Question: are half bloods allowed in slytherin?
Answer: There have also been definite examples of half-bloods sorted into the house, including Tom Riddle, Dolores Umbridge, and Severus Snape. Harry Potter was nearly sorted into Slytherin, but because he asked not to be, he was placed in Gryffindor instead. Also, pure-bloods are not automatically placed in Slytherin.