A male has XY gametes while the female has XX gametes. No matter what is the gender of the baby the mother contributes only the X gamete. If the X gamete of the male fusses, then the baby born is a girl. If the Y gamete fusses then the baby born is a boy. 
Hence there are 50 % chances to either have a boy or a girl.

Thank you.....!!!!
Mark as brainliest of helpful..
Yours, Jahnavi.