Your sex cells (sperm or egg) contain your DNA—genetic material that is like the thumbprint for who you are on a cellular level. Do you remember learning about the X and Y chromosomes? 

Well, a female is the result of two X chromosomes and a male is created by an X & Y pairing. 

Each person is created from the fusion of the female sexual cell that contains one sexual chromosome, the X factor, and the male sex cell that contains one other chromosome, either the X or the Y. if the sperm is an X carrier the baby will be a girl, if the sperm is a Y, the baby will be a boy. yes- you got it—the sperm (so, males) decide the gender of the new baby.  

Twins are created in two different ways. Usually only one egg is released during ovulation.  However, if two eggs are released and sperm find both, and a hundred other things go right, then non-identical twins (di-zygotic twins) are formed. If the egg is fertilized by the sperm, and then splits into two fertilized eggs, then identical twins (Mono-zygotic twins) are formed because they came from the exact same genetic make-up, the same egg and sperm. Contrary to popular belief, identical twins do not run in families and the causes of myno-zygotic twins are pretty much unknown

(Source: https://ghr.nlm.nih.gov/primer/traits/twins)