What is 4p+?

Chromosomes are found in the nucleus of all body cells and carry the genetic characteristics of each individual. Each chromosome consists of a short "p" arm and a longer "q" arm. Every person has 22 pairs of human chromosomes with an unequal 23rd pair (XY for males, XX for females).

When a person has an extra chromosome added to one of the normal pairs it is referred to as trisomy. Trisomy can occur with any chromosome, but most will result in miscarriage. The most common types of trisomy to survive to birth are Trisomy 21 (Down Syndrome), Trisomy 18 (Edwards Syndrome), and Trisomy 13 (Patau Syndrome). The presence of an extra chromosome results in a characteristic pattern of features, birth defects, and medical problems.
Trisomy 4p, also referred to as a 4p duplication or 4p+, is a partial trisomy involving an extra copy of the short p arm of chromosome 4.  Each case of 4p+ is unique in how much of the 4p arm is duplicated and where it is attached (often on another chromosome). This results in a wide variety of presentation and severity between individuals with 4p+; those with larger duplications generally being more severely affected than individuals with smaller duplications.

Image from rarechromo.org

In about 75% of cases of 4p+ one of the parents has a balanced translocation, meaning that they have the same duplication but it is located on both chromosomes, so it balances each other out and doesn't cause any problems. The other 25% are referred to as de novo, meaning it is a random mutation. If a parent carries a balanced translocation any of their children have the possibility of having a balanced translocation like the parent's, an unbalanced translocation (trisomy 4p), or normal chromosomes.

The occurrence of Trisomy 4p is extremely rare. The number that is most often quoted is "less than 100 documented cases", but currently there are no accurate statistics to indicate what the occurrence rate truly is. We know that with increased availability of genetic testing more and more people are receiving a 4p+ diagnosis.

Trisomy 4p is not a disease or illness. It is simply a unique set of chromosomes that leads to a person being programmed a little bit differently than most people. There are disorders and illnesses that often accompany Trisomy 4p that present many challenges for the child and their families. It's the differences and problems that are easily noticed on the outside by others, but that extra piece of chromosome also carries with it an extra amount of strength, courage, and awesomeness that the rest of us could only dream of possessing.