifies the likelihood of positive recessive traits. If the diseases can be eliminated through careful tracking of lineage to find and eliminate carriers, along with promotion of breeding between carriers of desirable traits, then the end result can be extremely reliable, fine-tuned, and optimized breeds of animals for particular tasks.

        I suppose it doesn’t work well in humans because we don’t manage our “herd” nearly so well. Sometimes I wonder how much stronger, smarter, and healthier we could be if we did, but that’s one of those taboos…

    2. The Church of England Book of Common Prayer contains a handy table which I puzzled over as a youth in the 1950s/60s: https://www.churchofengland.org/prayer-and-worship/worship-texts-and-resources/book-common-prayer/table-kindred-and-affinity
      My parents, who knew nothing of modern genetics, told me rather vaguely that this was to avoid the risks of “inbreeding”. First cousins are allowed to marry according to this. IIRC Henry VIII reversed the prohibition of the Roman Catholic church so that he could marry his own first cousin. Oddly, it was once illegal in Britain for a man to marry his deceased wife’s sister, the basis for which was surely never genetic.

      1. You reminded me of examining marriage rules in Central America years ago, which were based on the medieval Catholic rules in force at the time, in the 1500s, when colonization was imposed. The Roman Catholic Church updated its policies over 500 years, but Latin American Catholicism was often stuck back in the 1500s. In practical terms this meant that in some Central American countries, the Church banned marriage between 4th or 5th cousins — those are cousins linked by a shared ancestor 5 generations back, someone most of us in the First World would barely consider related. I also remember an Italian priest in Panama pointing out that those rules often neglect the fact that marriage is not the same as sex….

  4. This is yet another example of subjects that have become taboo in science, much to society’s detriment.

  5. Pakistani clan gangs oppose a ban of cousin marriage because it would unravel the control that a handful of clan patriarchs exert over a large ethnic immigrant group, which the leaders can transmute into political power. They don’t care that the clan system causes more illness and mental retardation in children. The only children that matter are those who grow up to do the clan’s bidding. Damaged children can be abandoned for the state or low-caste women to look after. It may not be a bad thing if many of the surviving children are not overly bright or capable of thought independent of what the imams tell them.

    Certainly a ban on cousin marriage would have to apply to all. The group that would feel the teeth of the law the hardest would be the Pakistanis, who are a powerful political force in the UK generally and the Labour Party specifically. Citing the DEI principle of differential impact, they would cry racist. But in the UK today they don’t need to claim their rights are being violated by an intolerant majority. They just have to vote.

    1. The proposed ban certainly would apply to all, but as you state, the groups that would feel the effect of it are the ones that frequently practices first cousin marriage, which are overwhelmingly immigrants. Hence The Critic suggesting it is a stealth attack on immigrants.

    2. I understand that at least some of these cousin brides are brought from Pakistan, to keep the family insular.
      Damaged babies are inconsequential as they will be taken care of by NHS, schools, disability payments, and social services, at no real cost to the tribe. They can always make more babies to increase power and influence.

    3. This raises an interesting question that I haven’t seen discussed. Do all cultures value intelligence? If so, do all cultures value it equally? Presumably not, as there are an Infinity of other values against which to weigh it. Query how one would even research this.

  6. Years ago someone wrote to an advice columnist asking about marrying her cousin. The answer was – don’t do it. Then James Crow, at that time the leading population geneticist in the US, wrote in to say that he thought the risks were low. Darwin married his first cousin. As it can put together ‘bad’ genes, it can also con concentrate good ones.

    Some noticed that the risk of patent ductus arteriosus was 3 times higher in Iran than in the US. Why? Cousin marriages.

    1. Einstein’s second wife was a first cousin on one side (I think. his mother’s) and a second cousin on the other, which doesn’t seem to suggest that outbreeding was exactly encouraged in his family. No children from that marriage though, unlike the huge brood raised by the Darwins.

  7. Are all recessive genes harmful? I recall reading a long time ago that the Huxley and Darwin family trees contained many cousin marriages. The speculation was that perhaps this may have contributed to their reputed high intelligence level.
    I have also heard the same speculation about Ashkenazi Jews.

  8. Very interesting post, I just have one pedantic comment :v

    ” sister and I share half our genes”

    It is the most probable, but not necessarily. Lets say for 2 loci, your dad is AB CD and mom is ab cd. Possible gametes are A C, A D, B C, B D from dad, and a c, a d, b c, b d from mom.

