Twin Family

Identical twins are genetically identical (unless there has been a mutation during development) and they are always the same sex. On rare occasions, identical twins may express different phenotypes (normally due to an environmental factor or the deactivation of different X chromosomes in female identical twins), and in some extremely rare cases, due to aneuploidy, twins may express different sexual phenotypes, normally due to an XXY Klinefelter's syndrome zygote splitting unevenly.
Identical twins actually have only nearly identical DNA, and differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification. A study of 80 pairs of human twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences between identical twins increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. However, certain characteristics become more alike as twins age, such as IQ and personality. This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behavior.
• Phenotype similarity
Contrary to common opinion, identical twins are not always of the same phenotypical sex. There have been described cases where monozygocity resulted in 46,XO (i.e. female with Turner syndrome) and 46,XY (i.e. male). This is thought to be due to unequal distribution of zygotic protoplasm. However, as a rule, traits and physical appearances of MZ twins are very similar.