Equivalence Between Concepts¶
VRS allows for the expressive representation of variation concepts. Sometimes this allows for forms that can be reduced from one to another, and sometimes bi-directionally. Examples of this include the bi-directional translation of chromosomal bands to sequence coordinates via a sequence-band mapping, the uni-directional translation of a gene to one or more sequence location(s), and the use of different Sequence Expression instances that would resolve to the same sequence. Similarly, authority-based concepts such as Gene are entirely dependent on the definition of the concept by that authority–we provide no guidance on how to translate or relate such concepts to one another.
We provide no guidance or mechanism to enforce “equivalence” between these concepts, because the semantics of one representation to another are distinct, even when there exists functions that equate or translate between two distinct concepts. Instead, we encourage communities to adopt policies about how and when to use the various concepts provided by VRS to represent different forms of variation. To assist in that effort, the GA4GH Genomic Knowledge Standards Work Stream is developing a specification for resource-defined Variation Concept Origination Policies (VCOPs). You can learn more about VCOPs in the VRSATILE framework.
Using Sequence Expressions¶
When using Sequence Expressions, our general recommendation is to use LiteralSequenceExpression for when the precise sequence state is of importance to the Variation concept; this is the most common use case. When the precise state is not important but instead it is desired to refer to the general sequence derived from a location on a reference sequence, we recommend using a DerivedSequenceExpression; this is typically used when describing large sequences that are approximately reference for use in some large-scale Molecular Variation or Systemic Variation concepts. RepeatedSequenceExpression is typically used for the semantic importance of describing a specific, repeated subsequence by count, such as description of CAG repeats in the ATXN7 gene, where the repeat count is a diagnostic biomarker for severe neurodegenerative disorder spinocerebellar ataxia type 7 1.