Republican Senator Rick Santorum has dropped out of the presidential race following a recent hospitalization of his 3-year-old daughter, who suffers from a chromosomal defect called Trisomy 18.

Trisomy 18, also known as Edward's syndrome, is a condition caused by a person inheriting three versions of the 18th chromosome instead of the usual two. The defect occurs in about one in 3,000 live births, and more often results in miscarriages.

The developmental issues caused by Trisomy 18 are more life-threatening in the early months and years of life than those caused by Down syndrome, a related condition resulting from the inheritance of three copies of chromosome 21. Only 10 percent of children born with Trisomy 18 survive to their first birthdays, and very few live to adulthood.

The presence of excess genetic material from the extra 18th chromosome can cause a range of complications that vary in severity from case to case. Some of the worst problems are heart defects , kidney dysfunction, an inside-out digestive tract, disconnection of the esophagus and stomach, fluid on the brain and severe developmental delays.

Trisomy — whether it be 18, 21 or one of the rarer forms — occurs when cell division proceeds abnormally in one or the other parent. Generally, humans have 23 pairs of chromosomes in each of their cells, and these get split up to create sperm or eggs that each contain only one set of chromosomes; during fertilization, the sperm and egg combine to create an embryo with 23 pairs of chromosomes, like each parent has.

However, if one of the parent's chromosome pairs fail to separate properly during cell division, a sperm or egg may end up with a second copy of one of the chromosomes. If that sperm or egg makes it to fertilization, the resulting embryo will also have that extra copy of the chromosome (the 18th one in the case of Trisomy 18).