The chances of having a boy or a girl are decided by the father, and the chances are 50/50.
Normally, the fetus has two sex chromosomes, one from the father and one from the mother. The mother can only provide the fetus with X chromosome, while the father can provide the fetus with X chromosome and Y chromosome.
If mom’s X chromosome combines with dad’s X chromosome, the fetus will have XX sex chromosome, which is a girl; if mom’s X chromosome combines with dad’s Y chromosome, the fetus will have XY sex chromosome, which is a boy, therefore, the birth of a boy or a girl is decided by dad.
Although the father dominates the sex determination of the fetus, the chances of sex chromosome combination are equal, and the chances of having a boy or a girl are half and half.