Kennedy disease

Kennedy disease is inherited in an X-linked recessive manner. A condition is X-linked if the mutated responsible gene is located on the X chromosome (one of the two sex chromosomes). Males have one X chromosome and one Y chromosome, while females have two X chromosomes. In males, one mutated copy of a disease-causing gene on the X chromosome is enough to cause the condition. In females, a mutation usually must be present in both copies of the gene (one on each X chromosome) to cause the condition.^[1]

A characteristic of X-linked inheritance is that affected males cannot pass X-linked traits to their sons, because they only pass their Y chromosome to their sons.^[1] However, all daughters of an affected male will be carriers of the condition. A female carrier of an X-linked recessive condition has a 50% chance to pass the mutated gene on to each daughter (i.e. there is a 50% chance each daughter will also be a carrier) and a 50% chance to pass the mutated gene on to each son (i.e. there is a 50% chance that each son will be affected).

Females who are carriers of an abnormal CAG expansion in the androgen receptor (AR) gene are usually unaffected, but some may have mild signs and symptoms including muscle cramps or occasional tremors.^[2][1][3] No reported female carriers have had significant motor neuron disease. Females who are symptomatic may have an abnormal electromyogram or abnormal findings on muscle biopsy.^[2][3]