An Adam’s apple, five-o-clock shadow and rippling muscles may be some of the masculine qualities we think of when we consider what makes men manly.

But when it comes to our genetics, what makes a man a man boils down to one simple letter … Y. Women inherit two X chromosomes from their parents, whereas men inherit an X and a Y.

However, a new study from researchers in Sweden shows that in some men, the Y chromosome may be in danger. One bad habit indulged in by 20 percent of American men may be chipping away at their Y chromosomes in their blood cells, potentially putting them more at risk for developing and dying from certain types of cancer. What’s the bad habit? You guessed it … smoking.

Previous studies have shown that male smokers tend to fare worse when diagnosed with certain cancers than their female counterparts, and researchers think this new genetic wrinkle may help explain why.

The new study included blood samples and data about the lifestyles of about 6,000 men.

The chance of having blood cells that lacked Y chromosomes was quadrupled in smokers when they were compared to nonsmokers. The good news for smokers is quitting smoking seems to reverse these effects.

Researchers can’t yet confirm whether missing Y chromosomes are to blame for the higher risk of death men face from certain cancers. Age could play a factor as well. However, the researchers are busy on the next phase of their work — developing a test that will examine chromosome loss and men’s risk for cancer.

For men who want to snuff out cigarettes, resources are available to help you develop a plan to quit. From counseling and nicotine patches to acupuncture and hypnosis, there are a variety of ways to combat nicotine addiction. Visit smokefree.gov for tips and solutions to help you put down cigarettes for good.