wizard geneology - Genius or Baloney? (moved from main list)

[kkersey_austin]

Steve proposes a genetic model for magical inheritance here:
http://groups.yahoo.com/group/HPforGrownups/message/148089

I'm moving discussion over here to OT, since my post doesn't have much
canon relevance...

Steve, I'm having a bit of trouble with your notation and terminology,
but I think what you are proposing is a set of four distinct genes
that determine magical ability - each gene has two variations
(alleles): e.g. A and a, B and b, with capital letters designating the
"Magic" allele and lowercase designating "muggle" alleles.

If that's the case, each individual should have two copies of each
gene, one from each parent (with the exception of any genes that are
located on the y chromosome, in which case males get only one copy -
from their father; IMO there is no evidence for any magical abilities
being sex-linked so let's forget that as a possibility). 

Now, under your model, if I understand correctly, a sqib/muggle would
have aabbccdd genes. If someone has any magical allele, they are Magic
- e.g. aAbbccdd, aaBbccdd, AABBCCDD, or whatever, as long as there's a
capital letter in there somewhere.

The problem I see with this model is that squibs would not be rare at
all in the case where a witch or wizard marries a muggle. For
instance, for  AaBbccdd x aabbccdd, the possibilities are:

AaBbccdd
aaBbccdd
Aabbccdd
aabbccdd

That's a 25% chance of a squib. If the magic parent only has one Magic
gene, and is heterozygous for it, say Aabbccdd, the chances of having
a squib offspring is 50%! 

On the other hand, if the magic parent has more Magic alleles in their
genetic arsenal, the chances of having a squib offspring do go down
precipitously; in particular anyone who is homozygous for one of the
magic genes would be guaranteed to have no squib offspring. barring
mutation, AAbbccdd x aabbccdd can only have Aabbccdd offspring. 

AaBbccdd x aabbCcDd would have a 25% chance of producing aabbccdd
offspring (i.e. a squib).

All that assuming no mutation, of course. And assuming that each gene
follows a simple dominant/recessive pattern. And that is not taking
into account that the genes in question may be on the same chromosome,
in which case they wouldn't be independently inherited. In real life,
of course, things are rarely that simple... For example, some genes
can suppress other genes; the DNA molecule is mostly twisted up in a
knot and has to be straightened out and partially unzipped along the
stretch where a particular gene is in order for it to be expressed. So
in many cases the dominant allele isn't expressed even though it is in
there. One set of genes may interfere with or compensate for the
expression of others. Then there are all the enviromental interactions
(e.g. hormones and nutrition during gestation or growth) that might
affect the expression of certain genes. 

One more thing:
bboyminn:
> Unless I'm mistaken the DNA chain splits in half length-wise, and half
> of your mother's combines with half of your fathers. That is all of
> your fathers 'halves' combine with all of your mother's halves, and
> the combination creates a complete DNA ladder. When two specific DNA
> halves mate, they create the genetic characteristic of blue eyes,
> brown eyes, prone to cancer, or whatever.
>
Elisabet responds:
The familiar picture of a DNA ladder you cite is of a *single* DNA
molecule - when it divides by splitting down the middle that is the
process of replicating, that is, making an exact (hopefully!) copy of
itself. Each chromosome contains *two* DNA molecules, one from each
parent, each with its own copy of each gene. (Actually, it is a bit
more complicated than that, e.g. the y chromosome, and the fact that
the two strands can break and recombine during the process.)

In short, it's not two DNA halves mating, it's two complete DNA
molecules mating. 

One of the lovely things about genetics is that expression of traits
rarely can be analysed simply, so JKR can get away with waving her
hands and saying that there's a gene for magic and then she can make
it work however she wants without worrying about "the maths". :-)

Elisabet, who hasn't stopped by here for a while and is finding all
sorts of interesting discussions!