Transversion
a mutation changing a purine into a pyrimidine, or vice versa (i.e., changes from A or G to C or T and changes from C or T to A or G).

Trilobite
An extinct marine arthropod common from the Cambrian to Permian eras (570 245 million years ago). Trilobite fossils are abundant in rocks of this period. Trilobites were 10 675 mm long, and their flattened oval bodies were divided into three lobes by two longitudinal furrows. They had a single head shield, which bore a pair of antennae and in many species, insect like compound eyes. This was followed by more than 20 shortbody segments, each with a pair of forked appendages. Many trilobites apparently burrowed in sand or mud, preying on other animals or savenging.

Trisomy 18
A chromosome condition whereby the individual has three instead of two copies of chromosome 18. Individuals with Trisomy 18 have severe mental retardation and usually severe heart disease with 90% dying within the first year of life.

Triplet Repeat (Trinucleotide Repeat)
A pattern where a given set of nucleotides is repeated multiple times (many times more than in an individual without the condition) which causes a specific disease.

Trisomy 21
Scientific term for Down Syndrome. The individual has three copies of chromosome 21 instead of the usual two copies (one from each parent).