Genetics
The androgen receptor gene that is mutated in SBMA is located on the X chromosome, and the effects of the mutation may be androgen-dependent, thus only males are fully affected. Females are rarely affected; female carriers tend to have a relatively mild expression of the disease if they show symptoms at all.
Please note that the diagrams will be different if 1) the father is affected AND the mother is unaffected OR 2) if the father is affected and the mother is a carrier.
Read more about this topic: Spinal And Bulbar Muscular Atrophy