Unfortunately, that is the most common situation in the families we see. There is no absolute biochemical, DNA or pathology test which can tell conclusively the pattern of inheritance in an isolated case. This becomes extremely important to help predict how a child with MTM will do and what the chance of having another affected child is within a family. The genes are organized on larger structures called chromosomes, which occur in pairs. Women have a pair of X chromosomes, one received from their mother and one received from their father.
Boys have only one X chromosome which they receive from their mother.
They receive a Y chromosome, determining maleness, from their father. Thus we have two copies of all of our genes, except boys have only one copy of any gene located on the X chromosome. In most cases, since we have a pair of genes, if one gene is not functioning properly, the other can compensate and the individual has no problem. However, if she passed that abnormal X onto her son, there would be no good copy of the gene and he would have a genetic disorder. There are two ways a boy can have an X-linked genetic disease.