Name: Barbara M.
If a man commits incest with his daughter and a child results,
then that same man fathers another child with his granddaughter,
what kind of genetic-health problems might that
child - the great-granddaughter - have?
Most of the deleterious effects will be eliminated in utero by miscarriage
is my guess, and chances are probably, better than not, that there will be
no noticeable defects in the offspring. In some cases there might be an
advantage...sounds heretical but think of what inbreeding has yielded in
other mammals. Also in some countries consanguineous marriages are quite
common with little immediate and short term problems.
Peter Faletra Ph.D.
Office of Science
Department of Energy
Humans are genetically diploid. Setting aside the special case of genes
that are located on the X chromosome, we inherit one copy of each of
these genes from our father, and one copy from our mother. Of the at
least 35,000 to 40,000 human genes, each of us, on average, carries
about 3 to 6 defective ones. But, we are rescued from disease because
we also carry a functional copy of the same gene, which we inherited
from the other parent.
By bad luck, two unrelated persons may both have defective versions of
the same gene. If these "meet up with each other" in the genome of
their child, a genetic disease may result -- there will be no "good"
copy of the gene to cover for the defect in the "bad" copy. When
inbreeding occurs, there is the additional problem of genes that are
"identical by descent." In this case, the very same defective gene may
"meet up with itself" in a later generation.
Half of the daughter's genes originated with her father. Statistically,
the granddaughter shares 3/4 of her genes with him, and the
great-granddaughter shares 7/8 of her genes with him. Accordingly, the
chances that a recessive disease-causing gene, originally contributed by
the man, will account for both copies of that gene in the
great-granddaughter are quite high.
The most likely health problems would be either mental retardation or a
metabolic disease (an "inborn error of metabolism"). The actual outcome
would depend on which defective genes were present in this particular
man to start with, and, by luck, how they segregated into the particular
egg and sperm cells that united to produce each of the offspring.
There may not BE any genetic problems. Committing incest only increases the
CHANCES of genetic disorders because they run in families. Incest increases
the chances that two people with recessive traits will mate and produce
children with the trait.
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Update: June 2012