When Joan is going down to the basement of the brownstone to tell Sherlock that Baskerville's alibis confirm his whereabouts she comes down the stairs. She had previously nailed that door shut so they would have to go around the back way to enter the basement.
Sherlock says a chimera is a creature with the DNA from two different species. This is incorrect.
A chimera in mythology was a creature with parts of at least two different species, eg. sphinx - human head, feline body. Gryphons- half eagle, half lion. Manticore - man's head, lion's body, tail of a scorpion.
But in science and reality, a chimera is a creature that possesses the cells (therefor the DNA) of one, or more, other creatures. Since different species can't mix in nature, a chimera possesses only the DNA of its own species, eg. heterochromia, two different coloured eyes can mean the cells began in two separate animals (including humans). Conjoined twins are technically chimeras. As are women who have been pregnant as traces of fetal DNA remains in their bodies.