Behavioral and physical differences between identical twins are fascinating because they are unexpected. Many people assume that when identical twins differ they do so because they are treated differently by others, or because they try to differentiate from one another in order to carve out a separate
identity. These explanations may all be true at some level-but it is important not to overlook a hidden, yet powerful source of difference between identical female twins: X chromosome inactivation.
X chromosome inactivation is a process that affects all females, not just twins. Very soon after conception, one of the two X chromosomes in each cell shuts down. One X chromosome is received from the mother and one X chromosome is received from the father-and which one becomes inactive in a given cell is a matter of random chance. Typically, half the X chromosomes from mom and half from dad are inactive, which also means that half from each parent are active. If the X chromosome from dad carried the recessive gene for color-blindness, the daughter would still have normal color vision if she inherited the dominant color vision gene from mom. However, things do not always work out that way. If there were skewed X-inactivation (meaning that a high proportion of Dad's X chromosomes were active), then a daughter might express the color-blindness trait.
This process has some curious consequences for identical female twins. If the twins result when the fertilized egg splits very early after conception and before X-inactivation take place, then the twins may differ in traits located on the X chromosome. This is exactly what happened in a case report I came across recently.
Bennett, Boye & Neufeld (2008) described a difference in hemophilia A in a pair of identical female twins. The female twins in question were born to a father who had two normal genes and a mother who carried one recessive gene for this condition and one normal gene. Medical attention was sought when one infant twin developed excessive bruising and bleeding from minor incidents. Blood studies revealed skewed X-inactivation toward the paternally derived X chromosome in this twin. In contrast, her unaffected twin sister showed random X-inactivation. This is the first reported case of hemophilia A discordance in MZ twins. Other conditions for which identical female twins have differed include color-blindness and the Lesch-Nyhan syndrome.
It is plausible that psychological differences between identical female twins might also emerge in this manner. This is another example of how twin research can add to what we know, and can know, about human behavioral development.
Bennett, C.M., Boye, E., & Neufeld, E.J. (2008). Female monozygotic twins discordant for hemophilia A due to nonrandom X-chromosome inactivation. American Journal of Hematology, 83, 778-780.
