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And now it seems that a surprisingly common genetic mutation might decrease a man's chances of having viable sperm even 

further. According to a <a href="http://stm.sciencemag.org/content/3/92/92ra65.abstract" target="_blank" class="news_link">study</a> published online July 20 in Science Translational Medicine, roughly a fifth of the male 

population may suffer lower fertility because of this mutation, and half of all men may be carriers of the trait.
 
 
The frequency of the variation is surprising because natural selection quickly removes most genetic traits that limit 

sexual success from the gene pool.
 
 
Not so with this mutation, according to Gary Cherr of the department of environmental toxicology and nutrition at the 

University of California, Davis and his co-authors on the study. They estimate that this allele, or form of the gene, is 

present in some 50 percent of men. In the team's investigations, having just one of the mutant alleles did not seem to 

decrease a man's likelihood of fathering a child. But about 20 percent of men in the study samples had two copies of the 

gene mutation, which, especially when combined with other factors—such as low sperm count or decreased sperm swimming 

ability—diminished chances for conception.
 
 
The gene in question, DEFB126, helps to manufacture a protein that coats sperm in a sugary carbohydrate. Without that 

protective coating, sperm seemingly have trouble evading the immune defenses inside the female reproductive tract and 

"are typically rapidly dispatched," said Cherr at a press briefing on the paper Wednesday.


The recent research followed hundreds of couples in China who were trying to have their first child and had no known 

fertility issues. Couples in which the man had two copies of the genetic mutation were about 30 percent less likely to 

become pregnant or have a child within two years. Although the infertility link will need to be tested in other 

populations, the researchers said that they had also found similar frequencies of the genetic mutation in European and 

African samples.


Why would such a seemingly disadvantageous genetic mutation remain so common in the population? The researchers are 

scratching their heads over that question, too, but they suggested at the press conference that there might be some 

as-yet undiscovered advantage to having the mixed genetic makeup of one mutated gene and one normal one. If so, natural 

selection might favor retaining the mutation at a fairly high frequency despite its disadvantages.


About 13 to 14 percent of all couples in their reproductive years experience infertility, which is defined as being 

unable to conceive after one year of trying. The roots of infertility are split fairly evenly between the sexes, but 

about 17 percent of cases have no apparent biological basis. Men with the newly discovered genetic mutation often have 

sperm that appear perfectly normal to standard evaluations. Thus this genetic variation "potentially clouds 

interpretation of male fertility," said another of the paper’s co-authors, Theodore Tollner of the department of 

obstetrics and gynecology at U.C. Davis,at the Wednesday briefing.


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As Steve Rozen of the Duke Graduate Medical School in Singapore notes in an <a href="http://stm.sciencemag.org/content/3/92/92ps31.short?rss=1" target="_blank" class="news_link" >essay</a> in the same journal about the new 

paper, it is "too soon to predict" how knowledge of the mutation will affect efforts to overcome infertility. But 

Tollner and his colleagues suggested that screening for the mutation (possibly even with an at-home genetic <a href="http://www.scientificamerican.com/article.cfm?id=new-test-for-male-inferti" target="_blank" class="news_link">test</a> in the 

near future) could become a first step for suspected low fertility or infertility. And if all the sperm need is the 

protein coating, an answer to the problem could be as simple as a vaginal gel that could deliver the missing layer to 

sperm (a method that has already been shown to work in monkeys but has yet to be tested in—or approved for—humans).


So if the new link is supported by future research, it might help doctors and couples get to the bottom of their 

conception conundrum sooner—and recommend the appropriate remedy, whether that might be artificial insertion of sperm or 

<a href="http://www.scientificamerican.com/article.cfm?id=nobel-medicine-2010" target="_blank" class="news_link" >in vitro fertilization</a> (IVF). This clarity could reduce "costly protracted clinical evaluations and emotional stress" 

that often accompanies difficulties conceiving, the team wrote in their paper. "Knowing right up front if this mutation 

is present can give you a lot of important information and lead you down the right path," Tollner said. 


Image courtesy of iStockphoto/ktsimage; video courtesy of Theodore Tollner

Lowered Male Fertility Linked to Common Genetic Mutation (Video)