    Thus, two siblings can be Aa Cc and Bb Dd, sharing nothing. I am aware that with a big amount of loci, it tends to be 50%, but may be different in some cases.

    2. To nitpick further. This is only considering sites with variation. At most loci the sequences will be identical. So in fact siblings are likely well over 50% identical overall. We are a very inbred species, especially those of us with ancestry outside of Africa.

  9. Patrilateral Parallel Cousin Marriage. A concept well known to the Cultural Anthropology set. It is by no means confined to Pakistanis.

  10. There is an interesting discussion of the case for a ban on cousin marriage in an open access essay by Patrick S. Nash in the Oxford Journal of Law and Religion, available online.

  11. In Persian Gulf countries is was so prevalent they instituted compulsory genetic counselling (Qatar and UAE) to prevent cousin marriage some decades back.

    D.A.
    NYC

  12. While reading Adam Rutherford’s,
    “A Brief History of Everyone Who has Ever Lived.”
    The Story in Our Genes
    toward the end of the chapter on the English I remember thinking, jesus they seriously needed to outbreed.
    As I understand this is true of many societies, villages, around the globe.
    The woke seem to think diversity is only skin deep but on the other hand, centuries of aristocrats, religious, or mere geographical impediments, breeding to suit their needs seemingly makes for a high wall to jump and to use the tools and knowledge known in THIS centuries science.
    It’s is as they deliberately want to remain ignorant.

  13. Interesting technique to keep scientific research from coming out in both this case as well as the other article referencing the WaPo article discussing puberty blockers. In both cases, the argument is made that publishing research that does not align to the woke agenda will give ammunition to the right. As someone not in the field, I have to ask if this is effective – are researchers so beholden to leftist ideology in general that they see this as a logical reason to not publish?
    I know we saw a similar phenomenon in the comment section for this site during the runup to the election with people telling Dr. Coyne that he needed to squelch criticisms of the administrations to prevent Trump from winning.
    This is quite a manipulative fear tactic: don’t tell the truth or you’ll be helping the other guy who might use the truth to upset our social agenda (which is not based on truth, and thus must be a lie).

    1. the argument is made that publishing research that does not align to the woke agenda will give ammunition to the right. […] are researchers so beholden to leftist ideology in general that they see this as a logical reason to not publish?

      Yes, absolutely!

      A lot of researchers will avoid topics for this reason. Many others will then avoid the topics for fear of ostracisation by their peers.

      These taboos can be very powerful. For example, no mainstream scientists will touch the topic of genetics as related to race and IQ. It would be career death to do so.

      1. Why is there no #RESIST movement on the research front to oppose this?

        I understand – no one wants their grants rejected. But nothing is going to change if no one pushes back in a serious way.

  15. I live in a majority Muslim country. Spare me from mentioning its name since it doesn’t matter. I was also born to a Muslim family. Unfortunately consanguineous marriage is very much prevalent with an overwhelming stat of 40 percent. A horrific number indeed. Much of marriages result in death in infancy with the families suffering the most, despite being warned about the possible horrific outcomes. These days I witness one of these outcomes with my own eyes. A relative of mine committed this evil selfish act of marrying another relative, the result is 2 deaths of infants in a row and counting. Unfortunately I see no other solution than to out law such act.

  16. Slightly off on a tangent, but a historian I was listening to recently made the point that the outlawing of cousin marriage in Europe (in the 13th century) was instrumental in the rise to dominance of the west. It meant a move from trust being implied due to family links to the development of societal structures that allowed many people to work together, eg. the development of, and respect for, contract law.

  19. Am I reading and understanding that tweet by Ceballos correctly – ‘we discovered a population with 6k (ie, 6,000) times the background level of father-daughter incest’?

  20. Jerry: you wrote “For example, if I carry a gene for Tay-Sachs disease, the chance that my sister would have the gene copy would be 50%, and if that were to be the case, if we mated the chance that the offspring would get both copies of the gene would be 1/4. Thus for every bad recessive gene a person has (and most of us have one or two) incest with a sibling would give a probability of 12.5% that our offspring would get the genetic disease.” Aren’t you equating 1/4 with 12.5% here? Shouldn’t you have written 25%?

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