Orthomyxoviridae

Orthomyxoviridae is a family of viruses with Negative-sense RNA divided among multiple genome segments. Their particles have a viral envelope; Influenza A virus provides a familiar example with eight RNA segments. Segmentation describes the genome's physical organization, while negative sense describes its relation to the messages used for translation.^[ref]

A segmented viral genome allows reassortment: progeny can inherit segments of different parental origins when the necessary biological conditions are met. For a purely combinatorial model with eight distinguishable segment positions and two possible origins at each, there are $2^8=256$ origin patterns. This counts arrangements under the stated assumptions, not viable strains or equally probable biological outcomes.^[ref]

The distinction is useful when interpreting experiments: detecting a changed segment combination differs from observing a new change within one RNA sequence. Likewise, an enveloped particle's external appearance cannot reveal the number or origins of its genome segments. Structural, sequence and replication evidence answer different questions about the same virus.