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They may all look like tadpoles, but even sperm cells from the same man have big genetic differences, a study has shown.

For the first time, scientists have obtained genetic blueprints of almost 100 sperm from a single individual.

The results confirm what scientists already know, that every sperm is different because of the way their inherited DNA is shuffled. The process, known as recombination, mixes up genes passed down by a man's mother and father and increases genetic diversity.

Each of the 91 sperm studied showed an average of 23 recombination, or mixing, events. But individual sperm varied greatly in the way they experienced spontaneous genetic mutations, found the US study published in the journal Cell.

Every sperm contained between 25 and 36 "new" mutations, not seen in other body cells. Two sperm 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 sperm.

"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.

"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."

Genetically sequencing sperm could provide a "new kind of early detection system" to identify men who may have trouble conceiving, he added. 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.

Recombination occurs before the chromosomes are partitioned. During a process called meiosis, pairs of chromosomes line up along the midsection of the cell. Portions of the chromosome pairs may then be randomly swapped, mixing up the inherited DNA elements.

This generates much more genetic variation than possible if only intact chromosomes were segregated into reproductive cells. It helps ensure a baby inherits a blend of DNA from all four grandparents. Sperm and egg chromosomes pair up to produce the usual two-copy complement after fertilisation.

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