One of the two types of sex chromosomes, present twice in female cells and once in male cells.
I learned this last year I should know this
In X-linked genetic disorders (such as Duchenne's muscular dystrophy), males are usually more affected than women because they only carry one X chromosome. If the genetic mutation is on one X in a woman, the mutation is "turned off" (X inactivation), which is ok because they have another X chromosome to compensate. In males you cannot "turn off" the X, because you only have one, so you may get the full phenotype of the disease (all the relevant symptoms). For autosomal disorders (chromosome 1-22), gender differentiation may just be random, and not actually connected to the sex of the person. However, males may have worse symptoms in some diseases, but the biological basis for this is case specific.