Twins are two babies born from same pregnancy. There are three basic forms of twins recognised in gynaecology. They are;
(a). Identical twins
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First, identical twins are two babies born from the same pregnancy and with absolute and perfect resemblance. They are also called monozygotic twins. Now, how are they concieved? Well, it is like this; After the egg or ovum released from the last ovulation has been fertilised to form a zygote in the fallapian tube, the zygote splits to form two daughter zygotes under the influence of some physiological factors. The two daughter zygotes then swim simultaneously to the uterus where they become implanted as embryoes precisely in the endometrium section of the uterus each by its own placenta for anchorage and material exchange between its own circulatory system and the mother's. Each also has its own amniotic fluid and chorion. The two embryoes develop into babies which at the end of the pregnancy are born as identical twins. Identical twins are always of the same sex the reason being that they are formed from the splitting of the same zygote fertilised by a single sperm which could be the Y-chromosome carrying sperm for which case we will be talking of boy-boy twins or the X chromosome carrying sperm for which case we will be talking of girl-girl twins. Identical twins have the same genotypes( i.e DNA-exclusively controlled traits, like blood groups, rhesus factors, sickle cell trait, haemophilia, etc.) and phenotypes(i.e outward and observable physical traits like intelligence, height, skin color, muscle size, weight, etc). It should be noted that although identical twins have the same phenotypes, their phenotypes could overtime change as they grow up especially if they are brought up in different environments. For example, exposure to different diets, creams or sunlight intensity, schools, and exercise could precipitate or expedite variations on weight or size, skin color, intelligence, and muscularity respectively.
(b). Non identical twins
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Second, non identical twins are two babies born from the same pregnancy but with partial or no resemblance. They are also called dizygotic twins, although yet another name for them is fraternal twins. Now, how are they conceived? Well, it is like this; Although the monthly ovulation cycles in women are such that only one of the two ovaries release an egg at a time, some women however in some ovulation cycles have the two ovaries release an egg each. When this happens, the two egg are each fertilised to form two separate zygotes in each fallopian tube. The zygotes swim to implant in the uterus as two separate embryoes that eventually develop into two different babies which are delivered as non identical twins at the end of the pregnancy. Non identical twins have different genotypes and phenotypes the reason being that they are formed from different eggs and different sperms. They could be of the same sex or different sex depending on whether the the two separate sperms involved in the formation of their zygotes were both Y sperm or X sperm.
(c). Siamese twins!
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Thirdly, what about siamese twins? Watch out for a later post on siamese twins!
