= Reassortment
<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 $A_1A_2A_3$ and $B_1B_2B_3$ allow the reassortant combination $A_1B_2A_3$. 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>.<Evolution of human influenza A viruses in nature: recombination contributes to genetic variation of H1N1 strains by J. F. Young and P. Palese>[Abstract; A/Cal/10/78 segment-origin inference]{ref}
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