On Dec 18, 10:47 am, Mark & Steven Bornfeld
<bornfeldm...@dentaltwins.com> wrote:

And I suppose the chld would be, genetically, the child of his/her
grandparents. So the genetic and genealogic relationships would all be
one generation from each other.

I think you are correct.

Actually, they nominally share the mtDNA, too. mtDNA is inherited from
the mother, and both mothers have the same mother -- so have the same
mtDNA. If I followed you. :-)

bob

I meant, if a DNA test was run, you could say "this could be A sibling
of him/her" w/ a good degree of certainty.

Well, not really.  If you used the usual set of alleles of a few
genes, as is the current method for paternity testing, in most
cases this hypothetical person wouldn't have a pattern that could be
explained by parentage by its parent's parents.  There would be
some genes for which both alleles derive entirely from it's parent's
mother, and none from it's parent's father.

For example, suppose one person has alleles a and b for some gene
and their partner has alleles c and d.  The children have to get
one from each parent, so they can be ac, ad, bc or bd.  A child of
your hypothetical hermaphrodite's offspring can only get a choice
of two alleles, and in half the cases, it will be two copies of the
same one, so one of the grandparents would be ruled out as a parent.

Such a child would not only be closer to its parent than a sibling,
it would be twice as close as a child of two parents.  Multiple children
would be closer than normal siblings to each other on average.  Probably a
good thing such children aren't possible, since they'd be quite sickly.
IIRC, each human is estimated to be heterozygous for something like 8
lethal traits, so such a kid would have to be awfully lucky to avoid
being homozygous for any of them.

Most hermaphroditic animals also go to some length to avoid
self-fertilization.

AFAIK, the only known self-fertilizing vertebrate hermaphrodite is a
small fish, a Rivulus sp.  It has ovotestes and lays fertilized eggs.
On average, about every seven or so generations, it produces males
so there's some stirring of the gene pool.  

Time separation is a very common method of avoiding self-fertilization,
too, very popular in  invertebrates.  There are quite a few fish spp that
are one sex for the first part of their lives and then change irreversibly
to the other, including those cute little clown fish like Nemo.  It has
been observed as an anomaly in chickens -- an old hen will sometimes develop
male characteristics and in rare cases may even become a fertile male.

In self-fertilizing hermaphrodites, the amount of heterozygosity drops
by 50% each generation, so it doesn't take too long to get practically
clonal lines.  This is the case with many plants, e.g. tomatoes, so it
doesn't take long to stabilize a new cultivar.

Then we have parthenogenesis, production of non-haploid, activated eggs
without benefit of meiosis.  This is very common in invertebrates,
but also occurs in fish and lizards, where there are entire all-female
clonal species.  This usually happens when a mismating between related
species produces a viable triploid, which can't manage meiosis but is
otherwise functional, although there are other methods.  It can be a
big win in some contexts (e.g. colonizing remote islands from drifting
vegetation) but for geckos at least, it's been shown that once a
species with both sexes shows up, it tends to crowd out the all-female
species because of the aggressive defense of hunting and breeding
territories by males.  I guess this means that you only need males if
your competitors have them, as long as everything else stays the same
and you don't need to evolve. ;-)

There's a lot of science fiction, both well-written and otherwise, about
societies of humans and/or other intelligent species that have some of
the less familiar systems of reproduction found in other animals (or
plants!) on earth.  It can be amusing or thought-provoking or both.