Tag Archives: Sickle cell trait

The wages of inbreeding

Saguenay Lac St. Jean is a beautiful region of Quebec. It’s fairly isolated. Once you get to the top of the lake there is no way that you can drive farther north (no road).  We spent part of our 25th anniversary there.  The population bears a heavy load of genetic disease (through no fault of their own).

The reason is historical. Only 8,000 people emigrated from France to Quebec between 1608 and 1763. After the English victory that year  only 1,000 emigrated in the next 90 years.  In 1992, the population of the Saguenay  region was around 300,000 and Quebec itself 2,000,000.

This means that once the population began expanding with relatively little outside input, recessive genes began to meet each other, as in a large population there are so many more ways to make this happen than in a small one.

To keep the the nonBiologists reading this aboard, here is what recessive means. Our genome has 46 chromosomes.  We all have two sex chromosomes (either X and Y or X and X).  The other 44 chromosomes come in pairs.  This gives you two copies of each gene.  The classic recessive gene is that for sickle cell anemia.  If just one of the pair has the Sickle trait you are OK, if both have it, you have sickle cell anemia (which you definitely don’t want to have).  Actually if you live in Africa it is better if you have one gene with the trait as it makes you more resistant to Malaria.  This is why the trait became so common in Africans.  It’s natural selection in action (and in a human population to boot).  Just one good sickle gene (not carrying the trait) is enough to mask the effects of the bad gene, so the carrier is normal.   This is why sickle cell trait is called a recessive gene.

Here is one example.  The incidence of a muscle disease (myotonic dystrophy) worldwide is 2 – 14/100,000.  In the Saguenay region it is 189/100,000.

Even 20 years ago, the carrier frequency of many genetic disorders up there was quite high [ Proc. Natl. Acad. Sci. vol. 95 pp. 15140 – 15144 ’98 ]

Spastic ataxia 1/21

Type I tyrosinemia 1/22

Sensorimotor polyneuropathy 1/23

Pseudovitamin D deficient rickets 1/26

Cytochrome C oxidase deficiency 1/26

Cystinosis 1/39

Histidase 1/32

Lipoprotein lipase 1/43

Pyruvic kinase 1/64

Then again, there are all sorts of genetic diseases found only in this region.

Similar conditions may apply to the ancestors of today’s native Americans — for details see the previous post — https://luysii.wordpress.com/2019/07/16/the-initial-native-americans-were-quite-inbred/.  Incredible as it may sound, the rape and pillage of the conquistadores may have actually been good from a genetic point of view.  Similar considerations may apply to any pair of populations meeting each other for the first time.  Hard stuff indeed, but you can’t repeal biology.

So, from a genetic point of view, it’s good if you reproduce with someone from a different group.  It’s why I’m glad to have a Chinese daughter in law, 2 grand-nephews whose father is Hindu, and a Russian woman about to marry our nephew.