Expert: John Locke - 1/2/2007

Question
I recently returned from a trip to Hawaii, while on an excursion of Maui, the tour guide told us the royal families had kept their bloodlines pure by brothers and sisters marrying each other for 45 generations or until they were Christianized by the missionaries. What dangers would there be in such a practice? I know the European royals use to marry their cousins, however this really seemed bizarre. Would there not be some serious genetic defects?

Answer
Thanks for using AllExperts, Kay. The answer is very simply, absolutely, the practice is dangerous and contributes to higher incidences of a variety of genetic diseases.

Let me explain why: one of the strengths of sexual reproduction in organisms is the variation that develops in their genetics. Every time a child is produced, half of that child's genetic material comes from their father and half from their mother--twenty-three chromosomes from each. Each chromosome contains specified units called genes; the DNA code of these genes is read and transcribed so that a protein can be produced from it, and these proteins go on to perform essential cellular functions. This is the fundamental responsibility of DNA in every organism: to provide the genetic code for protein manufacture.

Since half of the genetic code comes from each parent, you've probably figured out that we have two copies of each gene--one received from mom and one from dad (the only exception to this is the sex chromosomes in men, where there is an unpaired X and Y chromosome). Now, many traits are controlled by the interaction of multiple genes (eye color, hair color, height, and skin color, to name a few); however, many genetic diseases are controlled by a single gene. Cystic fibrosis, Huntington's chorea, and sickle-cell anemia are examples of these; these types of diseases may be dominant (requires only one copy of the diseased gene to cause the disease) or recessive (requires two diseased genes copies to cause the disease).

Beyond that, a number of genetic diseases may be carried as  on the X chromosome. As noted above, men have one X and one Y chromosome; if a man receives a diseased X chromosome, he will display the genetic disease. For women, however, the process works as described earlier: if the disease is recessive, both of the gene copies--called alleles--must be diseased for the disease to be present. If the disease is dominant, one affected allele is enough to cause the disease. Hemophilia is carried in this way, most famously between the royal families of Europe in the 19th century due to intermarriage between cousins!

This is a bit of a simplification; there are other ways for genetic diseases to be transmitted, and some are controlled by multiple genes. Having too many chromosomes can also result in a genetic disease (Down's syndrome is caused in this way), while the chromosomes themselves can break or be damaged, also causing a genetic disease. I'm going to stick with the first two examples, however, generally referred to as autosomal and sex-linked genetic diseases.

The danger of incestuous marriage is that it increases both of these types of diseases. It reduces the available gene pool, so that any existing genetic disease alleles will become more common. Recessive disease alleles get paired up more often, and dominant disease alleles spread more quickly, than in the wider population. Take the example of recessive, autosomal disease: the likelihood of a carrier (a person with one dominant and one recessive allele; this person does not carry the disease) matching up with another carrier or a person with the disease from the population at random is fairly low. However, marriage between a brother and a sister who are both carriers carries a 25% risk of producing a child with the disease. Marriage between an affected person and their carrier sibling carries a 50% chance of producing an infected child.

This analysis doesn't even consider what happens after a single generation; more intermarriage reduces the gene pool even further, and the effect of 45 (!) generations practicing incestuous marriage would be severe. The concept of a "pure" bloodline, as referenced by your guide, is a misperception: the way to maintain a robust bloodline is intermarriage between unrelated spouses. An additional complication is that closely-related individuals are more likely to produce embryos that spontaneously abort, for reasons that I won't go into here.

Here's a website that explains the concept as it applies to fish, for whom exactly the same genetic principles apply:

http://www.nwfsc.noaa.gov/publications/techmemos/tm30/lynch.html

The moral of the story is, as your intuition told you, this practice is a dangerous one.