Which one of the following conditions of the embryo would lead to the birth of a normal human female child?

1. Only one X chromosome which is functional
2. Two X chromosomes but only one is functional
3. Two X chromosomes and both are fully functional
4. One X and one Y chromosome

The correct option is B Two X chromosomes but only one is functional
Human females contain two X chromosomes as compared to one X chromosome in males. This gives a potential to produce twice gene product as compared to males. A mechanism operates here to solve the problem called dosage compensation. In humans, dosage compensation is achieved by the inactivation of one X chromosome in the female somatic cells. This inactivated chromosome is known as the Barr body.