Why do Muggles have Wizard Children?

[ceefax2002]

No: HPFGUIDX 69464

--- In HPforGrownups at yahoogroups.com, "Ximena Valdivia"
<xvaldivia at y...> wrote:
> --- In HPforGrownups at yahoogroups.com, "Steven" <sngoing at y...> wrote:
> > I have a theory that for Muggle families to have wizardlings, they 
> > must have had an ancestor who was a witch or wizard. This might 
> give 
> > something to the theory that Lily is Harry's connection to 
> > Gryffindor. 
> > 
> > "Steven"
> 
> I'm not so sure considering that in the books, more than one student 
> have stated that they are the first wizard in the family. I believe 
> Hermione and Collin said that (I'm not so sure though) unless that 
> the ancestors had hide it to the rest of their family that they were 
> wizards. I imagine someone like Hermione as soon as she got her 
> letter will have been researching like crazy in her family tree if 
> there was someone that was a wizard.
> 
> Of course, that's just my thought
> 
> Ximena

Speaking as someone who wasting a considerable portion of her life
studying Biology...

If we assume magical ability is a recessive trait (which we pretty
much have to, given the ratio of muggles to wizards), then for a child
to be born a wizard, BOTH their parents will have to carry the
resessive magical genes. However, this doesn't mean that their parents
would have wizards (um... a wizard and a witch. You know what I mean),
or that thier parents would have wizards, they just have to have been
carrying the genes. For (a very simplified) example:

Parent A (let's call her Mrs Granger) has the genotype M m (where M is
Muggle and m is Wizard), meaning that she doesn't display magical
ability herself.

Parent B (let's call him Mr Granger) also has the genotype M m.

Their children could either be: M M (completely Muggley - doesn't
carry the magical genes, will never have wizard childen); M m (no
magical ability, but still carrying the magical genes); or m m (let's
call her Hermione - with no dominant Muggle genes, no her recessive
magical genes will be expressed).

On the other hand, parent C (let's call him Lucius) with a genotype of
m m, and parent D (Narcissa) also m m, would have nothing but m m
children, with nothing to override the recessive genes.

Therefore magical genes could be passed down through a muggle family
for generations, but unless one of this family had kids with someone
else who was carrying the magical genes, they wouldn't have wizard
kids. And even then, they'd only have one chance in four.

(It's been a while and I burnt all the notes when I passed - if I got
any of that wrong I'm sorry!)

Ceef