How are twins, triplets, etc, born (the fertilised egg splits into separate masses, each of which develops into a baby)? Are all twins born identical (no, it depends whether they have come from one egg, identical or monozygotic, or two eggs, fraternal or dizygotic)? Are identical twins exactly the same (although they come from the same egg, their development may begin to diverge slightly in the womb, a bit like the differences between the left- and right-hand sides of our body)? What are the odds of having twins (higher in some families where there is a tradition of twins, but usually around one in 80)? What are the odds against multiple births (fertility drugs increase the likelihood, but the number 80 keeps recurring: the odds against triplets are 80 x 80, ie one in 6,400; quads are 6,400 x 80 - one in 512,000, and so on)?
What are conjoined twins (identical twins whose bodies are joined together in some way)? How did this happen (instead of the zygote (fertilised egg) splitting early into two different babies, it separated later in their development in the womb)? Are all conjoined twins similar (some are symmetrical, split exactly in the middle, others are asymmetrical, consisting of a "host", the stronger one, and a "parasite", the weaker one)? Why were they called Siamese (the original pair were born in Siam, present-day Thailand)? Some conjoned twins have become circus exhibits - do you think this is right?
How can you use twins (not conjoined) to study the influence of heredity and environment on us (look at brothers and sisters, non-identical and identical twins, to see how different they are: although twins start off identical, their environment may become different, especially if they live in different families)? Are there any snags with this sort of research (when twins are adopted they may be put into similar families, so their environment may not be too different)?
Imagine you have an identical twin brother or sister (not conjoined). You grow up in two different families and then meet when you are older. Describe your first meeting.
Ted Wragg is professor of education at Exeter University * TALKING POINTS
Should conjoined twins be separated, if this is medically possible?
Conjoined twins are entitled to live separate lives, like any other twins. Being joined together rules out many possibilities, and medical science is now well developed and safe. Ethically it is no different from any other kind of surgery. Unseparated twins are eventually going to quarrel and resent each other, rather than be friends.
Unless their condition is life threatening they are better together than separate, as this is the way they were meant to be. There is bound to be some risk and often one twin has to die so the other can survive. The original conjoined twins showed that a near normal life can be lived, even to the point of both twins having a family.