There are several types of twins, but monozygotic and dizygotic twins are the most common. Monozygotic twins occur when an egg splits into two equal parts into the early stages of embryonic development for unknown reasons. Monozygotic twins are always the same sex; they have the same set of genes, the same blood types, and are very similar in appearance. Unlike dizygotic twins, monozygotic twins are born equally often in all nationalities and age groups.
One egg fertilized by one sperm is divided into two, less often three or four, parts, and these parts begin to develop independently. It is pretty essential at what time stage in the development of the embryo this division occurred. Separation can occur during the first two weeks of embryonic development, resulting in several forms of monozygotic twins. In a third of cases, separation occurs within the first three days (Jonsson 29). In this case, each embryo forms its placenta and its fetal bladder. If separation occurs at this stage, the children will develop as two separate organisms but with identical genes (Jonsson 28). In most cases, separation occurs on the 4th-9th day of development, then one placenta and two fetal bladders are formed.
Since identical genes are formed in children, the sex will always be the same in monozygotic twins. Identicals are born approximately every three hundred births. It is believed that the predisposition to the birth of twins is transmitted through the maternal line (Jonsson 31). This statistic is valid for natural insemination, while artificial insemination, such as IVF, ICSI, has different statistics. However, regarding the birth of twins of the same sex, the type of fertilization also does not matter – all the same, the twins will be of the same sex if they are monozygotic.
Work Cited
Jonsson, Hakon, et al. “Differences between germline genomes of monozygotic twins.” Nature Genetics, vol. 53, no. 1, 2021, pp. 27-34. Web.