From what degree is there no longer a risk of hereditary disorders in children of parents from the same family?

Answer

Hi Appi,

hereditary disorders due to “inbreeding” has to do with the more frequent occurrence of “homozygous recessive alleles”. What does this mean?
– “alleles” are variants of genes. For example, there is a gene that produces skin pigment, but albinos carry a mutation in that gene. Scientists say that the “healthy” and “mutated” genes are alleles of each other. Humans carry two sets of chromosomes, which can vary among themselves, but carry the same information. We are ‘diploid’ in scientific terms and we have two alleles available for each gene.
– a recessive allele is an allele that only has a certain effect if it is present in both sets of chromosomes. For example, someone who carries one mutated pigmentation gene and one healthy pigmentation gene will not exhibit albinism. The healthy gene can compensate for the mutated gene.
– homozygous for a particular gene means that each of the sets of chromosomes of that specific gene contains the same allele. For example, twice the healthy pigmentation gene or twice the mutated pigmentation gene are thus homozygous for the recessive allele.

What does all this have to do with the prevention of hereditary disorders through inbreeding?

Well, there are certain recessive alleles in the population, although they are only rarely expressed. In the case of inbreeding, the probability that two recessive alleles coexist is greater. Let me clarify that with an example. Let’s take the random and nonexistent disease X as an example, assuming that this disease occurs in about 1 in 40,000 people in the population (to give you an idea, albinism occurs in 1 in 15,000 people).
– Without inbreeding this means that about 1 in 100 people carry the gene in one allele. After all, the chance that both father and mother carry the gene is then 1 in 10,000, the chance that father will pass on his one disease X allele is 1/2 and mother also has a 1/2 chance of passing on the disease X allele. to give. Together that makes a chance of 1 in 40,000.
– However, if inbreeding occurs, that number is suddenly very different. Now suppose that a woman carries the gene for disease X (this probability is 1/100). The probability of passing a gene on to both her daughter and son is 1/4, so that means 1/400 for the disease X allele. If son and daughter then interbreed, then the probability that their offspring are homozygous recessive for the disease X allele is 1/1600 (because there is a 1/4 chance that they both pass on their one disease X allele) .

Compared to the 1/40000, the 1/1600 is a big difference! That difference is why genetic disorders manifest much faster in inbreeding.

Now, in my example, I was talking about brother and sister procreating together. However, let us take a less extreme form, for example cousins. They have the same grandparent, but different parents. Using the same example as above, the probability of each of them getting the disease X allele from their grandfather is 1/16. With the same reasoning as above, you see that the chance of a homozygous recessive offspring is 1/6400. That’s four times less than brother and sister. A generation further, it would be even less: 1/25600, which is already very close to 1/40000 for non-inbred. A generation later, the effect of inbreeding in this example would no longer be statistically noticeable.

The above calculation is of course only an approximation and example, but I think you can see that it is now a logical conclusion to state that the closer two partners are genetically related, the more likely their offspring are to have certain genetic abnormalities. . The number of generations it takes to make that extra risk smaller than random increases as the condition becomes rarer. In other words, it is not possible to say: “From the umpteenth generation, there is no more extra risk”. But if you juggle some numbers as above, you will see that the extra risk becomes much smaller than the natural risk from the 4th, 5th, 6th, or in extreme cases only the 7th generation.

Regards,
Benjamin