Genetic illnesses can be coded for either on the autosomes (non-sex chromosomes) in which case if you're unlucky enough to have one (in the case of a dominant) or both (in the case of a recessive) chromosomes carry the thing, then you have it, regardless of gender.

If the gene is carried on the X chromosome and you are a girl, then you have two options. If it is a recessive allele you need both of your X chromosomes to have a copy of it before you will show the disease. Otherwise you are a 'carrier' but not a sufferer. If it is a dominant allele, having just one copy is enough. You will have the disease.

If it is carried on the X chromosome and you are a male, then you will have the disease, because there is not a corresponding 'slot' on the Y chromosme to mask it in the case of a recessive (in the case of a dominant, well, you only need one copy to have the disease).

This pattern also follows for non-disease-y traits that are carried on the X chromosome. If a gene is carried on the X chromosomes it is termed X-linked.

If the gene is carried on the Y chromosome exclusively, then only males can get it. Very few diseases are carried this way - the Y chromosome is very small. Traits carried on the Y chromosome are termed Y-linked.