The genetic changes that cause trimethylaminuria (on chromosome 1
) are very different from those that cause Down syndrome
(and involve chromosome 21
) and the two conditions are completely unrelated. Although it is uncommon, it is possible for a person to have Down syndrome, as well as an autosomal recessive
condition such as trimethylaminuria.
Trimethylaminuria is usually caused by changes (mutations
) in the FMO3 gene
and is inherited
in an autosomal recessive manner. This means that to be affected, a person must have a mutation in both copies of the responsible gene
in each cell
. The parents of an affected person usually each carry one mutated copy of the gene and are referred to as carriers
. Carriers typically do not show signs or symptoms of the condition, although carriers of trimethylaminuria may experience mild symptoms or have temporary episodes of fish-like odor.
When two carriers of an autosomal recessive condition have children, each child has a 25% (1 in 4) risk to have the condition, a 50% (1 in 2) risk to be a carrier like each of the parents, and a 25% chance to not have the condition and
not be a carrier.
Most cases of Down syndrome result from trisomy
21, which means each cell in the body has three copies of chromosome 21
instead of the usual two copies. The extra genetic material
disrupts the normal course of development, causing the characteristic features of Down syndrome. A small percentage of people with Down syndrome have an extra copy of chromosome
21 in only some of the body's cells (mosaic Down syndrome), and others may have it because part of chromosome 21 becomes attached (translocated) to another chromosome during the formation of reproductive cells
(eggs and sperm) or very early in fetal development (translocation
Last updated: 3/26/2015
A diagnosis of trimethylaminuria is often suspected based on the presence of characteristic signs and symptoms. A urine test and genetic testing
can then be ordered to confirm the diagnosis. The urine test measures the level of trimethylamine in the urine, as affected individuals would be expected to have high trimethylamine urine levels. This test may be done after a person is given a dose of choline by mouth.
Genetic testing is available for FMO3
, the gene known to cause trimethylaminuria. Carrier testing
for at-risk relatives and prenatal testing are possible if the disease-causing mutations in the family are known.
GeneReviews offers more specific information about the diagnosis of trimethylaminuria. To access this resource, please click here
Last updated: 1/24/2017
We hope this information is helpful. We strongly recommend you discuss this information with your doctor. If you still have questions, please
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