Since long it has been considered that females are responsible for determining the sex of the developing foetus. However, it is evident that human females are homogametic.  They produce the same types of ova, each having the X chromosome. The human males are heterogametic. They have XY as sex chromosomes and so they produce 50% sperms with X chromosome and another 50% with a Y chromosome.  If the sperm having X chromosome fuses with the ovum, a female foetus will be formed. If the sperm having Y chromosome fuses with an ovum, a male foetus is formed. So, the deciding factor is contributed by the males and the sex of the foetus depends on the probability.  Hence, we all must take efforts to stop female foeticide and mother is not responsible for a girl child.