sentence2_structure: Mammalian sex determination<>is determined genetically<>by the presence of chromosomes$$$chromosomes<>identified<>by the letters
sentence1: The X and Y chromosomes in human beings that determine the sex of an individual.
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?