The study, published in the journal Science Translational Medicine, found that couples who had the most trouble conceiving were those where the men inherited both copies of this mutant gene, one from their father and one from their mother.
The loss of this coating makes it more difficult for sperm to travel through fluids in the female reproductive tract, which in turn reduces the rate of conception.
Using semen donated from 19 participants, Tollner and colleagues observed that sperm from donors who had both copies of the mutant DEFB126 gene exhibited most mobility problems.
"We found that sperm from donors lacking the normal gene have difficulty penetrating or swimming in the mucus surrogate (on laboratory dishes)," wrote Tollner.
"The rate at which they are able to penetrate the mucus-like gel is only 15 to 20% of the rate observed for sperm from donors with the normal gene," Tollner said.
What is infertility?
The World Health Organisation defines infertility as the inability of a couple to conceive after a year of unprotected sex, and the problem occurs to around 13 to 14% of couples in many countries across the world.
In about half of infertile couples, the cause lies with the men and experts have traditionally blamed low sperm count.
To test their hypothesis, the scientists recruited 509 young couples in China and tracked them for nearly 2 years. The average age of the men was 25.8 and the women 23.4.
The couples were put into three groups depending on the DNA of the men: men without the gene mutation, men with one copy of the mutant gene and men with both copies of the mutation.
By the end of the study, wives of 71% of men with both copies of the mutant gene had conceived, compared to 81% of wives of men with either one or none of the mutant gene.
"Our key finding was that the rate of births among couples where the husband had two copies of the DEFB126 mutation was 30% lower than in other couples," wrote Scott Venners.
When helping couples conceive, doctors may consider using more direct interventions such as in vitro fertilisation or intrauterine insemination if they find that the male partner has this gene mutation, Tollner said.
Do you or your partner have the DEFB126 mutation?