The Smiths have four children, all girls. Their fifth child is a boy. Why did this change occur?

Answer 1

With each birth, there's about a 50-50 chance the child will be a boy.

When the girls were born, the father supplied an X chromosome. A Y chromosome from the father produced a boy.

Check this site.

http://health.yahoo.com/other-other/boy-or-girl-determining-a-baby-s-gender/harvard--COL082001.html