Woman has only daughters that means when fusion of sperm and ovum took place sperm contributed X  chromosome and ovum also contributed X chromosome. So when fusion took place daughters are born.

X And Y Chromosome

Explanation:

• Women have  two X chromosomes where is men have one X and one Y chromosome
• Consequently a female produces gametes which are the entirety of a similar kind and just contribute in one chromosome to the zygote
• The other chromosome originates from the male and it very well may be either X or Y chromosome
• Case 1. On the off chance that a X chromosome shows up from the male, at that point the zygote will have XX condition and the subsequent posterity will be a female (little girl).  
• Case 2. On the off chance that a Y chromosome shows up from the male, at that point the zygote will have XY condition and the subsequent posterity will be a male (child).  
• Female  has just little girls in light of the fact that lone the sperms containing the X chromosome prepared the ovum each time