Phasing - Re: Leah, Billy, Jacob
[Why Leah didn't phase when the Cullens first returned, was Billy Black ever a wolf and why Jacob phased after the Cullens left]
In the Twilight universe, some members of the Quileute Tribe can 'phase' into werewolves under a few conditions:
They must be a direct descendant of Taha Aki, the first Quileute to phase (early 13th century). These descendants "are born with twenty-four chromosomal pairs rather than the twenty-three pairs of a normal human"
"This extra chromosome will not have any effect on the carrier if he or she is not in close proximity to vampires during all or part of the critical time between the onset of puberty and the age of twenty-five. If the potential werewolf is not exposed to vampires within this window of time, transformation will never occur. It is the scent of the vampires that triggers the reaction.."
When in close proximity to vampires, it is always the youngest in each family's lineage to phase, becoming a "protector" against the vampires
If the youngest member in a given family's lineage is not between the onset of puberty and 25, they will not phase and neither will their parents/elders
(It is "based on two factors: heritage and the proximity of vampires")
The other factors worth considering are:
"One factor affecting which members of the tribe transform is the number of vampire scents in the area"
"... the pack has usually consisted of three members. With most vampires traveling alone or in pairs, there was never a need for more..."
Members with the strongest blood ties phase first. As more vampires are present, tribe members with weaker blood ties start to phase
Until 2006, only male carriers of the gene had gained the ability to transform
The Cullens first moved to Hoquiam (aprx. 100 miles from La Push) in 1936. At this point, the confirmed "protectors" are Ephraim Black, Quil Ateara II and Levi Uley. Tracing their lineages it is probable that none of them had children that had reached transformation age yet. Billy Black was not born until the late 1950's so he obviously couldn't have phased here. However, the Cullens stayed in the area until 2003 when they moved to Forks (only ~16 miles from the Quileutes in La Push). Though Ephraim Black was likely deceased by this time, his grandson Billy was well past 25 (about 50) and Jacob Black was born but was only 13, meaning neither of them could transform (cond. 2).
Let's put a pin in that to quickly talk about Jacob. It makes sense that he didn't phase in 2003 when the Cullens arrived in Forks because he was only 13. But the Cullens left Forks in September of 2005 and Jacob didn't phase until February, 2006. There were only 2 vampires present in Forks when Jacob first phased, Laurent and Victoria. The Cullen family consists of 7 vampires, so why didn't he transform while they were around? Well, the transformation had actually already started before they left. The first sign of transformation in male gene carriers is a "noticeable growth spurt". When Jacob goes to see Bella at the prom in May, 2005 she comments on how much Jacob has grown.
(Twilight, Epilogue)
So this leaves me with one question unanswered. Going back to 2003, when the Cullens moved to Forks, the only Quileute who phased was Sam Uley, who was 17 at the time. But so was Leah Clearwater. So why didn't she also phase? At this point in time, there had never been a female werewolf, but Leah did eventually become the first around March of 2006, also after the Cullens had left. We can assume that the transformation had began before they left, just like Jacob. So the question is:
Why didn't Leah phase when Sam did in 2003?
There is no definitive answer to this that I can find. The real answer is "Smeyer didn't think that far" but, in universe, it seems like female werewolves were supposed to only be possible under exceptional circumstances. Perhaps, it was supposed to be the passing of her father that triggered her transformation, but we can rule that out because, when she first phased, "She was arguing with her mother about her mood swings, and her father joined the conversation in support of her mother. Leah was so angry that her whole body started shaking, and then she exploded into a werewolf...". So if Smeyer's intended reasoning is "exceptional circumstances", she either forgot or there just isn't a real reason, at least not one that I could find.
- Edited for corrections
(Acc. to the official illustrated guide - sections "Werewolves" pg. 298+ and "Timeline" pg. 404+, Twilight and New Moon)
