Fraternal (dizygotic or DZ) twins sometimes run in families. In the past, researchers believed that identical (monozygotic or MZ) twins do not run in families. They said that MZ twinning was a random event that occurs equally as often in all places and populations around the world. But many people know or have heard of families with more than one set of MZ twins. When several relatives in a family have MZ twin pregnancies, it is called familial MZ twinning.
Familial MZ twinning may actually occur more often than we think. In some cases, familial MZ twinning may be misclassified as familial DZ twinning. This could happen when the twins in the pair don’t look totally identical. And sometimes MZ twin pairs are mistakenly assumed to be DZ twin pairs because they had two chorions at birth instead of one. The chorion is the outermost membrane surrounding the developing embryo. DZ twin pairs always have two chorions. Most MZ twin pairs have one chorion. But one-third of MZ twin pairs have two chorions. This happens when the embryo splits very soon after fertilization. So, in a family where several sets of twins with two chorions are born, they may be thought to be familial DZ twin pairs when they really are familial MZ twin pairs.
Scientists who study genetics (geneticists) believe that familial MZ twinning might be caused by a mutation (change) in a single gene that controls cell-to-cell connections. The cells of early embryos with this mutation may not stick together as well. This could cause the embryo to split in two before it is implanted in the womb. Some geneticists say that there could be more than one single gene that controls cell-to-cell connections. And they think these genes may be involved at different times in early embryonic development. The evidence for this is that they have only found two types of families with familial MZ twinning. In some families, all the twin pairs had two chorions, because the embryo split very early. In other families, all the twin pairs had one chorion, because the split occurred later. There were no families in which some twin pairs had two chorions and other twin pairs had one chorion. This suggests that mutations in different genes were present in the two different types of families.
Geneticists have analyzed the family trees of all the families with familial MZ twinning studied so far. They say that the gene that causes MZ twinning is inherited in an autosomal dominant pattern. This means that the trait (MZ twinning) occurs in multiple generations, and only one copy of the gene is needed in order for the trait to be expressed.
Shur N. 2009. The genetics of twinning: From splitting eggs to breaking paradigms. Am J Med Genet Part C Semin Med Genet 151C:105–109.
Machin G. 2009. Non-identical monozygotic twins, intermediate twin types, zygosity testing, and the non-random nature of monozygotic twinning: A review. Am J Med Genet Part C Semin Med Genet 151C:110–127.
Machin G. 2009. Familial monozygotic twinning: A report of seven pedigrees. Am J Med Genet Part C Semin Med Genet 151C:152–154.