This article will discuss some conditions that are not included in 23andMe reports.
The 23andMe PGS test is not intended for diagnosis of disease or condition, pediatric, prenatal or pre-implementation purposes. For testing related to a personal or family history of a particular genetic disease, please consult a healthcare provider in order to ensure that you are pursuing the most appropriate test for your personal situation. Although we recognize there is interest in reports on the following topics, reports are not available on any of the following topics. The list below is not comprehensive, and conditions not listed here are not in our current 23andMe reports.
- Copy number variations (CNVs), balanced rearrangements and mosaicism
- Trinucleotide Repeat Disorders
- MTHFR analysis, such as a targeted mutation analysis, methylation analysis or detox profile
Genes are recipes that provide the instructions for how our bodies work. The instructions are spelled out using 4 letters, A, T, G, or C, called nucleotides. The technology 23andMe uses looks for specific typos (variants) in the gene instructions (called SNPs or Single Nucleotide Polymorphisms). This approach to testing, called genotyping, is not designed to identify all potential variants.
23andMe is not designed to analyze for repeated, inserted, inverted, translocated or deleted segments of DNA. In most cases we cannot provide information about copy number or other genetic features that are related to the number or order of base pairs present. The 23andMe service cannot address:
- Trisomies (for example trisomy 21 also known as Down syndrome) and copy number variations
- Trinucleotide repeat disorders (for example Fragile X, Huntington’s disease)
- Repeats, insertions, rearrangements or deletions
Trisomies and copy number variations
Typically people have 23 pairs of chromosomes. The first 22 pairs are numbered 1-22 and the 23rd pair, known as the sex chromosomes, are either XX for females or XY for males. Within each pair you receive one copy from mother and the other from dad. In some cases, a person may receive an extra chromosome. The presence of three copies of a particular chromosome, rather than the usual two copies, is called a trisomy. Examples of trisomies include: Down syndrome (trisomy 21), and Klinefelter syndrome (XXY).
It is possible to have a trisomy, such as Klinefelter syndrome (XXY) and not be aware. In addition, it is possible to have a partial trisomy where an extra copy of just part of a chromosome has been received. This can be referred to as a partial trisomy or a copy number variant (because the number of copies is different than typically expected). When we analyze a person’s DNA and more than two copies of a gene or chromosome is present, our algorithms report a result for just two of the three or more copies. Thus, our algorithms cannot reliably determine from the raw genotype data whether three chromosomes are indeed present. A "Not determined" result may happen more frequently for markers in regions affected by copy number variation, but "Not determined" may also occur for other reasons.
Trinucleotide repeat disorders
We know that some genes have trinucleotide repeats, groups of three letters (e.g. CAG) that repeat over and over. However there is a limit to the number of times these trinucleotide repeats can be repeated without disrupting the gene and causing disease. Disorders that are caused by an abnormal number of repeats include Huntington's disease and Fragile X syndrome.
The genotyping platform 23andMe uses is not capable of detecting trinucleotide repeats and therefore 23andMe does not include any reports on trinucleotide repeat disorders. Nor is there relevant data related to trinucleotide repeat disorders in the raw data.
Repeats, insertions, translocations or deletions
Disorders resulting from the addition or loss of genetic material (from part of one gene to hundreds of genes) includes DiGeorge syndrome (aka 22q11.2 deletion syndrome) and Cri du Chat syndrome (5p- where part of chromosome 5 is missing). In addition, the majority of Spinal Muscular Atrophy (SMA) and Duchenne Muscular Dystrophy (DMD) cases are due to loss of genetic material (in each case just part of the gene is missing).
As our technology is not designed to analyze for repeated, inserted, inverted, translocated or deleted segments of DNA, in most cases we cannot provide information about copy number or other genetic features that are related to the number or order of base pairs present.
For more information about some of these conditions and where to inquire about testing, visit the resources below:
- International Huntington Association
- Fragile X Research Foundation of Canada
- Global Down Syndrome Foundation
- Lynch Syndrome International