Since Alpha-1 Antitrypsin Deficiency is an inherited disorder, it occurs when both parents pass on a abnormal gene to their child. A father and mother who are both carriers (MZ) could expect to have a 50% chance of having a carrier, and a 25% chance of having either a healthy or a deficient child.
With the birth of each child, the same percentages apply. Therefore, it is entirely possible for a MZ father and a MZ mother to have four children, all of which are ZZ or all of which are MM. Or on the other hand, it is possible for this same father and mother to have 1 ZZ child, 2 MZ child and 2 MM child as the percentages imply.
If one parent has the ZZ genotype, and the other has two normal copies of the gene (MM), all of their children will have the MZ genotype.
For information on who should be tested, please visit the Alpha-1 Foundation website: Get Tested