Half-blood Squibs? / Weasley cousin

[mullsym255]

No: HPFGUIDX 40382

I was just recently re-reading SS and I got to thinking about Seamus' 
comments about his muggle father and witch mother. Is wizarding blood 
considered dominant? That is, if you were a half blood (and not a 
half-blood like Harry might be considered, I mean like Seamus, one 
wizarding parent, one not) and you were born without magical 
abilities, would you be considered a squib? Or just a muggle who 
happens to have a witch for a mother or a wizard for a father?  

And another thing that has always bothered me.. forgive me, because I 
lent my books to a friend, but I believe Molly or perhaps Ron makes 
reference to a cousin who was a squib and ends up being an 
accountant? And that the family "doesn't speak of him". Why is this? 
The Weasleys strike me as incredibly tolerant and accepting people 
and Arthur obviously seems to have a fondness for muggles. So why 
would they shun a member of the family who happened to be a squib? Or 
is it that this cousin chose to exclude himself from the family? 

-Tracy