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Microbiology and virology 
A strain is a genetic variant or subtype of a micro-organism (e.g. virus or bacterium or fungus). For example, a "flu strain" is a certain biological form of the influenza or "flu" virus. Compare clade.
The term has no official ranking status in botany; the term refers to the collective descendants produced from a common ancestor that share a uniform morphological or physiological character. A strain is a designated group of offspring that are descended from a modified plant produced by conventional breeding by biotechnological means or result from genetic mutation.
As an example, some rice strains are made by inserting new genetic material into a rice plant, all the descendants of the genetically modified rice plant are a strain with a unique genetic code that is passed on to later generations; the strain designation, which is normally a number or a formal name, covers all the plants that descend from the originally modified plant. The rice plants in the strain can be bred to other rice strains or cultivars, and if desirable plants are produced, these are further bred to stabilize the desirable traits; the stabilized plants that can be propagated and "come true" (remain identical to the parent plant) are given a cultivar name and released into production to be used by farmers.
A mouse or a rat strain is a group of animals that is genetically uniform. Strains are used in laboratory experiments. Mouse strains can be inbred, mutated or genetically engineered, while rat strains are usually inbred.
Misinterpretation with the term race 
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While some people misinterpret the term subspecies to be interchangeable with race this is technically incorrect in a biological view point, since the specimens actually have to have different DNA and have conflicts with breeding, not merely different traits.
For instance, the extremely wide variance of dogs do not count as subspecies. They merely have traits, traits that can be shared between each other. But genetically, they're all basically the same, and can breed easily. They're all of the subspecies Canis lupus familiaris.
However, all Canis Lupus Familiaris are genetically distinct from their similar cousin, Canis lupus dingo, and, obviously, Canis lupus. They can breed in certain cases, but not always, and traits won't always be shared.
In humans 
Between all humans, all traits can be shared and any "race" can freely interbreed. Therefore, there is not enough genetic variation for races to be called subspecies. Races are social constructs, and are not separated by reproductive barriers, and so cannot be considered strains.
- Usher, George (1996), The Wordsworth Dictionary of Botany, Ware, Hertfordshire: Wordsworth Reference, p. 361, ISBN 1-85326-374-5
- Maugh II, Thomas H. (18 February 2008). "Geneticist shaped hybrid rice strains - Los Angeles Times". Los Angeles Times.