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Name: Henry H.
Status: Other
Age: 50s
Location: N/A
Country: N/A
Date: 2002

In incestuous conceptions, are the risks of mental and physical problems for the baby different in brother/sister unions from those in father/daughter (or mother/son) unions? What sort of problems are invited by such circumstances of birth, and are the chances of their occurring statistically significant?

Dear Henry:

Brothers and sisters are, overall, more genetically similar to each other than to a parent since they have both acquired half of their genetic complement from each parent. A parent would only be genetically homologous with half of their offspring's genetic complement. However, since that half of the offspring's chromosomes would definitely be homologous with half of the parent's, whereas they would not be homologous with all of that parent's chromosomes acquired by their sibling, the risk of inheriting any individual abnormal trait would be greater in a parent/offspring cross.

A list of the potential problems could actually be almost infinite, but I cannot tell you what the most common are. The statistical probabilities also depend upon the exact genetic relationship. However, in addition to the increased possibility for directly inheriting a particular genetic disorder, the risk of inheriting any general abnormality is exacerbated by the increased overall genetic similarity. In other words, in an unrelated cross, a particular weakness, e.g. a susceptibility for cancer, can often be compensated for by stronger associated traits, e.g. increased immune surveillance, contributed by the unrelated partner. In a related cross, the lack of available genetic diversity decreases the possibilities of compensating for such genetic weaknesses.

I hope that this explanation has not been too confusing and will at least give you a general idea of the genetics behind the problems with incestuous conceptions,

Jeff Buzby, Ph.D.
Children's Hospital of Orange County
Division of Educational Programs
Argonne National Laboratory

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