Do Female Identical Twins Appear More Different Than Male Identical Twins?

Yes, female identical (MZ) twins can appear more different than male identical twins! The epigenetic factors that can cause identical twins of either sex to look different are discussed in the answer to the question “Can identical twins look different?” In addition, the sex chromosomes are responsible for the greater difference in appearance of some female identical twins.

The sex chromosomes determine a person’s sex. Females have two copies of the X chromosome – one from her father (paternal) and one from her mother (maternal). Males have one X chromosome which came from his mother, and a Y chromosome which came from his father. The Y chromosome has mostly different genes than the X chromosome and is much smaller.

The greater difference in appearance of female MZ twins can be explained by the random inactivation of one X chromosome that occurs in every cell of every female’s body (not just in twins). Remember, females have two X chromosome in each body cell, but males have only one. So that females don’t have a double dose of the products of all the genes on her X chromosomes, one X-chromosome in each female body cell is “turned off” (inactivated).

X-chromosome inactivation occurs very early in development, when the female embryo has fewer than 1000 cells, and it is totally random. From that point on, all cells that develop from each early cell have the same inactive X chromosome – either the maternal X or the paternal X. The genes on the X chromosome that are expressed in each cell are either the maternal or paternal genes.

Since coat color genes in cats are found on the X-chromosome, the effect of random X-chromosome inactivation can be easily seen. The random inactivation of one X chromosome is what causes the coloring in a calico cat, and it is the reason that calico cats are always females.

Like calico cats, human females are mosaics of cells in which either the paternal or maternal X chromosome are active. The calico appearance is caused by different versions of just a single gene on the cat’s X chromosome. The human X chromosome has over 1000 genes on it. The maternal and paternal X chromosomes of human females often contain different versions of many of these genes on them.

Random X-chromosome inactivation in MZ 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 the paternal X chromosome active. Each twin will have a different pattern of maternal and paternal genes that can be expressed. This is why random inactivation of the X-chromosome can produce noticeable differences in appearance of MZ female twins.