Trisomy Awareness: Understanding the Role of Chromosome Translocations

March is Trisomy Awareness Month, which is fitting, since a trisomy means the presence of a third copy of a chromosome and March is the third month of the year. Down syndrome is the most common chromosome condition caused by a trisomy, specifically trisomy 21. While many people may be familiar with Down syndrome, the understanding of how it happens is less common. Trisomy 13 and trisomy 18 are examples of rarer trisomies. Below we discuss how trisomies occur and outline the role that chromosome translocations can play in the occurrence and recurrence of trisomies.

Individuals typically have 23 pairs of chromosomes, for a total of 46. Chromosomes contain our genetic information. Twenty-three come from the egg and 23 come from the sperm. Chromosomes 1 through 22 are the same in males and females. The twenty-third pair are the sex chromosomes; females usually have two X chromosomes, and males usually have one X and one Y chromosome. When someone has trisomy 21, for example, they have three copies of chromosome 21, instead of the typical two copies. Most trisomies occur randomly, when the egg or sperm has an extra chromosome. Maternal age is a risk factor for a trisomy to spontaneously occur in a pregnancy. For most couples who have a child with a trisomy, the chance of having another child with a trisomy is low. However, if one member of the couple has a balanced chromosome translocation, the chance may be higher.  

A translocation is when a chromosome, or a piece of a chromosome, is attached to another chromosome. Some translocations occur spontaneously, while others are inherited. Chromosome translocations can be balanced or unbalanced, depending on whether there is any extra or missing chromosomal material or just a rearrangement of it. Approximately one in 250 to one in 500 healthy individuals has a balanced translocation.¹ These individuals do not have any outward signs of having a translocation or have any symptoms, but the translocation may affect their reproductive health. A person with a balanced translocation has a higher chance of producing eggs or sperm with extra and/or missing chromosomal material (an unbalanced translocation), which can lead to infertility, miscarriage, or birth of a child with a trisomy or other chromosomal imbalance.

No. There are many different types of genetic testing that can analyze chromosomes, but they each have advantages and disadvantages. Some types of fluorescent in situ hybridization (FISH) tests can detect if certain chromosomes are extra or missing but may not always show how the chromosomes are arranged. Similarly, chromosomal microarray (CMA) can tell if there is extra or missing chromosomal material but cannot determine how it is arranged and cannot detect if there is a balanced translocation. Therefore, chromosome analysis, or karyotyping, is often the best way to identify balanced translocations. Some balanced translocations are easily detected through a chromosome analysis/karyotype, but others can be challenging to discern. It is best practice for healthcare providers to inform the laboratory of the particular chromosome(s) of concern to ensure the correct type of testing is performed.

If there is a family history of a chromosome condition, it is best to consult with a specialist, such as a genetic counselor, for a comprehensive risk assessment. A genetic counselor can review the family history and genetic test results to determine if additional genetic testing is necessary.

In the vast majority of cases, yes! That’s why it’s important to check with a genetic counselor about your particular test results and family history. By doing a comprehensive risk assessment, a genetic counselor can help determine the chances for a pregnancy with a trisomy and review testing options and medical procedures that exist before, during, and after pregnancy.  

1. Tšuiko O, Dmitrijeva T, Kask K, et al. Detection of a balanced translocation carrier through trophectoderm biopsy analysis: a case report. Mol Cytogenet. 2019;12:28. Published 2019 Jun 18. doi:10.1186/s13039-019-0444-2