When a baby is conceived, each parent passes on one copy of each of their genes to the baby. The sex of a child is determined by inheritance from their parents of X and Y chromosomes. Females have two X chromosomes, one from each parent. The sons of men with Kennedy's disease do not get the disease, and do not pass on the mutation. The daughters of men with Kennedy's disease are called obligatory carriers .
Females have two X-chromosomes, one from their father, and one from their mother.
Since males have only one X chromosome to pass on, their daughters always receive the X chromosome carrying the genetic mutation. Because they also have a normal X chromosome, they do not develop the disease. The sons of female carriers of the genetic mutation have a 50 per cent chance of receiving the mutated gene and developing Kennedy's disease. The daughters of Kennedy's disease female carriers have a 50 per cent chance of inheriting the mutated gene, but because they also have a normal X chromosome, they do not develop the disease. It is also important to note there is a 50 per cent chance that their female or male children will not inherit the mutant gene from their mother and therefore not transmit it to their children.