| Twins are multiple children who were born simultaneously to a single mother — twins, triplets etc. Twins can be monozygotic (identical) and dizygotic (fraternal) twins.
Monozygotic (Identical) twins:
Monozygotic twins are formed from a single zygote, which is divided into two (or more) embryos during cell division. From a genetic point of view, they are identical, i.e. possess identical genotypes. Monozygotic twins are always of the same gender.
A special group among monozygotic twins constitute unusual types: two-headed (as a rule , unviable) and “Siamese twins”. The most popular case of Siamese twins, were Chang and Eng, born in 1811 in Siam (now Thailand). They lived for 63 years, were married to sisters - twins; Chang gave birth to 10 and Eng - 12 children. When Chang died due to bronchitis, Eng also died after 2 hrs. They were connected by a tissue intersection with a width of approximately 10cm from the breast up to the navel. Later, it was established, that the connecting intersection contained hepatic tissue, connecting two livers. Any surgical attempt to separate the brothers at that time would have been unsuccessful. In modern times, even the difficult joints between the twins could be easily separated.
Studying monozygotic twins helps to understand what and how in a person is determined by genes and also what should is not.
Dizygotic twins (Fraternal):
Dizygotic twins develop if two ovums are simultaneously fertilized by two spermatozoids. Naturally, dizygotic twins possess different genotypes. They are similar between themselves as brothers and sisters and nothing more than that since they possess approximately 50% identical genes. The general birth rate of twins is about 1% and about 1/3 of them turn out to be monozygotic twins.
It is known that the number of births of monozygotic twins is the same in various populations while the number of dizygotic twins significantly differs. For example, in the USA, dizygotic twins are born more often among members of the Negro race than amongst the members of Caucasian races. In Europe, the occurrence rate of dizygotic twins is 8 per 1000 births. However, the birth rate of dizygotic twins happens to be more in particular populations. The lowest birth rate of twins is characteristic to Mongoloid populations, especially in Japan.
It is supposed that multiple pregnancies are caused generically. However, it is justified only for dizygotic twins. Factors influencing the birth rate of twins is studied arely now-a-days. Data exists which indicates that the probability of the birth of dizygotic twins increases with the increase in the age of the mother and how many deliveries she's had. The influence of the mother’s age is explained, probably, by the increased level of gonadotropin, which leads to a higher-rate of poly-ovulation. Data also exists about the decrease in the birth rate of twins in almost all industrial countries.
It is noticed that the frequency of congenital deformities in twins, as a rule, is higher than in the case of single births. |
