And this difference goes on to determine which sperm will finally make it to the female egg.
In the study, scientists scanned 100 sperms from one man. They found that every sperm was different because of the way their inherited DNA is shuffled, the 'Daily Mail' reported.
The process, known as recombination, mixes up genes passed down by a man's mother and father and increases genetic diversity.
Each of the sperms studied showed an average of 23 recombination (mixing) events. But individual sperm varied greatly in the way they experienced spontaneous genetic mutations.
The study found every sperm containing between 25 and 36 'new' mutations, not seen in other body cells. Two sperms were found to be missing entire chromosomes, the protein-bound packages of DNA that contain the genes.
Random mutations create genetic variation, but can be harmful if they occur in the wrong places.
The cells were donated by a 40-year-old man who has healthy offspring and normally functioning sperms.
"For the first time we were able to generate an individual recombination map and mutation rate for each of several sperm from one person," said Professor Barry Behr, from Stanford University in California, US.
"Now we can look at a particular individual, make some calls about what they would likely contribute genetically to an embryo, and perhaps even diagnose or detect potential problems," said Behr.
Genetically sequencing sperm could provide a "new kind of early detection system" to identify men who may have trouble conceiving, he added.
The research is published in the journal Cell.
Most cells in the human body have two copies each of 23 chromosomes, containing DNA inherited from both parents. However, sperm in men and eggs in women only have single copies.
