Unions contracted between persons biologically related as second cousins or closer (F ≥ 0.0156) are categorized as consanguineous. It is very confusing whether children of consanguineous marriages are genetically predisposed to any developmental defects. In my previous article ‘Human Inbreeding: What are the consequences?’ I tried to explain the deleterious effects inbreeding may cause. The articles I read at that time were not very conclusive whether inbreeding is beneficial or deleterious. So, I decided to dig up some more scientific journals to find out the effects of inbreeding.
One article I found, reasserted the former argument that inbreeding does not have any effect on fertility (spontaneous abortions and stillbirths) of women. However, higher rates of neonatal and post-neonatal deaths, and deaths of children younger than 5 years were observed in consanguineous couples. I then thought, does inbreeding affect the fertility of men? A study involving Middle-Eastern Males demonstrated that consanguinity and the familial clustering increased the chances of having infertility problems 2.6 times higher in cases than controls. So, I am thinking even though inbreeding does not cause any aberrant pregnancies, inbreeding does have adverse effects on male fertility and on survival of newborn children.
I have also wondered whether inbreeding plays a role in other diseases. While searching for answers to satisfy my curiosity, I found that researchers who have looked at Middle East and North Africa region have pointed out that in the Islamic Arabs residing in these regions, there is a high prevalence of first-cousin consanguinity. Hemoglobin disorders, inherited metabolic disorders, neurogenetic disorders, and birth defects are also relatively common in this part of the world. One can thus ask a question that may be these diseases can be credited to, at least in part to the inbreeding?
Also, medical professionals have found “significant association between parental consanguinity and congenital hypothyroidism.” Congenital hypothyroidism seems to correlate with the degree of consanguinity—children of first cousins are more prone to than those of second cousins.
Amid all the confusions, it is very well accepted that among genetic disorders, only autosomal recessive disorders are strongly associated with consanguinity. However, there are ample of studies showing that in a population with a high rate of consanguinity, there is a significant increase in the prevalence of common adult diseases like mental disorders, heart diseases, gastro-intestinal disorders, hypertension, hearing deficit, and cancer. Interestingly though, a study in Arab populations, however demonstrated that reduction of overall cancer risk was associated with increased coefficient of inbreeding (F). Both older and younger healthy men had significantly higher F but only women over 30 years of age with higher F had reduction of overall cancer risk. This study showed that being more inbred was associated with a reduction in overall cancer risk by about 25%.
I am not sure what to make of the relationship between inbreeding and cancer but I am certain that in breeding may have adverse affects in humans and it should not be encouraged.
Examples of inbreeding in humans
Probability of inbreeding
Kobeissi and Inhorn; Health issues in the Arab American community. Male infertility in Lebanon: a case-controlled study.
Kerkeni et. al.; Interplay of socio-economic factors, consanguinity, fertility, and offspring mortality in Monastir, Tunisia.
Saadallah and Rashed; Newborn screening: experiences in the Middle East and North Africa.
Hashemipour, et. al.; Parental consanguinity among parents of neonates with congenital hypothyroidism in Isfahan.
Hamamy, et. al.; Consanguinity and genetic disorders. Profile from Jordan.
Bener, et. al.; Consanguineous marriages and their effects on common adult diseases: studies from an endogamous population.
Denic et. al.; Risk of cancer in an inbred population.