Debbie Ross, membership manager of the Twins and Multiple Birth Association, described the Pearson family\’s two sets of twins as \’practically unheard of.\’
\’The chance of having non-identical twins is one in 67 while the chance of having identical twins is about one in 1,000\’, she said.
\’But the chances of having two sets of twins, identical and non-identical, must be one in a million. You will find very few families who have a pair of identical twins and a pair of non-identical twins.\’
That is nonsense. Gibberish in fact.
Firstly, there\’s the assumption there that having twins out of two pregnancies are independent events. But if you are predisposed to having twins (while this couple do not have a history of twins in the extended family we do know that having twins is indeed connected to genes) then the fact that you have had one set will mean that you are more likley than normal to have another set.
Now, this is complicated by the fact that they are non-identical and identical. The first is a result of two eggs being fertilised, the second of the splitting of the embryo (or blastocyst or whatever the accurate term is). So, perhaps they are indeed independent events?
In which case the chance this happening is a simply multiplication of the chances of the two independent events. One in 67 times one in one thousand. Or one in 67,000, not one in a million. (That is, assuming already that there are going to be two pregnancies carried to term.)