Researchers in Edinburgh say their genetic clue could lead to new ways of treating infertile men and to new forms of male contraception.
They have discovered that a proportion of the men attending a fertility clinic have a gene defect.
The breakthrough was made by doctors at the Medical Research Council's Human Genetics Unit in Edinburgh in collaboration with a fertility clinic at the city's Western General Hospital.
Male infertility accounts for about one-third of all infertility, and when men are unable to produce sufficient sperm - and sufficient sperm of good enough quality - to achieve conception, little can be done to help the couple.
It is estimated that between one in ten and one in fourteen of all couples have problems in conceiving and when sperm counts are very low, specialists have few solutions. One technique is to isolate the viable sperm and inject a sperm into an egg using 'micro' techniques. However, the use of donor sperm is the more common treatment.
In Edinburgh, screening among men at the fertility clinic showed that about 15 per cent of men with severe sperm production problems carried a mutation or deletion of a particular gene cluster.
They say that this gene is involved in the production of a type of 'information' protein in cells. The scientists believe that mutations in the gene bring about a loss of information which causes the abnormally low sperm production.
The Edinburgh work could explain the underlying cause of the impaired sperm production in a significant proportion of men with no other apparent cause of infertility, the researchers say.
Dr Anne Chandley, one of the leaders of the research team, said: 'The discovery of this gene family could be the first step towards diagnosing male infertility through DNA analysis.'
While some male infertility has obvious causes, resulting from injury, the majority of sub-fertile men are otherwise completely healthy. Researchers have speculated, in the past, that their abnormal sperm production could have been caused by infection in childhood. In some cases the sperm are malformed, or their lifespan is too short for them to travel far enough for them to reach the woman's egg.Reuse content