Why hasn't my pullet got a tail?

[Sussex19]

Well, I think she's started laying, it's a bit hard to tell but I'm pretty sure some of the eggs are hers. No change in the tail!

 

[Coops Dad]

This is a pics of one of our three week old Ameraucana chicks. Of its 8 siblings, this one has no tail and is 2/3 the size of the others. There's quite a bit of variance in the way living things manifest the info coded into their DNA.