Actually, no. Identical twins are created by one egg, fertilized by one sperm, splitting into two embryos. Fraternal twins are, as HistofEng states, two eggs fertilized by two sperm.Twins of the same sex are called identical twins. When one is a girl and the other a boy, they are called fraternal twins.
Male/female fraternal twins or male-female fraternal twins, a term used by scientists and also by others, I see, after having consulted Google. There appears to be no consensus about whether to use a slash or a hyphen.When twins are a boy and a girl, what do you call it? We call it "dragon and phenix twin." I wonder if this makes sense to you. Thanks.
Re gender and sex:(Reviving a very old thread)
I'd say "one of each":
My friend Jill just had twins, one of each.
More formally, I'd say "one of each gender". I love the dragon and phoenix expression though.