Vertical gene transfer occurs during
Vertical gene transfer conveys genetic information from parent cell to daughter cells through genome replication and division. In bacteria, this occurs during binary fission where chromosome replication followed by septation yields two genetically identical clones, except for rare mutations. All chromosomal genes and stable plasmids transmit vertically, defining lineage inheritance. This contrasts with horizontal gene transfer which moves genes between contemporaneous cells irrespective of lineage. Vertical transfer underlies phylogenetic trees, clonal expansion, and inheritance of essential housekeeping functions across generations.
Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Vertical vs Horizontal Transmission