inbreeding, why does it cause deformities?

Well, it doesn't cause them so much as make them more likely.

 

Imagine if a person has one copy of a defective allele of a gene (basically a bad copy) which is so rare that only 2% of the population carries it (has one copy) and a hundredth of a percent 0.0001 express the disease form (two copies).

 

Now, imagine this person, A, had a kid, B. This kid, because of inheritance, has a 50% chance of having the defective allele.

 

So now we have two scenarios:

 

In scenario 1, Kid B grows up and finds some random person, X, to marry. Statistically, B has a 50% chance of having the gene, while X has a 2% chance, give or take. When you work through the numbers, that means that there is only a 2.5% chance they'll have a kid who expresses the disease form.

 

In scenario 2, Kid B mates with mommy A. Because mommy A *definitely* has one copy, and kid B has a 50% chance of having one, the chances of the kid having the disease is 12.5%, 5x higher.

 

This repeats at *all* loci every time there is a mating, and given that most humans have some recessive disease alleles, this simply means that inbred kids are statistically more likely to express deleterious mutations.

 

Mokele

Right basically for the recessive defective allels you get from one parent there is a compensating working allele from the other. (And most people get a few defective ones)

 

So if your parents are the same, there is a 1/16 chance for every recessive allelle your grandparents have that you'll recieve a homologus copy of it. Now since the grandparents would be assumed to be a non-inbred that would means on average you'd probably have about

 

1/16 x (average amount of defective recessive triats per 2 non-inbred people)

 

amount of uncompinsated recessive alleles of "defective" traits

 

given that the average person has a few of these makes those pretty bad odds.

 

now for every generation up that the inbreeding occured there is an exponentially smaller chance of getting a defective recessive trait. The problem really amplifies with very close inbreeding.

 

Well it works like this till you go back close to the average amount of generations back that two non-inbred people share an ancestor, we all share one at some point.