To improve controls for confounding familial characteristics, we included analyses of siblings and estimated singleton-twin differences within families. Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development. This results in one normal male XY and one female with Turner syndrome 45,X. That will never change. Twinless Twins Support Group International.
That sequencing technology and more rigorous replication has transformed the study of individual variation and the work of the twin studies group.
The Twins Project