Recently we read an article that was released by the Washington State Twin Registry talking about the differences between identical female and male twins and I wanted to share what we learned with you.
Sex chromosomes determine a person’s sex. Males have one X chromosome while females have two copies of the X chromosome. Females have one X chromosome from their father (paternal) and one X chromosome from their mother (maternal). Males have one X chromosome from their mother and one Y chromosome from their father. The Y chromosome is much smaller and most of the genes are different.
Female identical twins (officially know as monozygotic twins) can have greater differences than male monozygotic twins.
Female monozygotic twins have a random inactivation of one X chromosome that occurs in every cell of every female’s body. This does not only occur in twins. As females have two X chromosomes in each body cell one is turned off (inactivated) to ensure females don’t get a double dose of all the products fo all the genes on her X chromosomes.
When the female has fewer than 1000 cells very early in development inactivation occurs. All cells that develop from this point on will have the same inactive X chromosome which will be either the material X or the paternal X. The genes on the X chromosome that are expressed in each cell are either the maternal or paternal genes.
As random X chromosome inactivation in monozygotic twins often occurs after the twins have been created and are developing it is extremely unlikely that the exact same pattern of inactivation will happen in each developing twin. Each twin will have a different pattern of cells that have either the maternal or paternal X chromosome active and different genes can be expressed.
Therefore random inactivation of the X chromosome can produce noticeable differences in appearance of female monozygotic twins.
Monozygotic twins – (of twins) derived from a single ovum, and so identical.
Chromosome – a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes. A sex chromosome, two of which are normally present in female cells (designated XX) and only one in male cells (designated XY).