Here is a brief description of how the sex of child is determined
The human body is made up of 46 chromosomes. (Chromosome is a DNA in simple sentence and DNA contains genes). Now out of these 46, 44 are actually pairs.......ie 22 pairs of chromosomes - one set from father and one set from mother. these 22 pairs are responsible for the development of different parts of ur body. Since u get half from ur father and half from mother, u'll have the features of both.
now the last 2 chromosomes are responsible for the gender of the child. These last two chromosomes can be of 2 types - X chromosome and Y chromosome. If a human being has X and X as last set, it will be female. and if it X and Y set, it will be male. As said above, u get half of the chromosomes from father and half from mother. So the kid will always get X chromosome from the mother. However, it can get X or Y chromosome from the father. If the kid gets x chromosome from father, then the last set of chromosome that the kid will have is X and X. so it will be a girl
If the kid gets Y chromosome from father, then the last set of chromosome will be X and Y, so it will be a boy.
So father of the child is responsible for the gender and not the mother