A female carries only one type of sex chromosome in its egg cell i.e. X chromosome. But a male produce sperms which have two types of chromosomes – X bearing sperms and Y bearing sperms. And the ratio of X bearing sperms to Y bearing sperms is 50:50. If X bearing sperm fuses with X bearing egg cell female will be produced (XX). But if Y bearing sperm fuses with X bearing egg cell it will be a male (XY). Hence the chance of getting either a male or female child is 50:50.