Identical, or monozygotic, twins develop from a single egg fertilized by a single sperm. Monozygotic twins are genetically identical because they originate from a single zygote that split into two. Caroline Lost and her colleagues examined nine measures of social, behavioral, and cognitive ability in 1000 pairs of both male and female identical twins. Their study found that pairs of male twins tended to be more alike in their prosocial behavior, peer problems, and verbal ability scores than pairs of female twins. Which statement explains this observation?

Female mammals undergo X-chromosome inactivation during early embryogenesis, thereby females are mosaic for the expression of X-linked genes

Explanation:

In mammals, males receive only one copy of the X chromosome and one Y chromosome, while females receive two copies of the X chromosome. The X chromosome contains approximately 1,000 key genes involved in development. In human female embryos, one of these two copies of the X chromosome is inactivated to prevent the imbalance caused by a double dose of X-linked genes, a phenomenon known as X-chromosome inactivation. The inactivation of one X-chromosome is a random process, thereby female cells are mosaics for all heterozygous X-linked alleles, where some embryonic cells have an inactive maternal X chromosome, while other cells have an inactive paternal X chromosome.