|MadSci Network: Cell Biology|
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. Hemophilia Inbreeding is usually implicated as the reason for the high incidence of hemophilia in the so-called royal families of Europe. A disease caused by a defect 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 of 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 (hydrometrocolpos-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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