Re: Von Willebrandt Disease

Yes indeed. Mutations occur by chance, by two basic mechanisms. In the

first a mistake occurs in the course of DNA replication. DNA replication

is the process that occurs every time a cell divides - each daughter cell

gets a copy of the DNA. The mistake occurs because the enzymes that 

performs the process aren't 100% accurate - imagine typing the complete

works of Shakespeare again and again, and very fast and making a mistake

only ince every few pages. The other way mutations occur is that the DNA is 

damaged, and then not properly repaired. Imagine spilling coffee on the

original that you are typing from, and then not being able to read a word

or two. Both of these mechanisms are essentially based on chance. However, 

they can become much more problematic in disease processes: for example

UV in sunlight increases the rate of damage to DNA and so the rate of 

mutation.



New incidences of such diseases occur all the time, though at a low frequency.

For example, the hemophilia that afflicted the Tsarevitch treated by

Rasputin can be traced back to Queen Victoria, who probably suffered

the original mutation.



Once a mutation like the one in the vWF that causes vWD occurs, it is often

recessive. Most organisms have two copies of their genome - one from mom

and one from dad. Therefore, most of the time a good copy of a given gene

can compensate for a mutated one. The mutant gene makes either no protein

or defective protein - but the good gene makes normal protein and that

is enough for the animal to live. But if you now inbreed animals, the 

frequency of the mutant gene in the gene pool increases. Therefore the

chance that an animal gets a mutant gene from both parents increases. Such

animals have vWD, because they have no normal gene for vWF. So inbreeding

does not cause vWD as such - it unmasks it however.