sentence2_structure: Mammalian sex determination<>is determined genetically<>by the presence of chromosomes$$$chromosomes<>identified<>by the letters
sentence1: The sex of most mammals, including humans, is genetically determined by the XY sex-determination system where males have X and Y (as opposed to X and X) sex chromosomes.
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?