Some females release two eggs (one from each ovary) and both eggs become fertilised resulting in non-identical twins. The releasing of two eggs is a genetics thing and the gene is carried only by women.
Not sure how tru this bit is but... If you have the gene and have (non-identical) twins, then the gene will skip a generation (lay dormant I guess) and your granddaughters will carry the active gene and may have twins... and this repeats every other generation.
Some women don't have the gene and on a one off occassion release two eggs instead of one...
Identical twins are rarer. This is when one egg is fertilised and the egg then splits forming two embryos that grow into babies. As the egg contains one set of genetic information, this information is duplicated in each embryo so they both have the same eyes, hair, height etc etc. The chances of having identical twins is about 1 in 285...
Update, my friend for whom I started this thread likes the name Ma'az for the boy. What would be a good corresponding name for the girl? It doesn't have to rhyme.
