hmmm. well Im only in grade 12 and wont be going into any biological fields, but some genes are dominant while others are recessive I think the word is. (there are two genes in every pair) so anyway, Ill use one gene for overall hotness (in reality it would be many genes that contribute). But anyways, say the "hotness" gene is recessive. Her father and mother could both have the hotness gene, but as it is recessive it was overpowered by the ugly factor. So it would look like this...
U= ugly (dominant) h= hot (recessive)
so there are 4 possibilities. because the ugly gene is dominant, any pair with it proves to be ugly. The combinations are UU (Liza Minelli), two Uh's (Kerry Daughters) and the hh (your girlfriend). So, there is a 25 percent chance of her being hot, despite the fact her family is a bunch of ogres. I hope I put this in terms you can understand.
PS- in reality, they use this method to find out probabality of things like colour blindness, heart problems and various diseases.