A male has one X and one Y chromosomes which means half the male gametes i.e. sperms have X and the other half will have Y chromosome (heterogametic), whereas in females both the chromosomes are X thus all the females gametes will have one X chromosome (homogametic). In humans the sex of the child depends on what happens at fertilization. If Y chromosome is inherited by the offspring from male then it will surely be a male because the child will have XY combination of sex chromosomes. If X from both the parents then the offspring will be a female having XX combination. So, the percentage probability is 50% for having a son or a daughter.