Trisomy 18: What Is The Disease Affecting Rick Santorum's Daughter?

Bella Santorum: What Is Trisomy 18?

Few politicians have wed their political and family lives quite like GOP presidential candidate Rick Santorum. Frequently hitting the campaign trail with his wife and children by his side, the father-of-seven has spoken publicly and intimately about tragedies his family has faced over the years, including a stillborn baby that became the focus of a 2005 Washington Post article.

Now the Santorum family is faced with another tragedy: the life-threatening illness of their 3-year-old daughter Bella, who suffers from Trisomy 18. The genetic disorder is in the same category of the disease affecting Sarah Palin's son Trig, who was born with Trisomy 21, or Down Syndrome.

Like Trig Palin, Bella Santorum has become a central figure in her parent's presidential campaign, inspiring both praise for Santorum and criticism from those who believe he may be using her condition to further his agenda of fighting healthcare reform legislation.

So what exactly is Trisomy 18?

In short, Trisomy means a person has an extra chromosome in each cell in his or her body, excepting reproductive cells. Most human beings are born with 46 chromosomes because during fertilization, 23 chromosomes from the sperm and 23 chromosomes from the egg unite to form 23 matched pairs. However, sometimes a sperm or an egg cell has an extra chromosome. If that particular sperm or egg ends up playing a part in fertilization, the embryo will end up with an entire copy of the extra, or third, chromosome (hence the term "trisomy").

What's so bad about having a third chromosome?

Chromosomes are responsible, in part, for encoding and synthesizing proteins. If a person has three chromosomes in each cell, protein levels can be out of balance and cause a variety of conditions.

According to an article on Trisomy 18 by the U.S. National Library of Medicine, children like Bella Santorum who were born with Trisomy 18 often suffer from severe growth retardation, malformed facial features, heart defects and other abnormalities.

Bella's suffering was particularly acute during the three days she traveled with her father to Iowa for the August straw poll, Santorum told the Washington Post.

"She has a lot of difficulty breathing, a lot of complications as a result of that," Santorum said, describing the ordeal as "gut-wrenching" because he "wanted to protect her privacy at that point."

What's Bella's prognosis?

Half of children born with Trisomy 18 die soon after birth, and less than 10 percent survive beyond a year, according to The National Institute of Health. However, babies with the disorder that do survive infancy can live to adulthood.

For Santorum's part, he plans to continue his presidential bid while his daughter remains at home.

WATCH: Santorum speaks with Christiane Amanpour about the decision: