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

a) the law of averages finally catching up
b) the conception classes taken by the parents
c) the father's contribution of a Y chromosome
d) the mother's contribution of a Y chromosome
e) the timing of the fertility cycles

(c) The occurence of four previous female births does NOT increase the chances for a boy the next time.

It is the father's sperm that determines the sex of the child.

two "Xs" will produce a girl and
an "X" and "Y" a boy.
Since the mother can only contribute an "X" chromosome, the sex of the child depends on whether the father contributes a "Y" (boy) or another "X"
(girl).

The change occurred because of option c) the father's contribution of a Y chromosome.

The correct answer to why the Smiths had a boy after having four girls is c) the father's contribution of a Y chromosome.

Explanation: The sex of a child is determined by the combination of chromosomes inherited from both parents. The mother always contributes an X chromosome, while the father can contribute either an X chromosome or a Y chromosome. When a sperm carrying a Y chromosome fertilizes the egg, the result is a male child. In this case, after having four girls (who all received an X chromosome from the father), the father finally contributed a sperm with a Y chromosome, resulting in the birth of a boy.

To understand this, it is essential to have knowledge of basic genetics and the role of chromosomes in determining the sex of a child.