Wizarding genetics

[Catlady (Rita Prince Winston)]

No: HPFGUIDX 180647

Susan McGee wrote in
<http://groups.yahoo.com/group/HPforGrownups/message/180612>:

<< And a squib of course is a muggle. But a squib differs from some
muggles because a squib would have been born into family where one of
the parents was a witch or a wizard. >>

Carol replied in
<http://groups.yahoo.com/group/HPforGrownups/message/180625>:

<< Actually, that's not quite accurate. A Muggle is a a person with no
magical powers and no (known) magical ancestors. (Evidently, genes
from those unknown magical ancestors are sometimes activated in Muggle
offspring, producing Muggle-born witches and wizards, but as JKR's
attempts to explain this phenomenon are all off-page and her knowledge
of genetics is apparently limited, I won't go there.)

A Squib, OTOH, has, as you say, either one or two magical parents, and
is therefore *not* a Muggle. He or she is a Pure-Blood or Half-Blood
who somehow fails to develop magical powers. >>

Susan McGee replied in
<http://groups.yahoo.com/group/HPforGrownups/message/180634>:

<< I think that pure-blood or half-blood are political constructs, not
genetic categories (see my prior post referencing JKR interview about
this)...and there doesn't seem to be any correlation between intensity
magical powers and being half-blood, pure blood, or muggleborn, so I
wouldn't describe a squib as pure-blood or half blood...so would
someone be considered a squib if one parent was a muggle, and the
other parent was a witch or wizard? Or do both parents have to be
magical for a non-magical person to be considered a Squib (I would
guess that this is the case). >>

Parts of this can work just fine with realiverse genetics. Let the
do-magic gene be recessive (m). Then people who are homozygous
dominant will be Muggles (MM) and people who are heterozygous will be
Muggles (Mm). In many cases, recessive genes don't just go into
stasis, so it's quite possible that heterozygous Muggles will be more
psychic or more imaginative or smell slightly different than
homozygous Muggles, something to make them more romantically
attractive to wizarding folk than the homozygous Muggles are.

Statistically, a large group of offspring who each come from two
heterozygous parents will be half heterozygous, one quarter homozygous
dominant, and one quarter homozygous recessive. The homozygous
recessive show a trait that perhaps none of their ancestors have shown
for a century or two. Randomness often makes small groups have
different proportions than the large group, so just because the
Creeveys (obviously both Mm) had two wizarding (mm) sons doesn't mean
they necessarily had 6 Muggle children as well. 

Even in a large group, the numbers can be skewed if one phenotype is
more likely to die off -- I recall crossing the white-eyed fruit flies
with the heterozgous red-eyed fruit flies to get an F1 generation that
was about one-third white-eyed, not the one-half asserted by the
textbook. The teaching assistants said that was fine, it's just that
white-eyed fruit flies don't survive very well. So it could possibly
be that magic is a high-survival phenotype so Mm x Mm crosses will
skew to more than one-quarter magic children.

Then all wizards and witches are homozygous recessive (mm) so all
children of a wizard and a witch will also be homozygous mm. Pureblood
and Halfblood and Muggleborn are a statement of genealogy, not of
genes (which agrees with both Susan and Carol). A Muggleborn or
literal half-Muggle has no higher probability than a pureblood of
having offspring who have the M gene.

A Squib is a person who is homozygous for magic but nonetheless can't
do magic. It's like a birth defect -- something that happens to the
body that wasn't caused by genes. (I have a theory, below, about the
cause of the birth defect.) I think that Squibs, at least Squibs with
two wizarding parents, are very rare (Ron in CoS: "Kind of the
opposite of Muggle-born wizards, but Squibs are quite unusual."). I
think there might be one or two per generation.

The child of one magical parent (mm) and one Muggle parent (Mm) could
be magic (mm) or Muggle (Mm). If it were Mm, it would be a Muggle, not
a Squib. If it were mm, it would normally be a witch or wizard, but if
it had the birth defect that made it unable to do magic, it would be a
Squib, but the wizarding community would *call* it a Muggle, because
that's how they are. 

The child of Mm and Mm could also be a mm Squib, but would definitely
be called a Muggle by wizarding folk.

It seems that usually the child of an Mm and an mm is an mm. When the
mother is mm, this can be explained by her body giving a disadvantage
to the M sperm. I don't have a pseudo-scientific explanation why it
would skew magic when the mother is the Muggle.

There's probably a lot of other genes giving increments of magic power
intensity at various kinds of magic, so one could statistically expect
that the child of two parents with strong magic powers would have
stronger magic power than the child of two parents with weak magic
powers. But Muggles, even MM Muggles, could be carrying the genes for
very strong magic power in many flavors and be just as non-magical
because they're not mm. 

Part of my theory is not scientific at all -- it is that the magic
takes an active role in its own perpetuation. When a wizard or witch
dies, the magic sees to it that another magic person is born at that
time -- I used to say that the dead person's magic finds a new person,
but I don't know that it's quite so specific. The magic will settle
into the closest most suitable newborn; having the mm gene pair is
part of being suitable. Being close to magic (and it's hard to be much
closer than inside a witch's womb) increases suitability. Perhaps
those hypothetical other genes that give one's magic its strength are
part of being suitable. That could account for the rate of very strong
magic being higher among Muggle-borns than among other wizarding folk.

I like this part because it makes the Voldemort Reign of Terror so
counterproductive. During the years when many wizards and witches were
being killed, their magic was going to newborns. Even if the wizarding
folk didn't postpone having children while they were afraid for their
lives, their normal rate of childbearing isn't enough to provide for
all those deaths, so the Death Eater attempts to eliminate Muggleborns
actively increased the number of Muggleborns.

Listies used to suggest that the wizarding folk had a baby boom after
the first defeat of LV. A wizarding baby boom without a simultaneous
wizarding death boom will result in an unusually large number of
Squibs, another result that the DEs didn't want. But it's only
superstition that makes the wizarding folk think that Squibs are more
likely than any other mm to have Squib children.