MadSci Network: Cell Biology

Re: I know that inbreeding causes deformation, but with humans.............

Date: Fri Oct 15 21:22:53 1999
Posted By: Steve Mack, Post-doc/Fellow, Molecular and Cell Biology, Roche Molecular Systems
Area of science: Cell Biology
ID: 940010884.Cb

Wow Michael, what a question!

In order to answer this, we will have to first look at your underlying 
assuption that inbreeding causes deformation. What you are talking 
about when you say deformation is really a physical defect that 
results from a genetic defect. This occurs when a person has 
inherited two defective copies of a particular gene, one from each of the 
parents, and as a result that gene does not do what it is supposed to do. 
In extreme cases, babies can be born with obvious physical deformities, but 
in many cases, the resulting defects are subtle. 

Remember, in most cases, genetic defects only result in physical 
defects when a person has inherited two copies of the defective gene. 
Because the effect of a single defective copy can remain hidden, these are 
called recessive genes, and people who have a single defective copy 
are often called carriers of the disease.

So, if the actual cause of a physical defect is often the 
inheritance of two defective copies of a gene, what does inbreeding have to 
do with the process? Well, defective copies of genes usually occurr 
randomly, due to mutation. Since we have two copies of most of our genes, 
such a mutation will only change one of them, and we will still have a 
functional copy. Since each of our children gets one copy of each gene, 
each child will have a 50% chance of getting the functional copy or the 
defective copy.

If that particular gene has only mutated once, in you, then your child will 
have a 100% chance of having gotten at least 1 good gene from the other 
parent, since there is no chance that the other parent could have the 
defective gene. Like you, your children could have one functional copy and 
one defective copy of each gene.

Say you have two children, and each of them has 1 functional copy and one 
defective copy of a gene. If they have a child together, or inbreed, then 
there is a 50% chance that the child will get the defective gene from each 
of them. That means that there is a 25% chance of them having a child that 
has two defective copies of that gene, since the overall chance is 50% of 
50%. This grandchild of yours might have some sort of physical defect 
because they are missing a functional copy of the gene in question.

I think from this example, you can see what inbreeding does. It does not 
cause the formation of defective genes, but it increases the chance 
that two defective copies of the same gene could wind up in one person. 

Imagine that everyone has about 10 genes for which one of each pair is 
defective. Since these defective genes are usually different in everyone, 
the chance of you having a child with someone who shares a defective gene 
with you is low, unless you are having a child with someone related to you, 
who has a much higher chance of sharing some of those defective genes.

There are some very good examples of how this works in the real world. Some 
of them involve obvious physical defects, and some are more subtle. Some of 
them are real examples of inbreeding resulting in individuals with two 
defective copies of a gene, and some are not. Lets take a look.


Inbreeding is usually implicated as the reason for the high incidence of  
 in the so-called royal families of Europe. A disease caused by a 
in a gene that permits blood clotting (a hemophiliac can bleed to death 
from a minor cut), hemophilia is an example of an "X-Linked" disease, 
because the defective gene is on the X-chromosome. Since women have two X-
chromosomes while men only inherit one X-chromosome from their mothers, a 
man who inherits a defective copy of the hemophilia gene will suffer from 
the disease, while a woman would have to inherit two defective genes before 
she suffers from the disease.

However, if you look at the geneology of the members of various "royal" families 
Europe descended from Queen Victoria, you will notice that the most inbred 
family members on that three, the children of Queen Elizabeth II on line 
VIII, do not suffer from hemophilia. Examining this tree closely, you can 
see that there is no real relationship between the inbreeding that has gone 
on in this family and the incidence of the disease.

If there were women who were affected by the disease in this family, then 
we could blame inbreeding. They would have inherited two copies of the 
defective gene that Queen Victoria first passed on to her children.

Von WilleBrandt Disease

Like Hemophilia, Von 
WilleBrandt's Disease (vWD) is an inherited disorder that results in 
heavy bleeding when two defective copies of a gene involved in blood 
clotting have been inherited. This disease is seen in both 
people and dogs, with three distinct types being seen in three distinct 
dog breeds. Unlike Hemophilia, vWD is inherited from both parents. This 
allows us to explain why certain dog breeds tend to get this disease. In 
order to create a breed of dog, you usually inbreed dogs that have 
particular characteristics until they produce uniform offspring.

This is why all German Shepherds or Dalmatians or Doberman Pinchers look 
similar. They are inbred. If one of the parent dogs that you started your 
inbreeding efforts with had a defective gene, their offspring will have a 
high chance of inheriting two copies of that gene. In the case of vWD, the 
three dog breeds (Shelties, Scottish Terriers, and Doberman Pinschers) each 
have a different type of defective version of the gene responsible for vWD. 
Those defective genes probably came from the dogs that started those 
breeds, and all of the dogs in that breed are closely related to each other 
because they are inbred.

Other Examples

There are many other examples of this phenomenon, where someone inherits 
two copies of a defective gene, which results in a physical defect or a 
disease. One excellent example is of the case of a man with a rare form of 

Osteoporosis that had never been seen before. He had inherited two 
defective copies of a gene that controlled bone growth. His parents were 
second cousins, they had each inherited the gene from the great-grandparent 
they shared.

There have been several studies of inbreed diseases in 

Amish and Anabaptist communities, where community members are closely 
related and 
there is little genetic contribution from other communities. The frequency 
of defective genes is much higher in these situations, because, as was the 
case with the "royal" European Hemophiliacs, everyone is descended from a 
small group of ancestors.

Here are three genetic disorders that are seen in some Amish populations, 
but are extremely rare in the general population.

Glutaric Acidemia is due to a defect in a gene which produces a protein 
that destroys certain amino acids. When you have two defective copies of 
this gene, these amino acids build up, resulting in brain damage.

People with Ellis-
van Creveld syndrome suffer from a rare form of dwarfism and have extra 
fingers and toes (this condition is called polydactyly). 

McKusick-Kaufman syndrome results in another form of polydactyly 

Note that the observation of these diseases in the Amish population does 
not mean that all Amish people suffer from these diseases, or that only 
Amish people have them. For example, Ellis-van Creveld syndrome has also 
been observed in families from Mexico, Ecuador, and Brazil which are 
unrelated to each other or the Amish. This indicates that the gene for this 
disorder exists at a low frequency around the world, and that its frequency 
can increase in inbred families.

So, I hope by now that you can see that your question, "How many 
generations would take before babies got deformations, and how many more 
before they were born dead?", needs to be asked in a new way. It is not the 
number of generations of inbreeding that is responsible for genetic 
defects, it is the function of the defective gene itself.The process 
depends significantly on random chance, and the way that the genes are 
passed to offspring. 

Look at Prince 
Charles in that geneology of European Hemophiliacs. He and his brothers 
and sisters are the most inbred people on that tree and they are completely 
free of hemophilia.

I hope this helps you with your questions about the inbreeding described in 
the bible.

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