Re: If one were to marry within their own blood line how many generations

If we assume all humans today descended from a common ancestor, we are all inbreeding to a certain extent; it's just very, very distant! Almost all human societies have an incest taboo; but there is variation in the legal limits placed on marriages within families. For instance, in the US, first cousins can legally marry in some states but not others.

From a genetics point of view, the reason to discourage inbreeding is to prevent undesirable genetics disorders. Genetic mutations occur in almost everybody, but the vast majority do not create any medical problems because the are recessive. Recall that most genes code for a protein of some sort, and that for most of our genes, we have two copies, one from each parent (an exception are the X-linked genes, for which males have only one copy). Thus, if a mutation occurs in one gene, there is a back-up copy. A person would only show the effects of missing the protein entirely if both gene copies carried the mutation. This is an example of a recessive trait.

Let's say a person carried a mutation for a recessive disease, such as sickle-cell anemia (which is caused by a mutation in the gene coding for a hemoglobin protein.) This person would not have the disease, but each of his/her children would have a 50% chance of inheriting the mutant gene and also being a carrier. For a grandchild to have the mutant gene, the parent must first have it (50% chance) and then the grandchild must inherit it from the parent (50% chance), so the odds of a grandchild having the mutant gene are 0.5 x 0.5 =0.25.

The odds of a great-grandchild inheriting the mutation are 0.5 x 0.5 x 0.5= 0.125.

In order for a child to inherit two copies of the mutant gene (in other words, be homozygous for the mutation) and therefore have the disease, both parents must be carriers. The odds of two children of a carrier both being carriers are 0.5 x 0.5 = 0.25. If these carriers marry, their chances of having an affected child are 0.25. Therefore, the chances of an incestuous union between two children of a sickle cell anemia carrier giving a child with sickle cell anemia are 0.25 x 0.25 = .0625, or 6.25%.

The chances of two grandchildren both being carriers are 0.25 x 0.25 = .0625. Thus, if two first cousins marry, the odds of sickle cell anemia are .0625 x .25 = .0156, or 1.56%.

The chances of two great-grandchildren both being carriers are 0.125 x 0.125 =.0156. Thus, if two second cousins marry, the odds of sickle cell anemia are .0156 x .25 = .0039, or 0.39%. This is about 1 chance in 250, as opposed to the sibling marriage, which was 1 chance in 16. As you can see, the more generations removed you are, the less your chances are for inheriting two copies of a recessive gene. How close we define "inbreeding" then, is rather arbitrary.

But remember, these are the odds only for one gene. Our hypothetical ancestor may carry any number of recessive genes that might be dangerous in the homozygous form.

One recent example of this: In 1994, the New England Journal of Medicine reported about a young man with severe osteoporosis who had never stopped growing. It turned out he was homozygous for a mutated estrogen receptor (the protein to which estrogen must bind to have its effects); and therefore entirely insensitive to estrogen; estrogen could not act to halt body growth or strengthen his bones. This condition had never before been reported in humans. This man's parents were second cousins, and both had inherited the mutant gene from their common great- grandparent. So, despite the slim odds, this man ended up with the homozygous recessive condition. The odds he would inherit estrogen insensitivity were slim (1 in 250) but much higher than in the general population, in which two completely unrelated individuals would have to carry a mutation in the same gene. Mutations in the estrogen receptor are apparently quite rare, so the odds of this are likely to be 1 in thousands or millions.

It should also be noted that genetics are not the only reasons for an incest taboo... close relatives are forbidden to marry in almost all human socities, and this prohibition predates any knowledge of genetics. In some cultures, first and second cousin marriages are the normal, though almost all prohibit marriage between siblings, or parents and children. But some encourage marriage to cousins on the mother's side while forbidding marriage with paternal cousins (or vice-versa); genetically there is no difference in terms of the risk of disease. Also, many animals, particularly social primates, avoid mating with close relatives. There are probably a number of cultural and biological factors that discourage inbreeding.

As for why cats and dogs can't interbreed, the answer lies in chromosome compatability. This site will show you how chromosomes must line up during cell division. For a viable animal to be conceived, an egg must fertilize a sperm with the same number of chromosomes, and the chromosomes must be homologous enough to pair up. Distantly related animals often have different numbers of chromosomes, and the egg can't be fertilized. Other, more closely related species (like donkeys and horses) have the same number of chromosomes, and can interbreed to produce offspring (a mule). But the chromosomes do not match well enough to pair up during meiosis (the process that creates eggs and sperm) so the offspring are sterile.