Regarding genetic disorders, a couple with no family history may have an affected child without knowing they were asymptomatic carriers. This situation is mostly common in related couples. It is estimated that each one of us carries at least one genetic disorder. Nevertheless, this does not mean suffering from them, as they are inherited with a recessive pattern. What does this mean? For an autosomal genetic disease with a recessive inheritance pattern to develop, it is necessary that each parental passes on one mutation to their offspring. Thus, people with only one mutation will never suffer from the disease. This way, when a couple randomly carries one mutation in the same gene the probability of having an affected baby is 25% (Figure 1). As much, related couples are more likely to be carriers of the same genetic disorder, since they probably share the same ancestral chromosomes. This scenario is more common in little cities or villages, where marriage between relatives were common not too long ago, just a few generations back. That is exactly what happened in the European royalty in the 19th and 20th centuries with Haemophilia B. This is a recessive disorder caused by a mutation in the X chromosome, which causes a bleeding disorder. Due to consanguinity marriages, the incidence of haemophilia B in the royal family was higher than in the rest of the population, which caused a great weakness in the royalty.