sentence2_structure: Mammalian sex determination<>is determined genetically<>by the presence of chromosomes$$$chromosomes<>identified<>by the letters
sentence1: Sex chromosomes (called the X and Y chromosomes) contain genes for sex determination and other traits unrelated to sex determination.
sentence2: Mammalian sex determination is determined genetically by the presence of chromosomes identified by the letters x and y.
question: Mammalian sex determination is determined genetically by the presence of chromosomes identified by what letters?