When sexual precocity is present, menstruation usually begins prior to development of breasts or pubic hair. The average age for the first period is three. Boys begin with enlargement of the testicles, followed quickly by enlargement of the penis and the development of pubic hair. McCune-Albright syndrome is unique among all genetic diseases. Some genetic diseases have dominant inheritance, some recessive.
Some diseases have sex-linked inheritance, some result from too many or too few chromosomes.
Some diseases are polygenic or multifactorial, inherited through changes on several genes. When all the cells in a person do not have the same genetic make up, that person is called a mosaic. At some point early in development, one cell mutates. All of us are mosaics to some degree. It is the basis for moles and freckles.