The latest survey, from December 2009, counted 265 pairs of twins in the village, which is home to about 3,000 families and 13,000 inhabitants. This equates to a twinning rate of about 30 to 35 per 1,000 live births within a radius of about 500 metres. The average in the rest of the country is 8.1 per 1,000 live births.
Unlike in CÃ¢ndido GodÃ³i, consanguineous (blood-related) marriages are not a factor in the Kodinji twin phenomenon. "The majority of the population is Muslim, but the incidences of twinning are present even among the small minority of Hindu families," Sribiju explains. "More importantly, brides who are not originally from Kodinji are giving birth to twins after they move to the village."
Are the twins mainly monozygotic or dizygotic?
Cosanguineous marriages can still be part of the cause within groups, but marriage is not a barrier to gene flow between groups. It only takes a few times over many generations...
Twinning is awesome and useful and understudied...it is obligate in some species (armadillos), and assumed to be random and/or environmental in humans...but I wouldn't be surprised if there was a genetic predispostion.
they don't know re: zoygosity, but in all the human variation studies i've seen it is in the dizygotic component that you get this difference. oh, and stuff like drinking whole milk might cause increased likelihoods:
Dizygotic twinning is heritable and varies widely. Some people believe that monozygotic twinning is heritable, though the conventional wisdom is that it is not. One theory is that it can be triggered by the father, that there are inherited properties of the sperm that cause splitting. (while it is hard for the father to induce multiple ovulation)
It is not so rare to confuse MZ and DZ twins (really!), so it is plausible that we miss variation in MZ twinning when DZ is much more common, as is true in most populations. But if there were a population where MZ were more common, people would notice the sex ratio.