Outdoor plant yellowing only 1 week into flower.

Hey buds!
So I've got this plant that just started flowering last week.
Fan leaves started turning yellow almost immediately. I removed a bunch of them the other day so I could gauge if it was progressing. Yes it seems to be progressing. But..., all the new growth is looking healthy.
Her sister (seeds from the same plant) is about 6ft tall and healthy as a mutha. They have identical soil, sunlight, water and nutes. Is it possible that you can get two different strains from seeds from the same plant?
Any thoughts would be appreciated. Thanks in advance.
Here's the yellow girl:


And... here is her sister: (she was topped) :

Lacks Nitrogen ?

feed it!!!!!!!!!!!!!!!!!!!!!!!! N cal mag yesterday

I did give it a booster of N and magnesium today. Hoping it helps.
It seems weird to me that two seeds from the same plant would come out to be such different plants.
Is that common?

yes http://en.wikipedia.org/wiki/Phenotype

I have two White Widows growing side by side, they look identical except one is about half the size of the other. Same everything. If you've ever made clones in a mister, you can get a good idea how variable some things can be, in seemingly identical cuts. Some will put out a few large tap roots, while others will put out a clump of fine feeder roots. Such variation can be seen in how well plants will uptake nutrients. Do the best you can and let nature take its course.