How is sex is determined in human beings?

Sex determining mechanism in case of humans is XX-XY type. Out of 23 pairs of chromosomes present, 22 pairs are exactly same in both males and females, these are the autosomes.

• A pair of X-chromosomes are present in the female. X and Y chromosomes are present in
  males.
• During spermatogenesis among males, two types of gametes are produced. 50% of the total
  sperm produced carry the X-chromosome and the rest 50% has Y-chromosome besides the
  autosomes (A+X, A+Y).
• Females, however, produce only one type of ova with an X-chromosome (A+X).
• There is an equal probability of fertilisation of the ovum by the sperm carrying either X or Y chromosome.
• In case, the ovum is fertilized by a sperm carrying X-chromosome the zygote develops into female.
• The fertilization of ovum with sperm carrying Y-chromosome results into a male offspring.
  
• Thus, it is evident that it is the genetic makeup of the sperm that determines the sex of the child. It is also evident that in each pregnancy there is always 50% probability of either a male or a female child.