Name: Henry H.
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?
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
you a general idea of the genetics behind the problems with incestuous
Jeff Buzby, Ph.D.
Children's Hospital of Orange County
Division of Educational Programs
Argonne National Laboratory
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Update: June 2012