Horizontal gene transfer is the movement of genetic material between organisms outside of normal parent-to-offspring (vertical) inheritance. In prokaryotes it happens through transformation, transduction, conjugation, and transposition, and it increases genetic variation that natural selection can act on.
Most organisms get their DNA the boring way: a parent passes it down to offspring. That's vertical inheritance. Horizontal gene transfer (HGT) is the rule-breaker. It's when an organism picks up genetic material from a source other than its parent, sometimes from a totally different species.
In prokaryotes (bacteria and archaea), HGT happens four main ways. Transformation is when a cell soaks up loose DNA from its environment. Transduction is when a virus accidentally carries DNA from one bacterium to another. Conjugation is the cell-to-cell handoff, where two bacteria connect and one transfers DNA directly. Transposition is when DNA segments (transposons) jump around within or between DNA molecules. All four show up by name in EK 6.7.C.1, so know them. A related idea: two similar viruses infecting the same host cell can recombine their genetic information, which is how new viral strains emerge.
Horizontal gene transfer lives in Unit 6: Gene Expression and Regulation, specifically Topic 6.7 Mutations. It directly supports AP Bio 6.7.C, explaining how alterations in DNA sequences contribute to variation that natural selection acts on. EK 6.7.C.1 spells out that horizontal acquisition of genetic information increases genetic variation in prokaryotes. That's the big-picture payoff. More variation means more raw material for evolution. This is why HGT is really a bridge concept: it sits in the gene-expression unit but its whole point is to feed the evolution storyline (genetic variation, selection, fitness).
Keep studying AP® Biology Unit 6
Conjugation, transformation, transduction, transposition (Unit 6)
These four aren't separate topics, they're the actual mechanisms of HGT named in EK 6.7.C.1. Think of HGT as the headline and these four as the methods. If a question asks 'how' DNA moved between bacteria, you pick one of these.
Mutations as a source of variation (Unit 6, Topic 6.7)
Point mutations and frameshifts create new alleles within a cell's own genome. HGT does something different: it imports already-existing genes from elsewhere. Both end up doing the same job for evolution, adding genetic variation, but HGT can spread a useful gene fast instead of waiting for a lucky mutation.
Natural selection and antibiotic resistance (Units 6-7)
A resistance gene that arises in one bacterium can hop to others through conjugation, then selection takes over. The antibiotic kills the non-resistant cells and the resistant ones reproduce. HGT is the reason resistance can sweep through a population so quickly.
Viral recombination (Unit 6)
EK 6.7.C.1 notes that related viruses infecting the same host can recombine their genetic material. It's the same theme as HGT, getting new gene combinations without standard reproduction, which is how flu strains and other viruses keep changing.
Multiple-choice questions love pairing HGT with antibiotic resistance. A common stem describes a bacterial population that becomes resistant to a new antibiotic over several generations and asks you to explain it from an evolutionary angle. You need to connect the variation source (mutation or HGT) to selection acting on it. Other stems test whether you can match the right mechanism to a scenario, like recognizing that two bacteria physically transferring DNA is conjugation, not transformation. No released free-response question uses 'horizontal gene transfer' by name, but it supports exactly the kind of variation-and-selection reasoning FRQs reward. If an FRQ asks how prokaryotes generate genetic variation beyond mutation, naming HGT and one of its mechanisms is the move.
Vertical inheritance is genes passed from parent to offspring (the standard way, generation to generation). Horizontal gene transfer is genes moving sideways between organisms that aren't parent and offspring, sometimes even across species. Easy memory hook: vertical goes down a family tree, horizontal jumps across it.
Horizontal gene transfer moves genetic material between organisms outside of parent-to-offspring inheritance, and it increases genetic variation in prokaryotes (EK 6.7.C.1).
The four mechanisms to know are transformation (uptake of DNA), transduction (viral transfer), conjugation (cell-to-cell transfer), and transposition (DNA segments jumping around).
More genetic variation means more material for natural selection, which is why HGT connects Topic 6.7 to the bigger evolution theme.
Antibiotic resistance spreading fast through bacteria is the classic exam example of HGT plus selection working together.
Vertical inheritance goes down a family tree; horizontal gene transfer jumps across it, even between different species.
It's the transfer of genetic material between organisms outside of normal parent-to-offspring inheritance. In prokaryotes it happens through transformation, transduction, conjugation, and transposition, and it boosts the genetic variation that natural selection acts on (EK 6.7.C.1).
No. A mutation creates a new DNA sequence by changing the cell's existing genome, like a point mutation or frameshift. Horizontal gene transfer imports genes that already exist from another organism. Both add genetic variation, but they do it in completely different ways.
Vertical inheritance passes DNA from parent to offspring, straight down the generations. Horizontal gene transfer moves DNA sideways between organisms that aren't parent and child, sometimes across species. Vertical goes down the family tree; horizontal jumps across it.
A resistance gene in one bacterium can transfer to others through conjugation or other mechanisms. When an antibiotic shows up, it kills the non-resistant cells and the resistant ones survive and reproduce. HGT lets the helpful gene spread through the population fast instead of waiting for each cell to mutate on its own.
Transformation (a cell takes up free DNA from its environment), transduction (a virus carries DNA between bacteria), conjugation (two cells connect and transfer DNA directly), and transposition (DNA segments move within or between DNA molecules). All four are named in EK 6.7.C.1.
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