Heterochromosomes are chromosomes that differ in size and shape from other chromosomes in the cell. They are often associated with gender determination in some species, with females having two different heterochromosomes (XX) and males having one heterochromosome and one homochromosome (XY). Synonyms for heterochromosomes include sex chromosomes, allosomes, and gonosomes. Sex chromosomes are responsible for determining the sex of an organism, while allosomes refer to any chromosome that differs from other chromosomes in the cell. Gonosomes are a more commonly used term for heterochromosomes in the study of genetics and biology, and are particularly relevant when studying sex-linked traits.