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P. Palese investigated the genetic evolution of Influenza A virus. A 1979 study with J. F. Young compared RNA fingerprints and peptide mapping to infer the origins of different parts of a viral genome.^[ref]

The strain A/Cal/10/78 retained the H1N1 designation, yet the analysis suggested that some internal genetic segments came from an H3N2 parent. The result supported reassortment: a surface subtype need not describe the ancestry of every segment in a segmented genome. This distinction lets a scientist ask two different questions about the same virus: what surface antigens it carries, and where its complete genetic material came from.^[ref]

Reassortment is the mixing of intact genetic segments from different parental viruses into progeny with a segmented genome. If parental viruses reproduce in the same cell, a progeny genome can inherit some segments from one parent and others from the other. This exchanges whole segment identities; it does not require a crossover within each RNA or DNA segment.

For a toy virus with three distinguishable segments, parents $A_1{A}_2{A}_3$ and $B_1{B}_2{B}_3$ allow the reassortant combination $A_1{B}_2{A}_3$. Independent binary choice at each position gives $2^3=8$ possible combinations, including the two parental combinations. This is a combinatorial count; packaging compatibility and viability can prevent many combinations from appearing.

In a 1979 study, RNA oligonucleotide fingerprints and protein peptide maps suggested that an influenza strain retained some segments from earlier H1N1 viruses while other segments most likely came from an H3N2 parent. The unchanged H1N1 surface subtype therefore did not imply unchanged ancestry across the whole genome.^[ref]

A segmented viral genome is a virus genome composed of two or more distinct nucleic-acid segments that together provide its genetic information. The phrase applies segmented genome organization specifically to viruses. In Influenza A virus, the eight Negative-sense RNA segments make inheritance a question of which complete segment set a progeny virus receives, as well as which mutations each segment carries.

When related viruses infect the same cell, progeny can receive segments with different parental origins through reassortment. This differs from exchanging sequence within a segment. J. F. Young and P. Palese compared oligonucleotide and peptide maps of influenza isolates and inferred that one strain combined surface-associated and other segments derived from different parental lineages. The evidence supported mixing of segment histories rather than explaining every difference by sequential mutation within one lineage.^[ref]

A three-segment toy model makes the combinatorics clear. If each segment type can come from either parent and exactly one copy of each type is selected independently, there are $2^3=8$ possible parental-origin combinations, including the two unmixed parental sets. This is an upper-counting illustration, not a prediction that all combinations occur equally or produce viable viruses: packaging, compatibility and selection can constrain the outcome.

Segmentation alone also does not specify the number of particles required to transmit the complete information. Genome organization and how components are packaged are related but distinct properties; they must be established for the virus being discussed.