Robertsonian Translocation and Down Syndrome

The effects of a this translocation depend on whether or not it is balanced

Chromosomes involved in Robertsonian translocation
Chromosomes involved in Robertsonian translocations. Credit: Courtesy of Human Genome Project

A Robertsonian translocation is one of two types of chromosomal translocations. Translocations occur when a piece of a chromosome breaks off and sticks to another chromosome. Chromosomes are shaped like the letter x. The top part of the chromosome is known as the short arms. The short arms are connected to the bottom half, the long arms, by a centromere.

In a Robertsonian translocation, the short arms are broken and the long arms fuse together with the centromere towards the end. 

Robertsonian translocations can be either balanced or unbalanced. Balanced Robertsonian translocations produce no symptoms, and does not affect life span. People with this type of translocation are considered carriers. Carriers of a Robertson translocation have 45 chromosomes, instead of the normal 46. Those with the unbalanced form may have significant health problems as well as a shortened life span. If a number 21 chromosome is involved in forming a translocation, this can cause Down syndrome.

Only certain specific chromosomes can form Robertsonian translocations –– chromosomes 13, 14, 15, 21, and 22.


In most cases, Robertsonian translocations occur randomly. Translocations occur while the sperm and eggs (more commonly the eggs) are forming. There is nothing you did to cause it and there is nothing you could have done to prevent it.


One in 1,000 people is born with a Robertsonian translocation. 

What Happens If I Am a Carrier? 

If you are a carrier, you most likely will not notice any symptoms until you try and get pregnant. Men who carry a balanced Robertsonian translocation are more likely to have a lower sperm count. Carriers also have an increased risk of pregnancy loss.

People who have two long arms of the same chromosome –– 13:13, 14;14, 15;15, 21;21, and 22;22 –– will produce sperm and eggs with unbalanced chromosomes, making it impossible for full-term pregnancy to occur.

For all other carriers, there are four possible pregnancy outcomes:

  1. The pregnancy and baby are both normal. The pregnancy is carried to term and the baby is born with the normal 46 chromosomes. 
  2. The pregnancy is normal but the baby carries a balanced Robertsonian translocation. In this case, the baby (like their parent) carries the translocation and does not have any noticeable health or developmental disabilities. 
  3. The baby is born with a chromosome disorder. In most cases, babies are born with translocation Down syndrome. 
  4. The pregnancy results in miscarriage or never fully forms. If the sperm or egg does not have balanced chromosomes, the pregnancy might not form properly, miscarry, or result in a stillbirth. 

If you carry a Robertsonian translocation and are trying to get pregnant, speak to a genetic counselor to learn more about your risks, specifically. 

Unbalanced Robertsonian Translocations

A Robertsonian translocation can produce health and developmental disabilities if it is unbalanced.

The most common type of unbalanced translocation is translocation Down syndrome. Translocation Down syndrome produces the same symptoms and physical characteristics of regular Down syndrome. In the translocation type of Down syndrome, the child has three copies of the long arm of chromosome 21 instead of two. Most children with this type of Down syndrome are born to chromosomally normal parents. However, balanced carriers can, less commonly, have children with this type of Down syndrome too. 

Other unbalanced Robertsonian translocations include translocation trisomy 13, which causes Patau syndrome; uniparental disomy, which results in the chromosome trying to correct itself; corrected chromosome 14, which can cause developmental delays; and corrected chromosome 15, which can cause symptoms similar to Prader-Willi syndrome or Angelman syndrome.



RareChromo. Robertsonian Translocation (2005).

MedicineNet. Definition of a Robertsonian Translocation. (2012).