Why is interesting that? So, for example, if you share more of your genome, you have more tendency of having complex diseases, diseases for which there are lots of genes that may give you a higher probability. In this case, you have an interesting plot of the likelihood, the probability of having schizophrenia according to the genes you share. We know that monozygotic twins they share all the genome, but they do not share always the phenotype of being schizophrenic. This means that environment is important for that. And you have the decreasing probability, till the 1% general to a population, according to the amount of genome share by them. Also, culturally, it has been interesting to recognize degrees.
And, for example, many religions and many laws, very clearly they allow only marriages given a small degree of relationship. And this is a way of calculating this degree of relationships. In general we could say that, traditionally, it has been in Western countries it has been forbidden always the marriages till first cousins, in some cases it has been allowed the uncle niece, but this is very rare but first cousins is important because in the first cousins you have enough far away but being always clearly within the family. And we can see, between a given individual and the expanding family, the amount of sharing or the degree of relationship we may have.
If you marry someone sharing the genome this is what we call, inbreeding. At the very end inbreeding is this tendency of being married to someone that really has in common parts of the genome, and it is important, because as we may have the two people marrying may have some genes coming from the same ancestor, it may be that in their descendants, the children are homozygotes for rare diseases. It is interesting to recognize that the amount of genetic diseases, will be clearly increased and this is an example, a clear example of a first cousin marriage in which you have several children having a given disease, given because they are homozygotes for a given deleterious trait.
So, the consanguinity is something that exists in some populations and it has been clearly regulated in most, if not all, human cultures and, even though in Western countries it has decreased in the last century, still now there are populations having a high amount of first cousins marriage mainly in the Middle East and North Africa. In these cases it has been studied, very exhaustively, to see the importance of that because the children will show up some of the diseases that otherwise would remain totally unknown.
And, maybe one of the famous cases is in the royal family of Spain, in which Charles the II, was the last in the dynasty that was really extremely inbred, with high consanguinity, and the facial traits, the prognathism of his face but many other diseases, anomalies, were at the base at the cause of the extinction of the family. What is the chance of having children with problems because of being relatives? We can give a general number, in which this is increased around twice. Let’s say, if the general probability of a child having some problem at birth is around two, three percent, the children of the first cousin marriages in general is higher than that, is around five to six percent.
And it is likely that, this is the base for which in many societies this has been strongly regulated, but we can also ask to each extent the behavior of avoiding mating with people in your family, or people with who you share your genome, also exists in other species.