GENE GENIE
Geneticist and broadcaster Dr Adam Rutherford on the science of inbreeding
What happens genetically when offspring are produced from two closely related individuals?
We have two sets of all our genes – one set inherited from your mother, the other from your father. Frequently only one of each gene will be actively expressed and some diseases require both versions of the gene to be the same, and these characteristics are called recessive. For example for a person to have ginger hair they need to have inherited a ginger hair gene from both parents. For recessive diseases, you have to have two copies of the defective gene to actually have the disease, which means that a person can have one copy and not show any symptoms – a carrier. The more closely related to people are the more likely their children will inherit the same versions of genes from each parent, which increases the chances of a recessive disease being expressed. The metric for this is called the inbreeding coefficient, basically, a measure of the proportion of each set of genes that are the same from each parent. If a brother and sister had a child together their inbreeding coefficient would be 0.25, that is, 25 per cent of their genes are the same from both parents.
How did scientists and historians determine that many of Charles II’S conditions were likely to have been caused by incest?
With Charles II of Spain, his inbreeding coefficient was calculated using his family tree back through more than ten generations, and it came out as 0.254 – he was more inbred than the child of a brother and sister. With that level of inbreeding, it’s not really possible to isolate any specific disease, as there would have been many conditions that emerged in him. It has been suggested that combined pituitary hormone deficiency fits many of Charles’s symptoms, and is a recessive disorder occurs when you have two copies of a mutated PROP1 gene.
How common was inbreeding in history?
As for how inbred humankind is, the answer is very, but mostly not enough for it to be problematic, most of the time. Pedigree collapse is what happens over a long enough time of consistent inbreeding, where offspring become inviable. In the Habsburgs, it happened over seven generations because of the cumulative effect of two centuries of inbreeding.