Final answer:
A zygote with only one X chromosome can survive because only one X chromosome is needed to be active for normal development. The inactivation of additional X chromosomes, through a process called X inactivation, allows females to compensate for having two X chromosomes.
Step-by-step explanation:
The reason why zygotes with only one X chromosome can survive is because only one X chromosome is active even when two are present under normal conditions. In each cell of a female, one of the two X chromosomes is inactivated early in development. This process, known as X inactivation, involves condensing one of the X chromosomes into a structure called a Barr body. The inactive X chromosome does not express its genes in the same way the active one does, although a few genes on the inactive X may still be expressed. If an individual has more than one X chromosome, the extra ones are also typically inactivated. However, a zygote with an absent X chromosome will not survive, as at least one X chromosome is necessary for development.